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DESIGN AND PLANNING MANUAL For

Cost-Effective Welding

International Standard Book Number: 0-87171-605-4 American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126 © 1999 by American Welding Society. All rights reserved Printed in the United States of America

NOTE: Although care was taken in choosing and presenting the data in this guide, AWS cannot guarantee that it is error free. Further, this guide is not intended to be an exhaustive treatment of the topic and therefore may not include all available information, including with respect to safety and health issues. By publishing this guide, AWS does not insure anyone using the information it contains against any liability or injury to property or persons arising from that use.

Photocopy Rights Authorization to photocopy items for internal, personal, or educational classroom use only, or the internal, personal, or educational classroom use only of specific clients, is granted by the American Welding Society (AWS) provided that the appropriate fee is paid to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, Tel: 978-750-8400; online: http://www.copyright.com ii

Contents Section

Page No.

1

Primary Concepts of Weldability..................................................................................................................1

2

Accepting Metal Fabrication Projects ..........................................................................................................5

3

Production Welding Cost Analysis................................................................................................................9

4

Modular Construction..................................................................................................................................17

5

Welding Process Selection............................................................................................................................23

6

Primary Concepts of Welding Design.........................................................................................................33

7

Fatigue Considerations.................................................................................................................................45

8

Welding Safety Considerations ...................................................................................................................51

9

Weld Joint Design Considerations ..............................................................................................................57

10

Weld Distortion and Control .......................................................................................................................63

11

Checklist for Sound Welding Decisions ......................................................................................................71

12

Defects and Discontinuities of Welding ......................................................................................................75

13

Nondestructive Examination .......................................................................................................................93

14

Information for the Welder........................................................................................................................103

15

Fitting Aids..................................................................................................................................................109

16

Welding Metallurgy: Practical Aspects ....................................................................................................119

17

Arc Stud Welding........................................................................................................................................129

18

Thermal Spray Fundamentals...................................................................................................................133

iii

SECTION

1

Primary Concepts of Weldability Contents Weldability ......................................................................................................................................................................... 2 Personnel Concerns about Fabrication Projects............................................................................................................. 2 Fundamentals of Welding Decisions ................................................................................................................................ 3 Coordination of Expertise................................................................................................................................................. 4 Education............................................................................................................................................................................ 4 The Welder, Technician, and Production Supervisor ..................................................................................................... 4 The 100% Quality Weld and Weld-Associated Failures ................................................................................................ 4 Weldability Can Be Determined by the Manufacturer .................................................................................................. 4 Bibliography/Recommended Reading List ..................................................................................................................... 4

1

SECTION 1—PRIMARY CONCEPTS OF WELDABILITY

Section 1—Primary Concepts of Weldability

Weldability

• From the customer:

Welding is used to make large and sometimes complex structures from smaller and simpler metal components. In general, a weld is a localized coalescence of metal that is used to join or repair metal components. Filler metal may or may not be added to the weld. The capacity and ability to join these metal components with available resources help define the term weldability. The definition of weldability is: “The capacity of a metal to be welded under the fabrication conditions imposed into a suitably designed structure, which will perform the intended service.” When the definition of weldability is reduced to its lowest common denominator, this definition is “it.” To ensure weldability, interaction of countless groups and individuals with specific needs may be involved. For a project to reach a successful conclusion, these needs must be identified, clarified, and, when in conflict, resolved. All metals may be considered weldable by the very fact that they exist as metals. Depending on whether you are the customer, the welder, the inspector, or the fabricator your definition of weldability may be different. Generally, it is the suitability for service, cost, and weldment aesthetics that determines weldability. With a fundamental understanding of welding and a knowledge of the resources available, “lack of weldability” causing inservice failures, production failures, cost overruns, and schedule delays may be avoided. In common usage the term “weldability” is somewhat ambiguous and about as varied as the people who use it. The “degree” of weldability can be defined in terms of: weld costs; the capacity of the fabrication to perform its intended service; the ability of the weldment to meet fabrication acceptance standards; or the difficulty of joining a material with a specific process.

“Does the fabrication perform the intended service and will it last as long as intended?” “Is the fabrication completed within cost and on schedule?” “Is it aesthetically pleasing?” • From the engineer: “Does the weld and its heat-affected zone meet the critical base metal requirements that are necessary for the intended service of the customer?” • From the manufacturer: “How much money does it cost to make the weld (i.e., what’s my profit)?” “Are there any inspections or requirements associated with the weld that will prevent acceptance of my work?” “Am I subject to any liabilities if my fabrication is not completed on time or fails in service?” • From the welder: “Do I have clearly defined instructions (such as drawings that specify weld size, joint design, material type, material size, stress relief, and welding procedures)?” “Is there adequate weld joint access?” “Is there adequate environmental protection?” “Do I have the proper equipment to accomplish the work?” “Are the fabrication requirements achievable from the standpoint of accessibility, position, distortion control, weld size, inspection requirements and fabrication sequence?”

Personnel Concerns about Fabrication Projects

“Am I trained and qualified to perform the welding required?”

Major concerns about a fabrication project and its weldability are expressed by those involved as follows: 2

SECTION 1—PRIMARY CONCEPTS OF WELDABILITY

and how they can affect the safety of personnel and the suitability of the part for service creates a desire by all individuals to follow requirements. Knowing the basics of welding engineering can prevent a company from extensive rework or a customer’s refusal to accept a manufacturer’s product.

It is obvious, since the weldability of a metal intersects so many disciplines, that the solution to many weldassociated problems and the ultimate success of a project lies in a fabricator’s ability to coordinate its available expertise. There can be no question that people are still the most important resource in the fabrication enterprise. Because of this, it is recommended that a preacceptance bid review group, including production as well as technical personnel, be established to review projects prior to acceptance. Formation of this group may be the single most important action taken to ensure a successful and profitable completion for all parties involved. Unless the same project is repeated over and over again, it is obvious that one person cannot resolve all the issues necessary for the project to be a success for all the participants.

The Welder, Technician, and Production Supervisor The resolution of many welding problems requires much more than technical solutions. Welding problems require practical solutions as well. For this reason, the practical expertise of the journeyman mechanic and his supervisor can never be overlooked. They are absolutely essential if a company expects to remain profitable. It is for this reason the use of welding engineering technicians is quite common in larger companies. Technicians frequently are trained individuals with journeymen production experience. A balance of welding engineers and welding engineering technicians is most desirable for resolving most welding problems.

Fundamentals of Welding Decisions One individual in a metals fabrication firm with a varying workload will find it difficult to make good welding decisions for all of the firm’s needs. The complexity of welding decisions is immense if the work varies. If an individual makes enough welding decisions, expertise will be required beyond that of filler metals, base metals, and welding processes. This expertise will require a background in, and integration of the technical fields.

The 100% Quality Weld and WeldAssociated Failures Designers often mistakenly expect the welds they design to have properties that equal or exceed the desired properties of the base metal. The major error in this logic is that the base metal and the weld metal do not perform independently. Rarely does the weld improve the properties of the metal it joins. The base material, the weld, and the weld’s heat-affected zone are not homogeneous regions. They are, in effect, structural and metallurgical discontinuities within the weldment. This is due to the fact that the base materials usually have undergone significant mechanical and/or thermal treatments to optimize their properties, the weld usually performs its service in the as-cast condition, and the heat-affected zone has undergone widely varying heating and cooling rates. In many cases a weld joint lowers fatigue properties and lowers impact properties; in some cases a weld joint also lowers the tensile and yield properties. However, large or complex structures must be welded with high joint efficiency, since large monolithic cast or wrought structures cannot be made in most cases. Since welds are usually located at changes of cross sections (commonly associated with highly stressed areas), the welds are often in areas where the structure fails. These failures may be wrongly attributed to welds that are perceived as inherently weak or flawed. These

Coordination of Expertise One person normally does not have expertise in all of the areas required to make sound welding decisions. It will benefit a company to have a welding engineer to handle the majority of the welding problems that will arise. However, this individual will find it necessary to coordinate with other available expertise to make the best decisions for the company. If a company is small, this may require hiring a part-time consultant.

Education There is no substitute for education of personnel involved in welding. Education is needed by the fabrication shop superintendents, the design engineers, the planners and estimators, the production supervisors, and the welding mechanics. Each has different needs. Often, a specification or procedural requirement will not be followed unless there is an understanding of why the requirement is necessary. Understanding the importance of preheats, postheat treatments, and filler metal selection 3

SECTION 1—PRIMARY CONCEPTS OF WELDABILITY

• Recognize the cause of existing and potential production problems.

failures are more likely caused by poor decisions on the part of engineers or company management due to a lack of knowledge or training. Improperly selected joint designs, welding processes, locations of welds, and filler metals are frequently associated with weld failures. The root cause of these types of failures is in the decision-making process and not in the quality of the weld.

• Properly utilize, coordinate, and develop cooperation of available expertise to make sound welding decisions. Often weld-associated failures are caused by lack of knowledge about welding and its effect on the base materials. These failures are wrongly viewed as resulting from inherent weaknesses of welds. In most cases, with proper utilization of available expertise, the failures could have been avoided.

Weldability Can Be Determined by the Manufacturer When a manufacturer designs and fabricates a weldment, it can truly be said that the manufacturer determines weldability. The manufacturer then controls most of the variables that determine weldability. The weldability of a metal, to a large extent, is determined by the manufacturer’s ability to:

Bibliography/Recommended Reading List Welding Handbook, 8th ed., vol. 1, Welding Technology (WHB-1.8). Miami, Fla.: American Welding Society.

4

SECTION

2

Accepting Metal Fabrication Projects Contents Background ........................................................................................................................................................................ 6 Defining Special Projects .................................................................................................................................................. 6 Establish a Prebid Review Group (Before Accepting Special Projects) ....................................................................... 6 Establish a Detail Planning Group (Prior to Issuing Job Orders)................................................................................ 7 Summary ............................................................................................................................................................................ 7

5

SECTION 2—ACCEPTING METAL FABRICATION PROJECTS

Section 2—Accepting Metal Fabrication Projects

Background

Special project should be defined as follows: (1) Not a repetitive project, even though a company may have procedures that are applicable to the work. This includes work that the company has accomplished before but has not done for a long period of time. (2) Any type of field work that is away from the company’s home office, and involves a significant number of man hours. Field work limits the company’s flexibility for scheduling work accurately. In addition, field work limits the company’s ability to combine work activities for economical use of materials, man hours and equipment. (3) Any project that requires many different disciplines from the same company to accomplish its task. (4) Unusual or complex work requirements (including inspections, paperwork, and records).

The critical question asked by a metals fabricator for a completed project to be considered successful is: Did the project make a profit and was it completed on time? When large or small companies have repetitive work, they are usually quite successful. They also know the pitfalls. They know what their minimum bid should be and what delivery or schedule times can be met. They probably have experienced the normal day-to-day items that threaten their profit and the quality of their work. Knowing these pitfalls, they can make allowances for them. However, for a company to be successful when it accepts work that can be defined as a special project (meaning a different type of work for the company), the engineer, the fabricator, and the welder need a fundamental understanding of the weldability of metals and the basics of sound welding decisions. During the acceptance of work, many decisions that seem innocuous to welding end up jeopardizing the weldability of materials and the service suitability of the completed weldment.

Establish a Prebid Review Group (Before Accepting Special Projects) Since a special project is a nonrepetitive project that may include multiple workshops having new or unusual requirements, the acceptance of such work should be reviewed by a small prebid review group before accepting special projects. It cannot be expected that a single person who plans and estimates jobs can assemble and understand all the technical, quality, material, and process requirements of unusual work. When the structure of a company permits, it is recommended that the membership of the preacceptance bid review group consist of comptroller, planner and estimator, design, production lead shop, and quality assurance representatives. A comptroller or planner and estimator should oversee this group. The primary goal of the group should be to assess the company’s limitations and capabilities in regard to technical, quality, and special process requirements before accepting work. The group should also determine the limitations and conditions that should be placed on the project for bid acceptance.

Defining Special Projects The critical first step for a company is the ability to recognize a special project. Nearly all special projects require high up-front costs. There are increased costs in determining: • • • • • • •

Should the company bid on the project? What should the bid be? To what extent are production instructions required? What are the costs to establish these instructions? How and when to monitor the work in production? Are the company resources adequate? Do company equipment and personnel capabilities adequately meet project requirements? • What additional training/equipment is required to meet project objectives? Resolving these issues should be accomplished with small review groups with specific tasks in mind. 6

SECTION 2—ACCEPTING METAL FABRICATION PROJECTS

chaired by engineering. After the production work starts, similar meetings should be held with the lead production shop chairing the meeting, to enable them to receive fast and direct technical resolution to unexpected fabrication problems. Internal quality reviews or audits should be planned for as work progresses. The first review should take place early in the project. This ensures that the work instructions are adequate to accomplish the work, that contract requirements are clear to the workforce, and the material that is to be received is correct. Trying to recover from incorrect material after a job has commenced is almost impossible without the company experiencing large material and labor losses. Therefore, the prebid review group should also be responsible for material acquisition recommendations. Any project is slated for failure where material requisitions are made haphazardly. Material must be ordered properly and receipt inspected accurately, if a project is to progress within the preestablished schedules. Simple, clear and concise job order instructions should be provided in the following areas:

Sometimes accepting a job just to keep the workforce busy can cost you the company. The company’s limitations and capabilities should be determined in regard to technical, quality, and special process requirements before accepting special projects. If deemed necessary from this determination, the company should place limitations and conditions on the bid acceptance, or not accept the work. To determine whether the company should bid or how much it is willing to bid, the prebid review group should evaluate the following items: • Quality requirements (establishment of special procedures, i.e., quality manual). • Inspection requirements. • Record requirements (degree of detail). • Material requirements certification—receipt inspection. • Material and labor losses in production (rework). • Cleanliness and shipping requirements. • Special process requirements (mockup testing or control samples). • Personnel availability and special qualifications. • Special welding equipment and fixtures required. • Health and environmental considerations.

• • • • • •

Establish a Detail Planning Group (Prior to Issuing Job Orders) After a special project is accepted, a planning group is needed for detailing the job requirements to the production workforce. This detailed planning effort is necessary, because the work is nonrepetitive, may involve multiple trades, and may have different quality requirements than the company normally uses. The greatest potential for noncompliance and error is when the project is nonrepetitive and has extensive quality and technical requirements. This detailed planning group should be headed up by a design engineer, if possible. Once a special project is accepted by a company, resources must be expended in advance planning. Work instructions should be clear, concise, and when practical, they should be listed as line items with specific directions and accountability. The referencing of general specifications should be avoided where contract requirements are obscure and can be easily missed. Establishing this group early helps resolve many critical issues that make a project a success. It also establishes early ownership in the project by the participants. It is crucial that this group has authority to make and implement decisions. During the planning and engineering time of the project, weekly or biweekly meetings should be held to discuss the impact of technical requirements on production with shop supervision. These meetings should be

Specification (material and fabrication). Fabrication documents. Quality requirements (receipt inspection included). Records. Inspection requirements. Special process controls.

Summary If a project has been determined (by a company’s definition) to be a special project, a prebid review group should examine all the job requirements. This group will determine whether the company wants to make a bid on the project and what considerations and limitations should be placed with the bid. The group should consist of a designer, a planner and estimator, a production shop leader or project leader, and a quality assurance expert. After the project has been accepted, a planning review group needs to be established to provide clear and concise work instructions. Job orders should provide details in areas that would be expected to cause production and record problems. Details concerning the receipt of material, quality assurance requirements, and production shop requirements must be provided. Early in the production cycle, work instructions and receipt inspection of material should be audited, or at least reviewed. During production, work surveillances should be made of workmanship, inspection, and records to ensure work instruction compliance. 7

SECTION

3

Production Welding Cost Analysis Contents Introduction ..................................................................................................................................................................... 10 Review of Cost Estimating .............................................................................................................................................. 10 Welding Cost Variables ................................................................................................................................................... 11 Rework.............................................................................................................................................................................. 11 Fire Protection ................................................................................................................................................................. 11 How to Control Costs ...................................................................................................................................................... 11 Joint Designs .................................................................................................................................................................... 12 Automation/Mechanization of New or Existing Thermal Joining Processes ............................................................. 12 Records: Accountability for Accomplishment of Work and Inspections ................................................................... 13 Cost Considerations of Nondestructive Examination (NDE) ...................................................................................... 13 Summary .......................................................................................................................................................................... 14 Bibliography/Recommended Reading List ................................................................................................................... 14 9

SECTION 3—PRODUCTION WELDING COST ANALYSIS

Section 3—Production Welding Cost Analysis

Introduction

of specific projects for a customer. As an example, this includes any personnel assigned to a specific end use project. Particular attention should be considered not to exceed the estimated manhours/mandays for project accomplishment.

The application of welding as a major fabrication process in United States industry began during the late 1930s. Prior to this time period, the welding process did not play a significant role in production applications. Welding was primarily utilized for small-scale fabrication, maintenance, or repair. Its use was accelerated by the military. The U.S. Navy required massive ship construction for deployment during World War II and the U.S. Army needed significant ordnance/weaponry equipment that could only be practically constructed by welding.

Overhead Costs Overhead costs are those costs of indirect services, materials, or other expenses that cannot be specifically identified to a productive effort/customer or do not result in direct productive work. Such examples include paid vacations, health insurance premiums, sick leave, electricity, water, building construction or maintenance, and capital investments not tied to a specific customer.

Review of Cost Estimating There are several fundamental variables (costs) that need to be considered prior to submitting a competitive bid for a specific project, in terms of welding costs. For purposes of this review, many of the following terms can apply to repair welding or new construction applications. These cost estimates include material, direct labor, overhead, fixturing, and welding. Definitions of these five expenses are listed below.

Fixturing Costs Fixturing costs are expenditures for tools, jigs, or fixtures that are directly utilized in the manufacturing process, which can be traced or charged to an end product.

Welding Costs

Material Costs

As noted in the AWS Welding Handbook, costs for welding basically include the same elements that are applicable to manufacturing costs, i.e., labor, material, and overhead. However, there are other variables that need to be considered:

Material costs involve those materials (i.e., steel plate, piping, or other hardware) that ultimately become part of a finished product. These individual components can be traced to the finished project, in terms of a unit of measure expended. Material costs also include consumables used during the manufacturing evolution. Required receipt inspection and traceability documentation should also be included.

• Joint design type. • Weld size. • Weld type. • Weld process. • Electrode deposition efficiency.

Direct Labor

• Special weld procedure development or verification of essential elements.

Direct labor is a level of effort or services that can readily be charged or identified towards accomplishment

• Heat and fire protection. 10

SECTION 3—PRODUCTION WELDING COST ANALYSIS

Welding Cost Variables

If at all possible, it is preferable to accomplish the majority of welding fabrication in the shop vs. in the field. For example, in one case history involving Cres piping, the field reject rate was 420% over those joints welded in the shop. For Monel®, the same reject criteria was 189%, and 107% higher for carbon steel. The average among these three materials welded in the field was 238% higher than the shop welds! The disparity between shop and field reject rates can be explained by reviewing the complexity behind field conditions. In many situations, the welder will encounter very tight space restrictions, which may require the aid of mirrors to complete the particular pipe weld. Other physical problems might include welding out-of-position, vs. the normal horizontal-fixed or rolled position welds found in shop welding. Some additional negative conditions may involve moisture or contamination in a piping system, or a difficult configuration that restricts the flow of the purging gas for piping welds. Another variable affecting welding reject rates/costs is the available skilled workforce. Promotion of experienced personnel into management positions, retirements, etc., can cause an increase in the reject rates. Replacement of skilled personnel with inexperienced welders can be one source of higher reject rates/costs. Regardless of the location of the particular project, job preplanning, definition of scope, and scheduling are essential. These should include an in-depth review of the applicable fabrication specifications, drawings, material lists, work documentation requirements, and schedules. Conduct meetings with the designer, shop supervisor, and other technical support staff to determine the proper direction. A strong preplanning effort will provide a solid project estimate regarding manufacturing costs and successful job accomplishment. As an example of required preplanning, determine what type of variables need to be evaluated prior to making decisions about utilization of a particular welding process for a specific job. The AWS Welding Handbook offers the following criteria for consideration:

There are a large number of welding variables that can affect the manufacturing costs. For example, the design engineer should be concerned with joint design, weld size or type, and weld process. The welding supervisor should be aware of the setup (field or shop), production welding time, finishing, and inspection costs. The welder holds a very key role. Prior to welding, he/she must be familiar with the applicable weld procedure that lists variables such as type of shielding or required flow rates, welding parameters (volts and amps), travel speed for automated/mechanized welding, preheat or interpass temperatures, and type of filler metals required.

Rework The welder, in many instances, has major control over the amount of rework. Since rework can be as high as 25% of the project costs, the existence or elimination of potential rework costs deserves a high degree of attention.

Fire Protection Currently, it is estimated that for every hour of actual welding time, a similar amount of time is required for related fire/heat protection. These costs may include the use of “firewatch” personnel and the installation of flame retardant material or cloth for protection from the welding, or burning evolutions. Finally, the company owner/management is concerned with all of the above and additionally: utility/labor rates, consumable costs, transportation, and overhead costs. It is very apparent why welding costs make a significant monetary impact in the overall manufacturing costs. Now, the next obvious question is, “How can these costs be controlled, and yet provide assurances for quality weldments?” Has the designer given adequate thought to “fitness for use,” or has the component been overdesigned which increases costs? The remaining portion of this section will attempt to introduce answers/suggestions to aid in solving these questions.

• Type of welding operation to be performed. • A clear definition of what physically can and cannot be done. • Nature of the material to be joined. Is preheating or postweld heat treatment required? • Joint geometry, thicknesses, and parts fitup/ tolerances. • Weld quality requirements. • Safety requirements: enclosures, shielding, scaffolding, etc. • Location/orientation of fixturing (positioners, etc.) is important.

How to Control Costs Many weldments involve a combination of shop and field fabrication. It is critical to understand that differences exist in job preplanning through project completion. For example, shop fabrication drawings are specific in nature, whereas drawings for field installation or modifications provide sectional views to show the location of numerous components. 11

SECTION 3—PRODUCTION WELDING COST ANALYSIS

sary down time to replace the spooled filler metal containers during the welding operation. Also, ordering filler metal in large quantities, whether it be covered or bare electrodes, decreases the basic costs per pound. Proper planning regarding future needs of specific electrode types, and their proper storage is required here.

In his discussion about estimating welding costs, in the Design of Weldments (published by the James F. Lincoln Arc Welding Foundation), Omer W. Blodgett states the following: “The cost of welding is directly affected by the amount of weld metal required. Very few people realize the great increase in weld metal and cost that results from a slight increase in weld size.”

Automation/Mechanization of New or Existing Thermal Joining Processes

Joint Designs

Another aspect of controlling manufacturing/welding costs, although it involves long-range preplanning with the possibility of only long-term payback on investment, is the introduction of automation or mechanization of new/existing thermal joining processes. However, before serious consideration is given to automate (i.e., use of robots), or to mechanize specific welding processes (GMAW, FCAW), it is critical that fabrication projects exist to support its development. Purchase of complex equipment is a large capital cost, not including associated operator training, engineering support, or maintenance. A projected return must exist. As an example, at one naval shipyard, after a successful development program involving a new welding process (Flux Core—Twisted wire), specific production applications were identified. Utilization of this new process resulted in major labor savings over the previous welding method in reduced joint design preparation, and increased welding deposition rate. The savings can be realized on future thick section weldments. In addition to the variables previously mentioned during the preplanning section regarding the utilization of welding processes, other variables need to be evaluated prior to making decisions on adapting or purchasing automated welding equipment. Additional criteria for consideration are:

In a further representation of Mr. Blodgett’s words, it has been shown that a decrease of 30° in joint design bevels can result in a savings of almost 50%, which includes the welding time, reduction in filler metal and distortion! In the inverse scenario, an increase in fillet weld sizes by a factor of 50% will require a significant increase in all the above factors. For example, an increase in fillet weld sizes from l/4 in. to 5/8 in. (using 1/4 in. as 100%) will increase the labor costs by a factor of 500%! This does not include the potential problem of weldment distortion due to the additional welding. Another factor involving design decisions, or joint design selections, involves the decision of when to utilize fillet or groove welds. There is obviously a significant difference between these designs, and equally so in costs. Based on deposited filler metal costs alone, it is evident that for 1/2 in. plate, fillet welds are the cheapest design per foot, but as the plate thickness increases up to three inches, a 45° double bevel groove joint design becomes the least expensive. The responsibility of the design engineer is very significant in controlling welding or manufacturing costs. Following a review of joint designs, another “tool” that needs to be evaluated is the applicable welding procedure/process to support fabrication. A welding procedure provides the welding foreman/welder with the essential welding elements including required gas shielding/flow rates, welding current, applicable preheat/interpass temperatures, and type of filler metals required. A careful review of available procedures/processes will afford an opportunity to determine costs, etc. When considering the filler metal selection, thought should be given to using the largest diameter suitable for the joint design, without violating heat input requirements, to improve the electrode deposition rate. Also, it should be noted when purchasing bare filler metal (spools) for Gas Metal Arc Welding (GMAW), Flux Cored Arc Welding (FCAW), or Submerged Arc Welding (SAW) processes, that the largest available spool poundage be considered if filler metal types do not need to be frequently changed. This will obviously avoid unneces-

• Processing speed of parts per unit of time. • Economic requirements: initial investment and operation costs, and payback period. • Location of the automated equipment should be as close to the prospective work as possible. • The path between the work area and control unit should be unobstructed. • Past experiences with similar automated equipment. • Efficient material handling and fixturing equipment requirements are important with a new process and higher production rates. • Part programming is a critical time aspect, as there is a need for a flexible software system. The necessary coordination between the welding and actual operation of the automated equipment is important to achieve the desired weld quality. 12

SECTION 3—PRODUCTION WELDING COST ANALYSIS

provides assurance that the quality level for the product or weldment is being met for critical applications. Specific definitions/applications of NDE are covered in another section, however, a brief overview of the six available nondestructive inspection/testing methods includes the following: Visual Testing (VT), Eddy Current (ET), Magnetic Particle (MT), Dye Penetrant (PT), Radiographic (RT), and Ultrasonic Testing (UT).

After review of the above criteria, one of the most crucial decisions remains: is there a high degree of assurance that utilization of the automated welding equipment or new processes will bring about a positive rate of return on the investment?

Records: Accountability for Accomplishment of Work and Inspections

Visual Testing For all weldments, VT is one of the best and most economical inspections employed. It can be accomplished quickly by qualified inspectors to verify pre or post welding criteria including, but not limited to, adequate joint designs, weld size, appearance (contour, cleanliness, etc.), or detection of surface flaws, (i.e., undercut, cracks, slag, etc.). The most significant disadvantage with this method is that it cannot detect subsurface or minute surface indications. Visual inspection of each weld layer can ensure quality, as it has been shown that if an in-depth VT is accomplished, there can be a 35% decrease in associated NDE inspections.

Maintaining accountability for accomplishment of work and inspection—nondestructive examination (NDE), receipt inspections, in-process, personnel qualifications, etc., is a tremendous, but necessary overhead cost. Several commercial code or military codes or specifications that might be referenced on drawings or be included in the contract provisions, require that an extensive quality control system for record origination/ retention be established. Some examples of these required records include: • Vendor certification of welding electrodes, or test results for compliance with the applicable specification/ receipt inspection requirements.

Eddy Current Testing ET is an electromagnetic inspection method in which small electrical currents are induced in a material. Flaws are detected by the interruption of these currents. This method can be used on a wide range of electro conducting materials, though the depth of inspection on ferrous materials is restricted to approximately 0.100 in. to 0.200 in., depending on the frequency used. It is excellent for measuring plating thicknesses and can be used on painted surfaces. This is ET’s main advantage over MT. The equipment for this technique is not expensive, easily suitable for automation, and the labor costs are minimal.

• Records that support test data for welding/NDE. • Records for welder/brazer/NDE operator qualifications. • In-process inspections or data collection (measurements, etc.). • Records of NDE inspections (ET, MT, PT, RT, UT, VT). • Records of casting fabrication, plates, shapes or forgings, and the applicable inspections. • Verification of thermal joining surveillance inspections.

Magnetic Particle Testing

Before bidding on a project, a thorough review of the required records, specified by the applicable fabrication documents, is necessary. The volume of required documentation may involve hiring of additional staff, thus increasing overhead costs.

MT can be accomplished only on ferrous materials. This method is reasonably economical, and serves as an excellent tool in detecting indications on joint preparations (including backgouged roots), root welding passes, and finished weld surfaces. Magnetic particle is basically a surface and slightly subsurface inspection process.

Cost Considerations of Nondestructive Examination (NDE)

Dye Penetrant Testing

The requirement to incorporate nondestructive testing during the manufacturing or welding evolution is another cost which has dramatic impact. Utilization of this type of testing has great potential for increasing costs, however, it

There are two types of dye penetrants: liquid and fluorescent. Both types can only detect defects open to the surface, and can be used on nonferrous and ferrous materials. Dye penetrant inspection is relatively economical, 13

SECTION 3—PRODUCTION WELDING COST ANALYSIS

manent record as the radiograph, improved equipment provides computerized printouts for records.

although more time consuming than MT. However, PT requires a better surface finish than MT for satisfactory inspection to be accomplished.

Summary

Radiographic Testing

With ever-shrinking budgets, and strong competition from abroad, controlling costs in the public and private industry is a major concern. As we have seen, there are several variables to consider when welding and related nondestructive testing is involved. This section has attempted to describe some of the welding variables that need to be considered. Table 3.1 is a condensed review. It is not intended to be all inclusive, rather a brief narrative about some of the major subjects which should be reviewed. Table 3.2 is a check-off sheet of costs to remember prior to the bid process.

Of the six NDE methods, radiographic inspection is the most expensive. This nondestructive method uses the penetrating radiation of X-rays, or the gamma rays of a radioactive source, to penetrate a specific object to reveal discontinuities. It is very labor intensive for the following reasons: • • • • • •

High cost of equipment and film. Stringent controls of the radiation source. Skilled personnel capable of producing exposed film. Required chemical processing of the exposed film. Trained personnel to review/interpret the film. Restriction of production efforts in the area of inspection, due to the presence of harmful radiation.

Bibliography/Recommended Reading List

This method can be used on most materials, and provides a permanent record of most surface or internal defects or flaws. RT is widely utilized in inspection of pipe and structural butt welds.

Blodgett, O. W. 1963. Design of Weldments. The James F. Lincoln Arc Welding Foundation, Cleveland, Ohio. Boyer, H., ed. 1976. ASM Handbook, 8th ed., vol. 1, Fundamentals of Quality Control and Quality Assurance. ASM International, Metals Park, Ohio.

Ultrasonic Inspection One of the most versatile NDE methods is ultrasonic inspection. UT has several advantages including location of internal flaws in components, and ability to be used on most materials. However, in terms of ease of operation, it is fairly complex, and requires the interpretative skills of trained technicians. In regard to cost, one individual can operate the machine and interpret the ultrasound wave signals. This makes UT considerably less expensive than RT. Because UT produces no radiation, inspection areas need not be evacuated. Though unable to provide a per-

Cary, H. 1997. Modern Welding Technology. Englewood Cliffs: Prentice-Hall. Cooper, N., Davis, W. J., and Pandjiris, A. K. 1968. Know costs then weld. Welding Journal 47(7): 561–568. Procedure Handbook of Arc Welding, sec. 12. 1973. The Lincoln Electric Company, Cleveland, Ohio. Welding Handbook, 8th ed., vol. 1, Welding Technology (WHB-1.8). Miami, Fla.: American Welding Society.

14

SECTION 3—PRODUCTION WELDING COST ANALYSIS

Table 3.1 Review of Production Welding Costs 1. Job preplanning, definition of scope, and accurate scheduling are three critical and necessary steps that must be accomplished prior to any project bid submittal or start of production. 2. Maintaining accountability of work accomplishments or inspections via the use of formal records is a necessary overhead cost that must be factored into the bid. 3. If at all possible, it is preferable to accomplish the majority of fabrication or welding in the shop vs. in the field. 4. Increasing fillet weld sizes beyond what is required can be detrimental. Aside from raising the filler metal and labor costs, the additional welding may contribute to serious distortion of the weldment. 5. Choosing the correct joint design for the particular plate thickness is very important. For example, for thicknesses over 1-1/2 in., the double bevel “T” joint design is more economical to employ than increasing the fillet weld size. 6. There are several factors that affect the efficient utilization of the many welding processes, i.e., deposition rates, joint geometry, fixturing, etc. Careful considerations must be made prior to utilizing a particular welding process or mode. 7. The selection of automated equipment requires a close review of several variables, including long-term preplanning and costs. There must be a projected positive rate of return, which will justify the capital expenditures. 8. Fire/heat protection costs have significantly increased. It has been estimated that for every hour of welding time, an equal amount of time is required for related fire/heat protection. 9. Weld procedure/personnel qualifications pose two problems. Both are expensive, and can require considerable time to accomplish. Early identification of specific needs is required. 10. Inexperienced welders or craftsman can affect the reject rates. Maintaining a skilled workforce should be a high priority. 11. Nondestructive testing during the manufacturing process has potential for increasing costs. However, it ensures that a quality product is being produced, and may in the long run actually reduce costs. 12. The roles of the design engineer, welding engineer, supervisor, welder, etc., are all varied, with each being critical for successful project accomplishment. Joint meetings to discuss particular attributes or suspected problem areas are necessary for unified direction. The results of these meetings will decrease costs and enhance the product quality.

15

SECTION 3—PRODUCTION WELDING COST ANALYSIS

Table 3.2 Checklist of Project/Welding Cost Variables ❒

Advance planning costs; i.e., meetings, accomplishment of tasking actions, etc.

❒

Costs of origination/distribution of funding or work standard documents.

❒

Cost of origination/distribution of pertinent engineering instructions or drawings for prefabrication support.

❒

Receipt inspection costs; i.e., issue instructions for base and filler metal inspections, etc.

❒

Weld procedure and welder performance qualification costs.

❒

Material and welding electrode costs.

❒

Special equipment purchases: welding machines, fixtures, etc.

❒

Inspection costs: in-process, nondestructive, and other, such as hydrostatic.

❒

Estimated rework costs.

❒

Formal record costs: origination, tracking, and retention.

❒

Possible post weld heat treatment costs.

❒

Costs for preparation prior to welding; i.e., base metal cleaning, joint designs, fitting, etc.

❒

Rigging or crane costs in the movement of the materials during and after the weldments are completed.

❒

Labor costs associated with welding, machining, casting or forging fabrications.

❒

Distortion control costs during in-process welding, etc.

❒

Fire/heat protection costs (including labor) associated with welding/burning.

❒

Final cleaning, preservation, and shipping costs.

❒

Surveillance and audit costs.

❒

Supervision (labor) costs.

❒

Overhead costs (vacation, electric power, medical insurance, etc.) associated with labor and operation of the required equipment for the fabrication of weldments.

16

SECTION

4

Modular Construction Contents Introduction ..................................................................................................................................................................... 18 Productivity Value ........................................................................................................................................................... 18 Dimensional Accuracy (Process Control) ...................................................................................................................... 19 Preoutfitting—A Major Advantage ............................................................................................................................... 20 Designing for Zone and Preoutfitting Construction ..................................................................................................... 21 Summary .......................................................................................................................................................................... 21

17

SECTION 4—MODULAR CONSTRUCTION

Section 4—Modular Construction

Introduction

Optimizing Resources

Modular construction or block construction was a natural evolution of large structures. It became advantageous for structures to be assembled in small blocks followed by joining these small blocks together until the entire structure was completed. This method of construction offered new opportunities to the outfitting trades. The assemblies could be turned over so work was at a more convenient physical location and much of the welding could be flat position work. Also, machinery and electrical components could be packaged in a less expensive shop environment and landed on the block, reducing the field assembly time. This concept initiated the movement of the design and planning functions toward the zone outfitting approach. By implementing zone methods, the blocks could be preoutfitted prior to erection and joining of additional blocks. Implementation of modular construction encompasses more than just assembly and fabrication work, it requires the engineering design and production work scheduling to be an integral part.

By designing and planning for the zone work, the scheduling of people and material is optimized. The amount of time lost for material movement and relocation of people is minimized. A crew of craftsmen can complete a work package that may include portions of multiple systems within a zone, prior to shifting to a new zone.

Productivity Value The efficiency of a work package can be defined by a “productivity value” which is a function of: • Time. • Resources. • Quality. These three variables must be balanced to obtain an optimum productivity value. Each of the variables is a function of the other two and therefore must be considered together.

Trial and Error Evaluation

System Approach

Productivity value is a unitless term that cannot be formulated mathematically. Rather, it is an expression that can be used to evaluate performance by comparison. Each work package process and operation can be evaluated by trial and error, until the optimum level of productivity is determined. This may require changes in work package definition until the proper balance of time, resources, and quality is achieved. This process should be repeated periodically to reevaluate the productivity value. As methods change or schedules are readjusted, the variable balance may be shifted and the optimum conditions may no longer be present. A proper review of a specific zone project must include the prior and following operations after the specific zone function is complete. The processes should be evaluated in relation to resources, time, and quality. The prior process must be included, as it establishes a baseline

Traditional methods used a systems approach to construction. For example, an entire pipe system would be designed, planned, and scheduled for production without consideration of the other systems that would be fabricated during the same time and in the same location. The systems method enables designers, material personnel, and planners to rapidly define the production of an entire system.

Modular Construction In modular construction it is essential to change from the systems concept to a zone concept to optimize productivity. The structure and its systems are converted into zones. These zones define work packages that define a quantity of work to be accomplished within boundaries. 18

SECTION 4—MODULAR CONSTRUCTION

condition for the measured operation. Variations in conditions from the prior process or operation will affect the productivity variables. Similarly, the following operation will be impacted by optimizing the evaluated process. The quality value is a good example of the effect of one operation on the other process steps. Each processing operation must result in a satisfactory consistent level of quality being passed to the following operation. Changes in the quality variable will skew the balance of the three productivity value components and cause lessthan-optimum conditions. Starting with a poor-quality product prompts the allocation of additional resources to meet schedule requirements; or when resources are not available, additional time is used to compensate for correction of quality deficiencies. Basically, each operation of the production process will provide a final product to the following operation. This process provides identifiable responsibility and accountability to individuals completing incrementally completed segments. This customer-supplier relationship must be measured and controlled to maintain an optimum productivity value.

system components are located by measurements from the nearest structure in lieu of the more traditional compartment or ship centerline. In addition, forcing structural block dimensional control, this now requires the matching blocks to consider the alignment of the distributive systems (i.e., electrical systems) in addition to the structural members. Deviations from nominal values can become cumulative and the larger block connections must be monitored for dimensional accuracy to ensure proper alignment. In some cases, this will force the use of less than the traditional tolerances. Process control is necessary to ensure that a zone approach to outfitting is successful.

Accuracy Control Program An important tool for controlling process performance in structural fabrication is an accuracy control program. With the use of statistical process controls, variations in process parameters can be identified and the causes evaluated. Two of the more common methods of displaying the data for accuracy control measurements are the histogram and the process control chart. Each of these methods utilizes basic statistical analysis.

Dimensional Accuracy (Process Control) Successful preoutfitting is greatly dependent on structural dimensional accuracy. Proper implementation of these advanced concepts requires all structural members and platforms to be within predetermined tolerances. The

(1) Histogram. Figure 4.1 is a typical histogram. The data shown represent a series of actual measurements

Figure 4.1—Typical Histogram 19

SECTION 4—MODULAR CONSTRUCTION

sure that productivity is being improved and the effects on baseline performance can be determined. An accuracy control program provides essential information for successful implementation of modular construction. When sequencing work by zones, it is essential that the systems continuing from one zone to another properly align at the boundaries. The structure must be accurate and within predefined tolerances to ensure that the matching systems in two zones are within dimensional tolerances to allow for proper connections.

taken over a period of time. By collecting the data in the histogram format, the variations in the process can be collected and displayed to provide a meaningful analysis. The histogram can be used to approximate a “bell curve.” The bell curve is a standard statistical representation of predictable variations in a process. By utilizing the bell curve, normal process inconsistencies can be estimated and a process can be evaluated to determine the frequency and magnitude of deviation from performance to established tolerances. (2) Process Control Charts. The process control chart shown in Figure 4.2 is another tool that is used to measure the performance of a continuous process. The chart is used to monitor an operation by comparing measured accuracy to the accuracy predicted by statistical methods. The variation between the upper and lower control limits are normal and are expected. Excursions beyond the limits are unusual and should be investigated to determine the cause. Adjustments in the process can actually move the upper and lower control limits under controlled conditions, thus redefining normal performance. Other similar types of charts can be used to assist in monitoring process performance with statistical controls. These methods are valuable tools for management decisions. By implementing a control program, process performance can be measured numerically and decisions no longer need to be based solely on management impressions. Changes in each process can be measured to en-

Preoutfitting—A Major Advantage One of the most important benefits of modular construction methods is the opportunity to implement preoutfitting work. The objective of this approach is to minimize the outfitting work after erection. Very thorough planning and scheduling is required to optimize this effort. When outfitting does not follow zone preoutfitting concepts, the result may be the erection of empty steel assemblies, which then require the more expensive conventional methods of installing the distributive systems.

Scheduling and Improved Work Environment Production defines several objectives of this planning effort. By advance planning, much of the fitting and welding operations can be scheduled to allow flat

Figure 4.2—Process Control Chart 20

SECTION 4—MODULAR CONSTRUCTION

and zone approach to design. For example, with a zone approach to design, systems arrangement drawings can be eliminated. Also, material lists can be generated to support the zone construction method. Advancements in the use of computer-aided design (CAD) will soon permit a design to be made with the traditional systems approach and then be converted to a zone configuration. This will maintain the advantages of a system design methodology. From this base, the CAD programming will enable a designer to define zones of a structure and the applicable portions of each system will be broken down into these zones. This will gain the benefits of a zone design concept without a large amount of duplication of effort.

position welding and horizontal fillets. As this positioning will allow greater use of mechanized and semiautomatic welding processes, labor will be reduced and productivity will be increased. Additionally, the quality of the work will be enhanced. By early zone planning, components can be identified so that different types of work can be scheduled and kept separate at the earliest manufacturing operation. This will simplify the manning and scheduling problems associated with multiple-operation manufacturing. Another objective is to schedule work in uncongested and more efficient work locations, rather than having operators perform their tasks in enclosed, high or narrow sites. Quality and productivity will be enhanced by this planning effort which incorporates important production personnel needs to accomplish their work efficiently. The time span for a production zone work package can be reduced by scheduling multiple tasks, which are not in conflict with each other, to be performed at the same time. This objective must consider work area space allowances to ensure that craftsmen have sufficient room to perform their tasks without interfering with other work. Similarly, compatible tasks can be scheduled to be performed in unison, and considerations of conflicting efforts must be discouraged. For example, it is not efficient to have a blast or paint operation being performed in the same zone as a pipe-fitting operation.

Summary To reap the benefits of this construction method, changes in many areas must be considered: • The design effort must support a zone approach. • Planning must be directed toward a zone work package that is producible, to ensure the proper balance of the key variables (time, resources, and quality). • An accuracy control program must be implemented to adequately monitor the fabrication process to ensure that the blocks and distributive systems align properly. Some of the benefits of preoutfitting include:

Designing for Zone and Preoutfitting Construction

(1) A higher percentage of shop fabrication and assembly. (2) Less congestion and worker interference for structure installation. (3) Reduction in lost material. (4) Less on-site rigging and material handling. (5) Less on-site cleaning and reduction in fire hazards. (6) Improved working conditions on the assemblies.

Effective use of modular construction must incorporate design efforts that enhance zone definition for material and production planning. Design by system has value in defining the capability of each functional individual system and in demonstrating the location and path of the distributive systems. However, the optimum base of zone planning and work packages requires a zone approach to design. Implementation of preoutfitting will not be optimized without the corollary design package. As stated in the introduction, there are differences between the systems

If properly implemented, preoutfitting and modular construction will be cost effective and improve quality and productivity.

21

SECTION

5

Welding Process Selection Contents Common Welding Processes ........................................................................................................................................... 24 Process Selection .............................................................................................................................................................. 24 SMAW............................................................................................................................................................................... 26 GMAW .............................................................................................................................................................................. 27 FCAW ............................................................................................................................................................................... 28 SAW................................................................................................................................................................................... 29 GTAW ............................................................................................................................................................................... 30 Bibliography/Recommended Reading List ................................................................................................................... 32

23

SECTION 5—WELDING PROCESS SELECTION

Section 5—Welding Process Selection

Common Welding Processes

cess the weld metal composition is an alloy combination of filler metal and remelted base metal. Weld metal composition varies with each process. It is difficult to predict weld metal composition since many elements are diluted when crossing the welding arc. Figures 5.1–5.5 describe the most common welding processes and summarize their advantages and limitations.

The most common welding processes used in industry involve arc welding. The electric arc is a very effective and portable heat source for melting most metals quickly. When the base metal and filler metal become molten, they mix together then solidify to produce one solid piece. Most welding processes utilize a consumable electrode, which conducts the welding current and melts as it is continuously fed into the arc and deposited into the joint. The most common manual arc welding process is the Shielded Metal Arc Welding (SMAW) process (see Figure 5.1), in which a fixed length of electrode is hand fed into the joint. Typical semi-automatic arc welding processes include the Gas Metal Arc Welding (GMAW) process (see Figure 5.2) and the Flux Cored Arc Welding (FCAW) process (see Figure 5.3), which automatically feed a spooled electrode filler wire into the joint while the welder manipulates the torch manually. The Submerged Arc Welding (SAW) process (see Figure 5.4) is an automatic process whereby both filler wire and travel speed are controlled by the machine which runs along a preset track. The Gas Tungsten Arc Welding (GTAW) process (see Figure 5.5) uses a nonconsumable tungsten electrode to establish an arc in the weld joint, and typically, a separate hand-held filler wire is melted in the arc and deposited in the joint. The molten weld pool created with all arc processes must be shielded from oxidation and contamination during the welding process. SMAW electrodes have a thick dry flux coating, which creates a gas shield and also provides many of the alloying elements to the weld. The SAW process obtains shielding and alloying with a similar type of dry flux in a granular form that covers the welding arc and molten pool. Spooled FCAW electrodes also use a dry flux contained in the center of a tubular wire and typically obtain additional shielding from an inert gas shield similar to the GMAW process. Both GMAW and GTAW utilize inert shielding gases that flow around the molten weld pool, while all alloying elements are supplied by the wire electrode alone. For each pro-

Process Selection When selecting a welding process for a specific application, several factors that affect productivity and weld quality must be balanced. This can become a complicated decision, due to the number of conflicting advantages and disadvantages which each process possesses in each situation. Productivity is usually a very important consideration on most jobs. The deposition rate is a significant part of this factor. Each process can be ranked in terms of its deposition rate in pounds of weld metal deposited per hour. However, there are other factors that must be considered. For example, it usually takes time for a shop to become familiar with a new welding process, new equipment, or even a new brand of filler metal. Until adequate training and acceptance by the shop personnel are accomplished, the actual productivity rate will be less than the average. Reject rates also have a significant impact on actual productivity, since a considerable amount of time is required to remove defective weld metal, replace it, and reinspect it. Therefore, many factors must be taken into account before determining which is the best process for a specific application. Process selection must be made by someone who is very familiar not only with the various processes, but more importantly with the shop’s capabilities and the job requirements. As a minimum, the following items should be considered prior to selecting a welding process.

Base Metal Type Some processes are better suited to certain base materials than others. The maximum heat input must be 24

SECTION 5—WELDING PROCESS SELECTION

Availability of Equipment

limited on some types of materials, i.e., quenched, tempered, age-hardened, and other heat-treated or coldworked materials. Processes that derive their advantage from high productivity and high heat input may be limited for these applications.

Most large welding shops have access to the welding equipment required for the processes discussed. There are times, however, when new equipment must be evaluated to determine if the increased productivity or versatility would offset the initial cost and training.

Joint Design and Thickness As the section thickness increases, welding productivity becomes more important. If possible, the process selection should reflect this. The length of the weld must also be taken into account, since a higher-productivity process may not realize this advantage—particularly if the operator has to frequently stop the process to set up for the next pass, as would be necessary with a small repair. Since some processes require more access to the joint root to avoid lack of fusion defects, the selection of some processes may also require a joint design change.

Fabrication and Inspection Standards Most welding and inspection is governed by welding standards that often limit the use of some processes on certain materials, and also may limit the maximum heat input used to ensure that specific mechanical properties are met. A high-productivity process such as SAW would lose much of its advantage if the heat input were restricted down to the range achievable by GMAW. Table 5.1 is a general comparison guide for the most commonly used processes in the industry, relative to some of the factors discussed above. While some of the ratings are subjective, they do provide a general comparison for many applications.

Welding Position The weld joint position plays a very important part when selecting a process, since many processes are limited to only a few positions. Whenever possible, the joint should be in the flat position, since the highest productivity and weld quality are attained when welding is accomplished in this position. Since most repair work is done on large weldments that cannot be repositioned and access to the joint is limited, use of high-productivity processes and filler materials is limited. The four basic welding positions are: flat, horizontal, vertical, and overhead, as shown in Figure 5.6.

It can be seen from this chart that many variables must be taken into account when selecting the best welding process for a given job. In practice, it takes experience to identify and accurately weigh all of the variables involved. Some factors, such as the attitude and perception of shop employees toward a new process or procedure change, may be more difficult to analyze than others. Another factor complicating process selection is the rapid change brought on by the computer age, which is taking place in the electrical power sources used for arc welding. It takes a considerable amount of study to take full advantage of the wide variety of machines available today and all of the functions each one is capable of performing. The GMAW and FCAW processes are probably the most influenced by these advances, in the form of “synergic” power sources that manipulate the waveform of the welding current through complex electrical circuits. These advanced machines automatically adjust all of the waveform parameters to optimize the mode of metal transfer across the arc with simple operator controls. Since productivity and weld quality are greatly affected by many aspects of the metal transfer across the arc, these machines offer a significant advantage over the previous generations of power sources.

Environmental Conditions Wind and rain are the two field conditions that typically affect welding. It takes very little wind to disturb the gas shield that is critical for high-quality GMAW and GTAW welding. This restricts their use outside of sheltered containments. The SMAW and FCAW processes can also be affected by wind, but to a lesser degree. No process is tolerant of direct rain. Proper placement of tarps, dams, or other temporary containments can rectify this situation. Due to the many problems associated with welding outside (i.e., being exposed to the elements or cramped inside of a structure), shop prefabrication is recommended where practical. Studies have shown that reject rates for field welding increased from 100% to greater than 400% over those for shop welding.

25

SECTION 5—WELDING PROCESS SELECTION

SMAW Shielded metal arc welding is a manual arc welding process. A covered electrode is used to deposit the filler metal and supply shielding to the molten weld pool. The 12-in. to 18-in. long electrode has a dry flux coating that decomposes in the arc to form a gas shield and a solid slag covering on the cooled weld deposit. The flux coating often provides alloying and deoxidizing elements.

Advantages

Limitations Higher potential for weld contamination than for other processes.

All-position capability. Many alloys are weldable.

Welding is frequently interrupted due to the short, fixed length of electrodes.

Thick sections are weldable.

The process creates spatter and smoke fumes and leaves a slag that must be removed before the next layer can be deposited.

Excellent for field and repair work. Equipment is simple, low cost, low maintenance, and is easy to set up. Good accessibility in space-restricted areas.

Not suitable for thin material but is suitable for thin-wall repairs on thick material.

Arc visibility is good.

Poor usability for aluminum and most bronzes.

The process is fairly tolerant of environmental conditions.

Special electrode care is required since most flux coatings absorb moisture.

Electrode travel, which helps to control heat input, is required.

Foreign material exclusion for nuclear applications is poor. Severe arc blow can be a problem when using direct current (dc)

Figure 5.1—Shielded Metal Arc Welding

26

SECTION 5—WELDING PROCESS SELECTION

GMAW Gas metal arc welding is a semiautomatic or automatic process. A small-diameter, continuous consumable electrode is melted in the arc and deposited in the joint. Shielding is provided by a gas that flows through the weld torch. Depending on the amperage and wave form used, four modes of metal transfer are possible: short circuiting, globular, pulsed spray, and spray transfer.

Advantages

Limitations

High-quality welds.

Gas shielding is easily disturbed during field work.

High deposit rates (three times that of SMAW).

Handling gas bottles for field work can be expensive.

Most metals are weldable.

Equipment is complicated and expensive.

Minimal stub loss.

Flash burns easily obtained.

All-position welding with some GMAW processes.

Excessive weld spatter with CO2, shielding.

No slag removal and minimum interpass cleaning.

Not used for thin metals except with pulsed spray and short-circuit transfer welding.

Easily adapted to robotics. Limited accessibility. Good weld puddle visibility. Spray transfer limited to the flat position. Restriction on the use of short circuiting transfer welding.

Figure 5.2—Gas Metal Arc Welding

27

SECTION 5—WELDING PROCESS SELECTION

FCAW Flux cored arc welding uses equipment similar to GMAW. The main difference is that the electrode is a tubular wire with a flux core. Some electrodes provide adequate shielding with the flux core alone, but others require the addition of gas shielding.

Advantages

Limitations

Large single-pass fillets possible.

Large amounts of smoke and fumes.

Tolerant of mill scale.

Weld spatter can clog nozzles.

Self-shielded process is good for field work.

Self-shielded process has lower-quality welds than external gas shielded.

High deposition rates possible. Flash burns easily obtained. All-position welding possible. Slag coating must be removed between layers. Less joint preparation required than for SMAW and GTAW.

The flux core is susceptible to moisture pickup, which may cause porosity and worm tracks.

Good penetration with gas shielding. Equipment is bulky, and accessibility can be a problem. Equipment is complicated and expensive.

Figure 5.3—Flux Cored Arc Welding

28

SECTION 5—WELDING PROCESS SELECTION

SAW Submerged arc welding is very similar to GMAW except the weld pool is shielded, deoxidized, and alloyed with a granulated flux that melts in the arc. The flux is fed through a hopper and a guide tube and deposited in the joint ahead of the welding arc. Two or more electrode wires can be used in tandem or metal powders can be added to increase deposition rate to obtain optimum mechanical properties.

Advantages

Limitations

High deposition rates.

Welding positions limited to flat and horizontal.

Excellent weld quality.

Limited to ferrous metals and high-nickel alloys (no agehardenable alloys).

No flash burns. Best suited to long welds and thick sections. Less angular weld distortion. Weld appearance excellent.

Special care required for the flux, since it absorbs moisture easily.

Maintenance relatively low.

Slag and unfused flux must be removed.

Smoke and fume quantities are low.

High toughness properties are hard to obtain.

Penetration is excellent.

Joint fit-up and design are critical to penetration control.

Less weld joint preparation.

Figure 5.4—Submerged Arc Welding

29

SECTION 5—WELDING PROCESS SELECTION

GTAW

DIRECTION O WELDING C CP

Gas tungsten arc welding is a gas shielded manual or automatic process that uses a nonconsumable tungsten electrode to establish the arc. The filler metal is added to the weld pool separately. Shielding of the weld pool and electrode is provided by predominately inert gases. Autogenous welding (no filler wire added) is possible with some materials.

ELECTRODE LED TORC OD SIELDING GS IN

COLLET OD GS NOLE

NONCONSLE TNGSTEN ELECTRODE GS SIELD ILLER ETL

RC

OLTEN WELD ETL

Advantages

SOLIDIIED WELD ETL

Limitations Lower deposition rates compared to SAW, GMAW, or SMAW.

Excellent arc visibility. Precise filler metal placement.

High-amperage, water-cooled torches are heavier and bulkier than air-cooled torches.

The best undercut control.

Both hands must be used to hold the torch and filler wire, which limits accessibility.

Wide range of thicknesses can be welded. Very clean welds can be achieved with no significant slag, spatter, or smoke.

Shielding gas is easily disturbed by gusts of wind.

Excellent weld quality on most metals.

Flash burns are easily obtained.

Amperage can be controlled by remote foot or hand switches.

Very limited for outdoor field work. Difficult to monitor heat input.

The process can be used either manual or automatic. All-position welding possible. Filler metal is not always required.

Figure 5.5—Gas Tungsten Arc Welding

30

SECTION 5—WELDING PROCESS SELECTION

Figure 5.6—Four Primary Welding Positions

Table 5.1 Process Comparison Chart Process

SMAW

GTAW

GMAW

FCAW

SAW

Good

Excellent

Excellent

Good

Excellent

Fair

Poor

Good

Good

Excellent

Excellent

Poor

Fair

Excellent

Poor

Equipment Maintenance

Low

Low

Medium

Medium

Medium

Smoke/Fumes

High

Low

Medium

High

Very Low

Excellent

Poor

Good

Good

Satisfactory

Good

Excellent

Satisfactory

Satisfactory

Poor

Satisfactory

Excellent

Good

Good

Fair

Quality Deposition Rate Field Work

Heat Input Control Arc Visibility and Filler Metal Placement Variety of Metals Weldable

31

SECTION 5—WELDING PROCESS SELECTION

Bibliography/Recommended Reading List

———. The Everyday Pocket Handbook for Shielded Metal Arc Welding (SMAW) (PHB-7). Miami, Fla.: American Welding Society.

American Welding Society. Recommended Practices for Gas Metal Arc Welding (C5.6). Miami, Fla.: American Welding Society.

———. The Everyday Pocket Handbook for Gas Metal Arc Welding (GMAW) and Flux Cored Arc Welding (FCAW) (PHB-4). Miami, Fla.: American Welding Society.

———. Recommended Practices for Gas Tungsten Arc Welding (C5.5). Miami, Fla.: American Welding Society.

———. Welding Processes and Practices (WPP). Miami, Fla.: American Welding Society.

———. Recommended Practices for Laser Beam Welding, Cutting, and Drilling (C7.2). Miami, Fla.: American Welding Society.

———. Welding Handbook, 8th ed., vol. 2, Welding Processes (WHB-2.8). Miami, Fla.: American Welding Society.

32

SECTION

6

Primary Concepts of Welding Design Contents Introduction ..................................................................................................................................................................... 34 What the Designer Should Know about Welding ......................................................................................................... 34 Service Requirements...................................................................................................................................................... 34 Base Metal Selection........................................................................................................................................................ 35 Welds Create a Discontinuity in the Base Material...................................................................................................... 35 Weld vs. Base Metal Properties ...................................................................................................................................... 35 Welds Rarely Improve upon Base Metal Properties .................................................................................................... 35 Keep Weldment Designs Simple ..................................................................................................................................... 35 Standardize Component Welding Design...................................................................................................................... 35 Minimize the Number of Welds...................................................................................................................................... 35 Location of Welds ............................................................................................................................................................ 36 Weld Deposit Mechanical Properties ............................................................................................................................. 36 Joint Designs—Amount and Type of Weld ................................................................................................................... 36 Partial Penetration Welds ............................................................................................................................................... 36 Full Penetration Welds .................................................................................................................................................... 38 Fillet Welds....................................................................................................................................................................... 39 Design Considerations for Change of Cross Sections................................................................................................... 39 Residual Stress—Fact or Fiction? .................................................................................................................................. 40 Summary for Improved Fabrication Design ................................................................................................................. 40 Bibliography/Recommended Reading List ................................................................................................................... 43 33

SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN

Section 6—Primary Concepts of Welding Design

Introduction

Service Requirements

Inexperienced engineers frequently believe that if their design can be conveyed to paper as a drawing, it can be welded. Welding is the most common means of joining two pieces of metal, but it requires special consideration if it is to be done economically. What appears to be a good design on a computer or drawing board may be a disaster when it hits the production floor if the fabrication requirements are not properly considered. Using quenched and tempered steel in a complex design for strength-to-weight savings may appear to provide cost savings. However it may be more expensive to fabricate than heavier section mild steel. The term “designer” is used in this section to address any engineer, technician, draftsman, or other person issuing instructions regarding metal fabrication requirements. These individuals are engineering type employees with cognizance of structural, machinery, piping, and pressure vessel fabrication. The intent of this section is not to provide detailed welding design information but to provide basic welding design concepts. Recommended reading is provided at the end of this manual for more detailed information on welding design specifics.

Service requirements are the first item that must be clear in the designer’s mind before making welding design decisions. Welding decisions will be less than fully adequate without clear definition of the service requirements, and the weldment may be subject to premature, in-service failures. Service requirements may be very simple or quite complex. Weldments experience high and low temperatures, high pressures and stresses, corrosive environments, and fatigue and impact loading. Often more than one of these conditions is involved simultaneously. The designer should consider only the service requirements that make the weldment (the combination of weld, HAZ and base metal) suitable for the expected service. Some of the service requirements the designer may consider are: • • • • • • • • •

What the Designer Should Know About Welding

Tensile, compressive, and shear stresses and strains. Corrosion resistance. Erosion resistance. Abrasion resistance. Impact resistance (stress and temperature dependent). Creep strength. Temperature (high and low temperature). Fatigue resistance. Aesthetics (color match, anodizing, appearance).

It is rare that a designer would be concerned with more than a few of these service requirements for a specific design. It is poor practice to seek to achieve certain properties from a weldment, based upon the material’s ability to achieve these properties, even though the properties are not related to the service. If a quenched and tempered material is used for its strength-to-weight ratio properties, and impact loading is not of concern, the designer should not impose requirements of impact properties just because the material used has the ability to achieve high impact resistance. If 300 Series stainless steel is used because of its high-temperature yield and oxidation

Welding design must not be based on isolating the weld alone. The designer must understand how the weld and the heat-affected zone (HAZ) next to the weld will affect the structure’s service. Many factors influence the effect of the weld on a structure’s performance including the filler metal type, the welding process and parameters, and how and where the filler metal is placed in the structure. It is essential for the design engineer to review the actual production site prior to and during fabrication, and to review the finished product. This provides some practical perspective to weld design that would be missed if the designer were primarily an analytical type. 34

SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN

• • • • • •

resistance, as required in steam boilers, the designer should not be concerned about weld sensitization of the base metal during fabrication. In many cases, boiler components will operate in the sensitizing temperature range in service anyway.

Base Metal Selection

Allowable stress. High-temperature service. Impact loading. Fatigue loading. Leak tightness. Corrosion resistance.

Welds Rarely Improve upon Base Metal Properties

Weldments should be made from the most economical and readily weldable base metals available that are suitable for in-service requirements. Mild steel is much less expensive to purchase and to fabricate than the quenched and tempered steel alloys or the heat-treatable steels and will usually require considerably less rework. Steels designed for special applications frequently require preheat, and interpass temperature controls during welding, and sometimes stress relief. Special electrodes and electrode care may also be required. Since fabrication is much more expensive with quenched and tempered steels, special consideration should be given to their use. From a design prospective, in some instances, quenched and tempered steels offer little advantage. Fatigue stress ranges are not improved with these steels, only the maximum allowable fatigue stress or static loads. If complete reversal of service stress is expected, these steels offer little advantage for fatigue. These steels also have lower buckling stability for similar cross sections of mild steel, because they are typically used for higher loads and will most likely require additional stiffening.

As compared to base metals, welds will cause the weldment to have: • Lower fatigue resistance than the base metal. • Lower impact properties than the base metal. • Higher residual stress than the base metal. The weld is a discontinuity which is both a geometric and metallurgical notch that decreases the suitability for service below that of the base metal alone.

Keep Weldment Designs Simple Weldment design should be kept as simple as possible. Weldments that involve multiple crossing of stiffeners on the same plane, multiple intersecting “I” beams, or complex patterns of stress flow should be minimized. Complex designs are expensive to fabricate and can lose serviceability, due to high residual stress and distortion. Complex designs also limit welding access and may cause the inability to meet post-weld inspection requirements.

Welds Create a Discontinuity in the Base Material

Standardize Component Welding Design

The designer always must keep in mind that a weld does not create a monolithic structure that has uniform properties throughout. Welds do not act the same as the base metal and they have different properties. Welds are, in fact, a discontinuity (a stress riser) in the structure— caused by their metallurgical, chemical, and mechanical property differences. Because welds are different than the base metal, they must be given special design consideration.

Standardization of design is necessary for automation and mass production of similar items. Standardization also reduces engineering analysis time and may include complete foundations, stiffeners, pad eyes, weld joint designs, or even the size and shape of structural members. Unique welding processes, specialty items, varied member size, and different materials are not economical in terms of fabrication or engineering time to analyze each structure. Standard design details will simplify fabrication and reduce costs.

Weld vs. Base Metal Properties Welds are used to join base metals because they are economical, have high joint efficiencies, and reliability relative to other fabrication processes. Service areas where welds can perform favorably toward base metals are:

Minimize the Number of Welds Welding and its related processing is one of the most expensive operations in fabrication. Shaping plate by pressing or breaking is usually preferable to welding one 35

SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN

than the welding filler metal specification requirements. In production the weld metal properties are dependent upon the base metal chemistry, how much it has been diluted into the weld metal, and the rate of weld metal cooling. The cooling rate of the weld can also affect the mechanical properties of the weld and the heat-affected zone. The cooling rate can be drastically affected by base metal type and thickness, the amount of weld deposit, the technique used to deposit the filler metal, and the temperature of the base metal prior to each weld pass (i.e., the preheat and interpass temperature). In general the designer should use the lowest possible yield strength filler metal compatible with the design. Designers will frequently use filler metals that produce a deposit that exceeds base metal properties. Often these higher-strength deposits, in combination with the base metal HAZ, result in the weldment having:

or more pieces of plate together. Reducing the number of component parts by improving the design or purchasing standard shapes is usually more economical than welding.

Location of Welds From a design point of view, a structure’s ability to carry service loads is only as good as the weakest part of the structure. In most cases, the weakest link is the weld deposit and the localized effects on the structure (especially for fatigue applications). The allowable loads that welds are permitted to carry are determined in many instances by a standard or code to which the item is fabricated. Examples of such controlling codes are the AWS D1.1, Structural Welding Code—Steel, and the American Institute of Steel Construction (AISC) Code. Welds should be located according to the cost of making the weldment, inspection requirements and the service requirements. Because welds represent a metallurgical notch and are stress risers, welds should be kept in the lowest stressed areas as practical. When fatigue, impact, or creep is a primary concern, the welds should be placed in low-stressed areas. When possible, the designer should arrange for the welds to be made in the flat position. The most economical, fastest and highest-quality weld is achieved in the flat position with standard welding processes. Flat position welding with simple designs allows high deposition rate mechanized, semiautomatic, or automatic welding processes at substantial cost and time savings. The location of welds should always provide the welder easy and open access. The most costly welds to make are predominately those where proper welding access has not been provided for by the designer. If welding access is difficult, then Nondestructive Examination (NDE) will usually be difficult, as will any necessary follow up repairs. Snipes or “rat holes” should be as large as practical to provide access for welding and NDE. Designs should avoid welding across flanges under tension as drastic reductions to allowable service loads are imposed by most codes. Avoid welds on edges of members subject to tensile fatigue. Edges of members subject to tensile loads usually experience higher loads than the rest of the member and, since a weld acts as a stress riser, premature failure of a component could occur due to unanticipated loads.

• • • • • •

Decreased fatigue resistance. Decreased impact resistance. Increased distortion. Increased base metal cracking. Increased amounts of residual stress. Increased costs.

Joint Designs—Amount and Type of Weld The type and direction of loading of the weldment will determine the type and amount of weld. The least amount of weld should be used to meet the necessary design service requirements. Specific weld joint design selection is discussed by another section of this manual. However, it will suffice to say the primary reasons for selecting a joint design are to provide the desired service requirements (i.e., the ability to accommodate service stress/strain and corrosion) and to accommodate welder accessibility. There are many other secondary factors that affect joint design selection, such as available processes and skill levels, cost, aesthetics, environmental conditions, system status, base metal thickness and properties, location of the weld, position of the weld, the number of welds, the schedule of the project, etc.

Partial Penetration Welds A partial penetration weld is a weld that intentionally does not completely penetrate through the thickness of a joint (see Figure 6.1). Partial penetration welds can often be used for tension, compression, and shear stresses that act parallel to the longitudinal axis of a weld and for

Weld Deposit Mechanical Properties The as-deposited mechanical properties of a weld in a fabrication should always be considered to be different 36

SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN

(1/2) 1/2

Figure 6.1—Partial Joint Penetration with(1/2) Joint Geometry Optional 1/2 1 1/4

WELD CROSS SECTION

SYMBOL

(1/2)

1/2

3/4 WELD CROSS SECTION

SYMBOL

1 1/8

1/2

(1/2) (1/2)

1/2

WELD CROSS SECTION

SYMBOL

(1/2)

1/2

WELD CROSS SECTION

SYMBOL

Figure 6.1—Partial Joint Penetration with Joint Geometry Optional

37

SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN

Full Penetration Welds

compression loads normal to a weld (see Figure 6.2). Making full penetration welds on very thick material for these types of loads can cause higher residual stress than the anticipated service loads. In some cases, particularly on heavy sections, weld failures will be experienced during fabrication before service loads are imposed. Often welds that are subject to shear stresses can be designed to be 50% efficient, thereby allowing partial penetration joint designs or lower-strength welds to be used. Welding defects that are placed in compression or shear while in service are much less critical than those placed in tension by service loads. In a practical sense, partial penetration welds have crack starters and continuous lack of fusion conditions at their roots. Open roots of partial penetration welds may be subject to crevice corrosion. They should not be used in corrosive environments unless the metal is resistant to crevice corrosion. These joints have poor resistance to fatigue and impact loading, and they should not be used when the weld roots are subject to transverse cyclic tension or bending tension loads. Partial penetration welds are less expensive and cause less distortion than full penetration welds.

A full penetration weld penetrates through the thickness of a joint (see Figure 6.3). Groove welds are used on butts, corners, and tees. Full penetration welds should be used only where required by the design service requirements. Services involving high shock, high temperatures, axial or bending cyclic loading (fatigue), and pressure vessels often require full penetration welds. Full penetration welds are expensive, often cause the largest distortion and have high residual stress. However, they are permitted the highest allowable stresses for design service by all codes. Full penetration welds on heavy sections require special joint design consideration to minimize residual stress and the possibility of cracking and lamellar tearing (see Figure 6.4). Mild steel structural plate has low ductility through its thickness and it cannot accommodate high through thickness strains. Lamellar tearing and cracking of the base metal adjacent to the weld is a common result of poor welding joint details on heavy structural steels.

Figure 6.2—Compression and Tension Loads for Partial Penetration Welds 38

SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN

NOTE OVERLAP NOTE OVERLAP OF 1/8 1/4

1/4

3/8

1/4

3/8

3/8

1/4

3/8

Figure 6.3—Full Penetration Groove Welds

Fillet Welds

penetration, the actual mechanical properties of the weld deposit and HAZ, the amount of weld reinforcement, and the actual failure path. These varying conditions lead to some conservative positions by design engineers. However, it is extremely rare that fillet welds result in service failures unless unanticipated service fatigue or impact loads are experienced. Great care should be taken by designers in not applying liberal safety factors and oversizing fillet welds. Oversize fillet welds are very expensive and can result in excessive distortion, welding time, filler metal, and possibly the need for flame straightening. Fillet weld sizing formulas and tables can be found in AWS D1.1, Structural Welding Code—Steel, or see AWS Design Handbook for Calculating Fillet Weld Size.

Fillet welds are generally triangular in cross section and are used to join two relatively perpendicular sections. Small fillet welds result in low residual stress and weld distortion. A 5/16-in. fillet weld can be readily achieved in one pass. Larger fillets will require a minimum of three passes, which greatly increases the cost of welding. If possible, it is better to keep the fillet size down to one pass and extend the length of the weld to achieve joint efficiency. The smallest typical production welds are generally 1/8-in. fillet welds, because a 1/16-in. fillet weld is nearly impossible to produce. Since fillet welds are used primarily in shear, they are unlikely to propagate weld defects into failures except under fatigue service applications. Intermittent fillet welds and continuous fillet welds made from one side only should be avoided where fatigue, corrosion, or debris and contamination traps are a concern. One of the common mistakes of engineers is that they will frequently increase the fillet weld by one size (1/16 in.) above their calculated values to play it safe. It also is not uncommon for production personnel to be conservative and go oversize due to increased attention being given to visual inspection. Increasing fillet weld sizes by two sizes (1/8 in.) has the potential of increasing welding time, distortion, and cost of filler metals by more than 100%. Fillet weld sizing is not strictly based on engineering calculations that are backed up by extensive laboratory tests. The various codes and standards each have different requirements based on different theories and assumptions. Laboratory tests and calculations can also vary by the selection of the welding process, the depth of weld

Design Considerations for Change of Cross Sections One of the fundamental errors made by design engineers is not allowing for stress and strain to take place through gradual changes of the cross section. Failures commonly occur as a result of thermal and/or mechanical fatigue associated with rapid changes of the cross section. Changes of the cross section must be gradual in both the weld metal and the base metal when fatigue is of primary concern (see Figures 6.5 and 6.6). It is preferable to have changes in the cross section take place in the base metal instead of in the weld metal through a weld joint. It should also be noted that, in general, those items that affect fatigue life are also of concern if impact loading is expected. Some common design practices allow tapered sections or changes of cross section to be a 2:1 ratio. However, a 4:1 ratio is considered to be the most desirable. 39

SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN

sidual stresses caused by welds? Yes! Residual stresses as high as the yield point do exist in weldments. The effects of residual stress can be seen in distorted weldments, and fractured welds and base metal. This metal fracturing can take place during fabrication or in service. Without stress relief, localized areas of yield point residual stress are common and should be expected. One way to allow for this residual stress in the design is by placing welds in low-service stressed areas. Some items that can minimize residual stress and metal fracture: • Use low-yield strength base metals. • Use low-yield strength filler metals with high ductility. • Use the minimum weld size or the least amount of weld metal consistent with the design requirements. • Avoid the use of notches, sharp corners, or rapid changes of cross section (stress risers). • Peen the welds lightly during the welding process. • Prestress the weld joints prior to welding. • Avoid full penetration welds on heavy wall structural assemblies (jumbo sections). • In general, observing preheating and interpass temperature for a quenched and tempered metal or quench-hardened material will lower residual stresses. • Stress relieve the weldment. • Sequencing the weld by back stepping, balancing the weld around the neutral axis or welding toward rigidity. • Avoid joint designs that cause high residual stress through the plate thickness. Joint details, when practical, should have the welds placed across the plate ends (through the plate thickness) on heavy sections.

Summary for Improved Fabrication Design (1) Only consider service requirements that make the weldment suitable for service. (2) Welds are geometric and metallurgical notches in base metal. (3) Welds in base metal frequently cause the designallowable stresses for fatigue to drop. (4) Welds can raise the residual stress in base metals to the yield point. (5) Keep weldment design simple. The weld is usually easier and less expensive to make. (6) Avoid welding when practical. Use standard shape material or use castings. Reduce component parts or the number of plates. (7) The weld and heat-affected zone are usually the weakest link in a weldment design. The weld is a stress riser that is preloaded by residual stress. (8) When practical, keep welds in low-service stress areas.

Figure 6.4—Weld Configurations that May Cause Lamellar Tearing

Residual Stress—Fact or Fiction? What every welder knows as a fact of life, the inexperienced design engineer questions. Do welding residual stresses exist and do they affect welding design? Are re40

41 SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN

Figure 6.5—Design Considerations for a Change of Cross Section (Tubular)

SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN

Figure 6.6—Design Considerations for a Change of Cross Section (Nontubular)

42

SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN

(20) Use partial penetration welds for joints subject to axial tension and compression loads. (21) Do not use partial penetration welds when the weld root is subject to cyclic tension or impact loads. (22) Base metal stiffness does not increase with increasing yield strength. Additional stiffeners may need to be added to a structure which uses high-strength material in order to decrease plate or section thickness.

(9) Design for prefabricating the material in the shop when practical. (10) Use flat position welding. It’s cheaper, faster and usually higher quality. (11) Avoid designs that require welding across tension flanges. (12) Avoid corner welds or edge welds that are subject to tensile fatigue. (13) “As deposited” welds will have mechanical properties that are different from their specifications due to base metal dilution. (14) Use the lowest yield strength filler metal compatible with the design. This reduces costs, residual stress, distortion, and cracking. (15) Use the least amount of weld metal necessary to meet design requirements. A 1/8-in. fillet is typically the smallest practical fillet size. (16) Use partial penetration welds in lieu of full penetration when the design permits (i.e., all compression loads, tension and shear loads that act parallel to the axis of the weld). (17) Use full penetration welds only when required by the service requirements. (18) Use fillet welds when practical. Use single pass fillet welds where possible (5/16-in. to 3/8-in. maximum leg sizes depending on the welding process). Some standards require a two-layer minimum for leak tightness if they are to be used for fluid containment. (19) Balance welding about the neutral axis.

Bibliography/Recommended Reading List American Welding Society. Design Handbook for Calculating Fillet Weld Sizes (FWSH). Miami, Fla.: American Welding Society. ———. Structural Welding Code—Reinforcing Steel (D1.4). Miami, Fla.: American Welding Society. ———. Structural Welding Code—Sheet Steel (D1.3). Miami, Fla.: American Welding Society. ———. Structural Welding Code—Stainless Steel (D1.6). Miami, Fla.: American Welding Society. ———. Structural Welding Code—Steel (D1.1). Miami, Fla.: American Welding Society. ———. Welding Handbook, 8th ed., vol. 1, Welding Technology (WHB-1.8). Miami, Fla.: American Welding Society.

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SECTION

7

Fatigue Considerations Contents Introduction ..................................................................................................................................................................... 46 Loading ............................................................................................................................................................................. 46 Stress Concentrations...................................................................................................................................................... 46 Crack Initiation Sites ...................................................................................................................................................... 46 Inspection ......................................................................................................................................................................... 48 Code Allowables and Actual In-Service Fatigue Life................................................................................................... 48 Repair of Fatigue Cracks ................................................................................................................................................ 48 Surface Finishing to Improve Fatigue Life ................................................................................................................... 48 Peening.............................................................................................................................................................................. 48 Surface Shape and its Effect on Fatigue ........................................................................................................................ 48 Grinding ........................................................................................................................................................................... 49 Base Metal and Weld Metal Changes in Cross Section................................................................................................ 49 Thermal Fatigue .............................................................................................................................................................. 49 Checklist for Fatigue Considerations............................................................................................................................. 50 Bibliography/Recommended Reading List ................................................................................................................... 50 45

SECTION 7—FATIGUE CONSIDERATIONS

Section 7—Fatigue Considerations

Introduction

caused by welding. The residual stresses become redistributed under the influence of cyclic loading and cracks propagate out of the tensile residual stress field.

Component failures result predominately from fatigue, corrosion and wear, or a combination of fatigue and corrosion (corrosion fatigue). A large percentage of failures are directly related to fatigue. Welded structures that are subjected to repeated fluctuating loads (cyclic) are candidates for fatigue failure at strain levels well below their yield strength. Many structures can sustain static service loads forever, but under cyclic conditions the structure may fail in a short period of time, with an average load no greater than the static condition. Essential elements of fatigue are fluctuating loads with a tension component, a stress concentration, and localized plastic strain. Failures are associated with a permanent physical change in the metal at and near the stress concentration (crack tip).

Stress Concentrations Fatigue cracks only initiate at stress concentrations. When they do occur in service on weld-fabricated structures, they are usually associated with welds and their heat-affected zones. Welds and their heat-affected zones are, by definition, discontinuities and stress risers. Welds that have been deposited for repair, buildup, or cladding must be considered stress concentration areas that may be subject to fatigue failure. As soon as a fatigue crack is initiated, the crack itself becomes the primary stress concentration factor. Progressed fatigue cracks have caused catastrophic failures, where the members have lost their ability to carry stress because of loss of load-carrying cross section. In some cases, members fail cataclysmically due to overload and/or brittle failure during their final stages of cracking (unless the stress and/or the cyclic loading is reduced).

Loading Fatigue loads are cyclic in nature. The loads may be induced by cyclic mechanical forces, temperature variations, pressure fluctuations, vibration from machinery or a combination of these loads. Under some circumstances, environmentally corrosive conditions may increase the fatigue crack propagation rate. The fluctuating fatigue loads must have a tension component but may reverse even into compression loading. Loads may also be placed on a component by fluctuating bending loads, or rotational bending as is common in pump turbines, and electrical motor shafts. Varied loads of tension or tension and compression may cause failure in fatigue in a matter of a few thousand cycles, well below a load that would cause yielding. In bending and torsional fatigue, surface discontinuities are most significant since the maximum tensile loading is at the surface. The fluctuating service fatigue load can be purely compressive in nature and still cause crack initiation. This occurs in regions of high tensile residual stress

Crack Initiation Sites Crack initiation sites are typically at weld-base metal interface surfaces located along the toes of fillet welds or edges of butt welds (see Figure 7.1). Also, hard rough flame-cut edges that have not been dressed by grinding are subject to fatigue. In addition, weld termination sites are common locations for failures. Few welds experience fatigue crack growth from the roots of fillet welds, or from the roots of partial penetration groove welds. However, partial penetration welds with insufficient throat thickness will typically fail through the center of the weld and the crack will not progress into the base metal. It is also uncommon that fatigue cracks initiate from internal weld defects. However, defects that are large in 46

SECTION 7—FATIGUE CONSIDERATIONS

Figure 7.1—Cracks

47

SECTION 7—FATIGUE CONSIDERATIONS

loading, the rate of loading, or the number of cycles should be examined. Welds should be kept in lowstressed areas. Reverse cyclic loading (tension-compression) should be avoided. High-yield base metals have little advantage if reverse cyclic loading is experienced.

area, typically associated with lack of fusion, cracks or slag, open to or very close to a weld surface, are most likely to progress into cracks if undesirable loading is present. Fatigue crack growth from the toes of welds will typically progress from the edge of the weld and through the heat-affected zone (HAZ) into the base metal at right angles to the fluctuating primary stress in the weldment. By proper examination of fatigue fracture surfaces, it can be determined where the failure initiation sites were, and the direction and amount of primary stress.

Surface Finishing to Improve Fatigue Life Since surface shapes can directly affect fatigue life, it is necessary to know how surface conditioning can improve fatigue life. A simple example of this is that a finely finished smooth machined plate or shaft will have an improved fatigue performance (number of cycles and stress limit) over base metal with mill scale. Also, plates or shafts with mill scale will have considerably better fatigue resistance than the same base metal that has been subject to surface pitting by corrosion.

Inspection Fatigue cracks are difficult to find during the early stage of growth since they are tight and exhibit no deformation. Visual inspection is of limited value during this stage of crack growth. The magnetic particle (MT), penetrant (PT), and eddy current (ET) tests would be the best inspection processes for discovering fatigue cracks.

Peening

Code Allowables and Actual In-Service Fatigue Life

Light peening of a surface, if accomplished properly, will provide a smoother surface than the as-welded condition, it will lower the residual stress in the weld joint, and place the surface of the weld under light compression which will increase fatigue life. Improvements as high as 20% in the fatigue stress range or the number of cycles can be achieved with proper peening. Some standards do not permit peening of the last layer of a weld because finished inspections may not be able to be properly accomplished. However, for fatigue applications it is critical that peening not be limited to internal weld passes. Fatigue cracks most commonly progress from the surfaces of components and peak service and residual stresses are on the base and weld metal surfaces. Peening of these surfaces is essential to improving fatigue life. If necessary to facilitate subsequent inspections, light contour grinding of the peened surface may be accomplished to facilitate subsequent inspections.

There are codes, such as the American Welding Society’s D1.1, Structural Welding Code—Steel, that designate certain allowable cyclic loads based on the type of weld, the location of the weld, and the direction of loading. It should be noted that some codes, such as the AWS D1.1, provide no advantage in the allowable stress ranges for high-strength steels in fatigue applications other than permitting higher maximum allowables for static or dead loads. To obtain actual fatigue life data, it must be determined by testing with controlled mockups, or the actual component. (The airline industry manufacturers subject complete airplane fuselages and wing assemblies to fatigue testing.)

Repair of Fatigue Cracks Since fatigue failures are common in machinery service, the repair of such cracks and their prognosis for success needs to be discussed. Repairs should completely remove the fatigue crack. Peen each weld layer to minimize tensile residual stress loads. Finish welds should be contour ground to a smooth finish. Weld reinforcement should be kept to a minimum. Any major abrupt changes in the cross section in the weld or the adjacent base metal area should be avoided. The final surface condition should be inspected and proven free of linear discontinuities. The original condition that initiated the fatigue crack must be changed. A reduction of

Surface Shape and its Effect on Fatigue The surface shape of a component is critical to its fatigue life. All as-welded surface shapes result in stress risers or stress concentrations. That means that the stress in the component in these as-welded surface areas is higher than the nominal cross-sectional stress of the component, just because of a change of shape. 48

SECTION 7—FATIGUE CONSIDERATIONS

The following weld geometry attributes are stress risers that directly lead to a lower fatigue life (see Figure 7.2). It is important to realize that the only thing that is being addressed here is the shape of the surface and not differences due to a weld deposit being present, such as residual stress or different mechanical properties. Reduction in fatigue life due to these surface conditions would be present regardless of whether a weld were present or not. If all base metal fatigue samples are machined into different shapes, such as increasing amounts of reinforcement or increasing re-entrant angles or machined in simulated weld ripples, the fatigue life will be decreased. For example, if simulated undercut is machined into the surface of a plate it will reduce the fatigue life of the plate even though a weld is not present.

gles is very beneficial for improving the fatigue life of a component. In every case, fatigue life should be expected to be improved over the as-welded condition by proper grinding. Grinding that is properly accomplished can improve fatigue performance by at least 20%. However, grinding should not leave gouges or heavy grind marks. Also, removal of weld reinforcement to nearly zero without creating a noticeable angle between the reinforcement and the base metal improves fatigue life. Contour grinding of welds should be performed prior to stress relief on components, such as pressure vessels. Since contouring of welds is very expensive, and in some cases it is more expensive than making the weld, it should only be accomplished for critical applications, or where known service problems exist.

Base Metal and Weld Metal Changes in Cross Section

Grinding Contour grinding of welds and the adjacent base metal to remove undercut, excessive weld reinforcement, weld ripples, weld defects and sharp weld re-entrant an-

Improper change in cross section is one of the most common causes of reduced fatigue life. Abrupt changes in the cross section (stress concentrations) must be avoided or minimized for improved fatigue resistances. Poor transitions in thicknesses cause higher local cyclic fatigue stresses, and consequently, lower fatigue life. These localized stresses and strains can be induced in the component by different sources. Fatigue is often experienced due to differential thermal expansion or vibration of structural and machinery components. Piping system fatigue failures are often caused by vibration of nearby machinery components, not allowing systems to freely expand during heating periods, or improperly installed pipe hangers that were used to support service loads and dampen the number of fatigue cycles.

Thermal Fatigue When a component is comprised of thick and thin sections and is subject to transient changes in temperatures, high thermal strains can easily develop at abrupt changes of the cross section without considering other external loads. During the heat-up cycle the thin material will heat up much more rapidly than the thick sections. The hot thin sections must thermally expand, but will be restrained by the thicker cooler sections. During the cooldown cycle, the thinner sections will cool-down much more rapidly than the thick sections, causing high thermal strains, again due to differences in temperature. Further, thermal strains can be induced as a result of the difference in thermal conductivity between materials used in a given joint configuration.

Figure 7.2—Surface Geometry Effects on Fatigue Life 49

SECTION 7—FATIGUE CONSIDERATIONS

Checklist for Fatigue Considerations

(13) Continuous fillet welds have better fatigue performance than intermittent fillet welds. (14) Full penetration welds have better fatigue performance than partial penetration welds. (15) Avoid the use of welds on external corners or plate edges when practical for critical fatigue applications. (16) Avoid welding transverse to primary tension fatigue stress (i.e., across tension flanges of beams). (17) Flame-cut edges on heat-treatable base metal should be ground to a smooth surface to remove peak hardness residual/stresses and stress concentrations. (18) Avoid forced alignment of flanged joints and weld joints, as this may result in fatigue cracks adjacent to welds during service.

(1) Keep welds in low-stressed areas. (2) Keep the maximum stress and the number of cycles as low as is practical. (3) Avoid reverse cycles (tension and compression). (4) Avoid rapid or abrupt changes in cross-sectional thicknesses especially in weld-associated areas. (5) Keep weld reinforcement and re-entrant angles to a minimum. (6) Contour grinding or peening will improve fatigue performance. (7) Keep components painted; it improves fatigue performance and minimizes surface corrosion. (8) Reverse thermal strains are common in piping and machinery components where abrupt changes of thickness are present or components’ piping systems are not allowed to freely expand. These internal loads can be reduced by providing gradual changes in cross sections, flexible supports, sliding foundations or bends in piping systems to allow gradual strain distributions over larger areas. (9) When repairing a fatigue crack, be sure the crack is completely removed. Contour grind the weld and inspect the weld for critical applications. (10) Unless the stress pattern, the rate of loading, the number of cycles, or the basic design is changed, repaired fatigue cracks will redevelop (usually at a more rapid rate). (11) Avoid the use of doubler or cover plates where fatigue considerations are critical. (12) For dissimilar metal welds involving thermal fatigue applications, the filler metal should have a thermal expansion rate approximately midway between the two base metals selected.

Bibliography/Recommended Reading List American Welding Society. Structural Welding Code— Steel (D1.1). Miami, Fla.: American Welding Society. ———. Welding Handbook, 8th ed., vol. 1, Welding Technology (WHB-1.8). Miami, Fla.: American Welding Society. Fatigue Assessment of Welded Joints Using Local Approaches. England: Abington Publishing. (Available through AWS.) Fatigue Strength of Welded Structures, 2nd ed. England: Abington Publishing. (Available through AWS.) Proceedings of the International Conference on Fatigue. Miami, Fla.: American Welding Society.

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8

Welding Safety Considerations Contents Introduction ..................................................................................................................................................................... 52 Design................................................................................................................................................................................ 52 Production ........................................................................................................................................................................ 53 Respiratory Protection and Ventilation ......................................................................................................................... 53 Use of Inert Gases ............................................................................................................................................................ 54 Planning for Safety .......................................................................................................................................................... 54 Bibliography/Recommended Reading List ................................................................................................................... 55

51

SECTION 8—WELDING SAFETY CONSIDERATIONS

Section 8—Welding Safety Considerations

Introduction

• Can the assembled unit be lifted and transported to the installation site? • Joint design as related to accessibility. • Fabrication processes. • Materials. • Process safety.

The purpose of this section is to provide the designer, planner, or mechanic with insights into potentially hazardous conditions that commonly exist in production and work environments, but unfortunately are often overlooked. All too often, hazardous conditions are present and overlooked or not properly considered. This results in unnecessary risk to personnel, equipment, and product. It can be said, with reasonable certainty, that no designer, planner, production superintendent, or mechanic ever plans to have an accident. Accidents are, by definition, unplanned occurrences, sometimes resulting in loss of life, serious collateral damage, or both. The information provided and discussed in the following pages is not intended to address all of the potential hazards associated with welding fabrication, or the subsequent safety precautions necessary to resolve every potential problem. This reference guide is intended to stimulate the imagination of everyone, from the designer to the mechanic, to look for potential hazards and eliminate them before an accident occurs and it becomes a costly lesson learned the hard way. Safety, as related to welding fabrication, must be considered from three specific points of view: design, planning, and production fabrication. Each area has specific responsibilities and concerns and will be looked at individually in this section.

All of the above are important aspects of a wellformulated plan. While these items do not directly address safety, it can be seen that several of the items will greatly impact the safety of the production personnel performing the specified task. The designer must use knowledge and experience to weigh production techniques against worker safety requirements. For example, are the weld joint design and component configuration consistent with the intended weld process, i.e., will a joint’s backside accessibility be in a tank, with a single access at the top of the tank, jeopardizing a welder’s safety by asphyxiation due to lack of oxygen? This resulting from the plan-specified weld process, “gas metal arc” welding which incorporates an inert shielding gas. It cannot be assumed by the designer that production personnel will take this type of potential hazard into consideration before starting work. There are many industry publications available that cover industry-associated health and safety hazards, e.g., AWS Z49.1, Safety in Welding, Cutting, and Allied Processes, and AWS Effects of Welding on Health I–X. These and other industry reference materials can be valuable sources of information if your organization is not fabricating under some other safety standard, such as those of the Occupational Safety and Health Administration (OSHA). The following are specific areas that designers should consider prior to issue of plans and instructions:

Design Safety during the design phase is usually not a field that is given independent consideration. Safety is usually limited to general areas of consideration, and more often than not, related to production capabilities and costs, more than safety, such as:

(1) Design and lay out welds which will allow for adequate and safe welder accessibility. (2) Design assemblies so that fabrication welds can be performed in areas where adequate ventilation can be provided.

• Can the component be handled or positioned for fabrication? 52

SECTION 8—WELDING SAFETY CONSIDERATIONS

found that the majority of fires have been caused by portable equipment. Permanent weld stations are made fire safe.

(3) Provide direction on plans and instructions where known fire and safety hazards exist, i.e., tanks which have contained or presently contain flammable or healthhazardous material and air-tight compartments or voids which have been closed for long periods of time. (4) Design assemblies to utilize automated systems wherever possible.

• Do not weld on tanks that contain or have contained combustible material or liquids unless they have been inerted according to IAW local instructions. • Do not weld when material or personnel are improperly supported or staged.

Production

• Do not weld with equipment that is improperly insulated or has broken electrical insulation.

While safety is everyone’s concern, the ultimate responsibility for a welder’s safety is the welder who is performing the work. Without a thorough understanding of the risks and the initiative to eliminate potentially hazardous conditions in the interest of their own and their fellow worker’s safety, injurious and costly accidents will occur. Training that instructs the welder on the types of safety hazards which exist in his/her particular work environment and even more important, how to spot unsafe conditions under various circumstances, whether related specifically to welding or some other trade, must be provided. As the safe working environment ethic becomes intrinsic to all workers, supervisors, planners and design personnel, accidents and losses will be greatly reduced. The following is a listing of many common hazardous conditions and practices that have caused and will continue to cause lost time, serious injury, and death.

• Do not oxyfuel cut, weld, or carbon arc gouge on bulkheads or walls without stationing a fire watch or taking appropriate precautions to protect against fire on the opposite side. Equipment and machinery in the area must also be protected. • Do not enter a previously sealed void without checking the atmosphere for the presence of oxygen and potentially harmful gases. • Do not weld in a tank or void without adequate ventilation and especially when using processes such as gas tungsten and gas metal arc welding that employ inert gases. NOTE: All inert gases will displace air. • Do not weld or grind without appropriate eye and skin protection. • Do not weld or oxyfuel weld/cut in an oxygenenriched environment, as the potential fire hazard is greatly increased.

CAUTION • Do not forget that argon, carbon dioxide, mapp, propane, and natural gas are heavier than air and will flood a space if stored inside a room or compartment. Always store in a well-ventilated area. Never store leaky cylinders indoors.

Respiratory Protection and Ventilation

• Do not dip a hot electrode holder in water to cool it off.

• The respiratory health hazards associated with welding operations evolve largely from the inhalation of the gases, dusts and metal fumes produced. Whether respiratory damage occurs during welding will depend on the use of precautionary measures that are indicated by an evaluation of the hazards involved. With only a few, relatively simple precautions, the chance of respiratory damage can be eliminated.

• Do not assume that because power sources are low voltage they are not dangerous. A welder should avoid standing on wet floors and never touch the bare metal parts of an electrode holder with any exposed skin or wet covering on their body. The danger of electrical shock is particularly present in hot weather where the welder may be wet from perspiration.

• The single most important factor influencing the amount of fumes inhaled by a welder is governed by the welder himself, by the position of his head with respect to the plume of the fumes.

• Do not weld while standing in water without properly insulated footwear. • Do not transport arc welding cables coiled around the shoulders when the conductors are carrying power.

• The nature of any toxic materials to which a welder is exposed will depend on the type of welding process, filler and base metal, presence of contamination on

• Do not weld in areas that are not fire safe, especially if the work can be moved to a safe location. It has been 53

SECTION 8—WELDING SAFETY CONSIDERATIONS

Use of Inert Gases

the base metal or the presence of volatile solvents in the air.

Due to the serious nature of the potential hazards associated with the use of inert and other gas mixtures required during welding, additional emphasis is warranted. The use of nonflammable (inert) gases within structural enclosures, piping systems, etc., creates a potential personnel hazard. The inherent danger in the use of inert gases is that sufficient quantities may be released into an occupied compartment or space to reduce the oxygen content of the atmosphere below that required to support human life. Inert gases are used in many production methods in the building and repair of ships, argon (Ar) for purging piping systems, nitrogen (N 2 ) for drying air piping, helium (He) for some types of welding and carbon dioxide (CO 2 ) for inerting tanks containing flammables, to name a few. An atmosphere deficient in oxygen (O2) usually gives little or no warning, particularly if the gas that has displaced the oxygen is odorless. A trained observer may notice an increase in pulse or breathing rate in time to escape; however, the average individual may fail to recognize the lack of oxygen until he is too confused and weak to save himself, especially when a considerable distance must be traveled to reach fresh air. To minimize the hazards, the use of inert gas must be closely controlled. Positive exhaust systems must be in operation to reduce the possibility of an inert gas buildup, should accidental leakage occur. Low areas where heavier gases tend to accumulate require special attention. In addition to general ventilation requirements to reduce this hazard, local exhaust ventilation should be installed in such a manner as to exhaust the inert gas. Each opening from which the inert gas is flowing must have individual ventilation. Personnel using and/or having access to inert gas should receive additional training about its potential hazards and the appropriate methods for handling and working with it.

• Zinc and magnesium oxide inhalation. — Toxicity produces chills, fevers, nausea four to eight hours after exposure. — Toxicity disappears almost invariably, within 24 hours. • Copper fumes toxicity. — Toxicity symptoms similar to zinc and magnesium except it is more severe, longer lasting and is precipitated by less exposure. • Nickel (1 mg per cubic meter), cobalt (0.1 mg per cubic meter), and mercury (0.1 mg per cubic meter) are: — Strongly suspected of causing a chemical pneumonitis which may have severe or even fatal results. • Cadmium oxide and beryllium oxide. — These oxides produce severe fume pneumonitis. — They are extremely dangerous. • Iron and aluminum apparently do not cause a metal fume fever although inhalation of very large amounts of iron fumes for a period of years may cause a condition known as siderosis, a deposit of iron in the lungs. • Materials coated, even accidentally, with toxic materials such as lead, cadmium, mercury or paints containing toxic materials produce what is probably the greatest health hazard in welding or cutting. Any such coatings should be removed prior to welding, or rigid ventilation control measures should be established. Welding on materials containing lead, or other toxic materials such as high-lead bronze, may produce a harmful exposure, depending largely on the amount of such material present and the amount of metal melted. Professional safety advice should be consulted prior to welding. Welding on materials containing Beryllium may produce an extremely dangerous condition. No such welding should be undertaken until complete control of fumes has been provided. Welding on tanks, pipelines or containers of any sort from which the contents have not been completely removed may introduce a serious hazard by volatilization or decomposition of the residue. This is particularly true when halogenated materials or plating solutions are involved.

Planning for Safety It is often the case during the planning and funding phase of a project, that safety is assumed to be a normal work practice and is just another unaccounted for expense of completing a project. This can be a dangerous approach to doing business. This attitude or lack of awareness of what is required to ensure the safety of the workers places an unnecessary burden on those production people trying to meet the schedules. Not properly identifying and subsequently funding safety requirements may cause production personnel, in their efforts to complete a job in a timely, cost-effective manner, to overlook serious safety considerations. Safe working 54

SECTION 8—WELDING SAFETY CONSIDERATIONS

conditions for your personnel are not an incidental part or expense of doing business. Staging, eye, ear, ventilation, respiration, skin, and fire protection add considerable cost to a job and must not be overlooked. Successful completion of any complicated task requires thoughtful coordination of assets and personnel—usually from a variety of trades. The same can be said for thorough planning and funding of that task prior to the start of a complex job. Without careful coordination between design, planning, and production personnel, a costly lesson to be learned would be in the making. Areas that require special safety considerations may be overlooked, resulting in a potential loss of both human and material resources.

———. Arc Welding Safely (AWS). Miami, Fla.: American Welding Society. ———. Effects of Welding on Health, vols. I-X (EWH). Miami, Fla.: American Welding Society. ———. Fire Safety in Welding and Cutting (FSW). Miami, Fla.: American Welding Society. ———. Lens Shade Selector (F2.2). Miami, Fla.: American Welding Society. ———. Safe Practices (SP). Miami, Fla.: American Welding Society.

Bibliography/Recommended Reading List

———. Safety and Health Fact Sheets (SHF), 2nd ed. Miami, Fla.: American Welding Society.

American Welding Society. A Sampling Strategy Guide for Evaluating Contaminants in the Welding Environment (F1.3). Miami, Fla.: American Welding Society.

———. Safety in Welding, Cutting, and Allied Processes (Z49.1). Miami, Fla.: American Welding Society.

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Weld Joint Design Considerations Contents Introduction ..................................................................................................................................................................... 58 Design................................................................................................................................................................................ 58 Weld Joint Design Considerations ................................................................................................................................. 58 Accessibility...................................................................................................................................................................... 59 Environmental Conditions .............................................................................................................................................. 60 Temperature Control....................................................................................................................................................... 60 Base/Filler Material......................................................................................................................................................... 60 Pipe Welding Considerations.......................................................................................................................................... 60 Summary .......................................................................................................................................................................... 61 Bibliography/Recommended Reading List ................................................................................................................... 61

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SECTION 9—WELD JOINT DESIGN CONSIDERATIONS

Section 9—Weld Joint Design Considerations

Introduction

Weld Joint Design Considerations

Manufacture/assembly of most products requires joining of component parts. When welding is used for joining, the assembly is called a weldment. As in any assembly process, the basic objective of a weldment design is to perform the intended function of the product with minimum fabrication costs.

When selecting welded joint designs, the following items need to be considered.

Application What is the function of the weldment? Generally all fabrication is covered by a standard or code that is invoked by the customer. Standards/codes have specific sections that define/clarify structural welding, piping systems, pressure vessels, machinery components, turbine components, etc. Information is also given for allowable joint designs/limitations, required nondestructive tests, and the acceptance standards that have to be met.

Design During the conceptual design phase, the design engineer needs to review items such as the fabrication standard/code being invoked, operating condition/environment, material selection, assembly techniques, required inspections and maintenance/repair. The design engineer should have basic knowledge or seek consultation from related engineering and production personnel in the following areas:

Cost The designer should keep in mind that variables in joint design, material type, welding process, nondestructive testing, and fabrication sequence have an effect on cost. During the initial design/planning stage, meetings should be held to review the projected fabrication with related engineering, funding, and production personnel who will be involved with the work. Preplanning helps minimize fabrication cost. Items to discuss at these meetings should include:

(1) Mechanical and physical properties of base metals and weld metals. (2) Weldability of metals and the effect of their strengthening mechanisms (alloyed, cold worked, age hardened, heat treated). (3) Welding processes (advantages and limitations). (4) Preparation and fabrication of welded joints. (5) Thermal effects of welding, cutting, gouging, and grinding. (6) Effects of restraint, stress concentrations and residual stress. (7) Distortion control, prevention and cause. (8) Nondestructive examination methods and the applicable acceptance standards. (9) Applicable welding and safety codes/standards. (10) Assembly and erection methods. (11) The proper use of welding symbols and terms.

(1) Review of the fabrication standard/code requirements including weld type and size, and nondestructive testing. (2) Review of possible base materials. (3) Review of assembly sequence including weld joint accessibility and position. (4) Review of possible weld joint designs and welding processes with consideration to mechanized processes. (5) Welder and procedure qualification requirements. (6) Post weld stress relief and machining. (7) Alternate fabrication methods. (8) Staffing and equipment requirements. (9) Documentation of job performance.

The AWS Welding Handbooks are a good source of information on material properties and welding. 58

SECTION 9—WELD JOINT DESIGN CONSIDERATIONS

Welding Processes

lack of penetration or weld surface roughness should all be avoided to maximize fatigue life. Weld reinforcement sized to the members being joined will increase fatigue life. Welds are often designed as full penetration and fillet welds designed as continuous instead of intermittent in order to maximize fatigue life.

When selecting a weld joint design, it is necessary to consider what welding processes are to be used. Will the welding be accomplished in the field or in the shop? Each welding process has its limitations. Gas Tungsten Arc Welding (GTAW) should not be used on heavy structural sections, but rather on piping or sheet metal. Submerged Arc Welding (SAW) will only be used in the flat or horizontal position preferably in a shop-type environment. If weld deposition rates are of concern in the field then the Flux Cored Arc Welding (FCAW) process should be considered rather then Shielded Metal Arc Welding (SMAW). Joint access and position may determine which welding process can be used. This in turn determines the weld joint design.

Impact Resistance Impact loading may result from any sudden application of a load. During impact loading, a member is required to absorb energy rapidly. Designs that are susceptible to fatigue failures are often also subject to impact failure. The weld and base material should be ductile and have suitable impact resistance at the lowest possible service temperature. Weld joints subject to impact loading will gain increased resistance by design incorporating many of the guidelines used for “Fatigue Resistance.”

Corrosion Resistance What type of service (operating media such as hightemperature steam, seawater, acidic fluids, etc.) will the joint experience? Weld joints with backing strips or backing rings and partial penetration joints with open roots will be subject to crevice corrosion and should not be used in the above-mentioned service without specific consideration.

Distortion Control There are a number of distortion origins during the manufacture of structural sections and piping systems. Distortion is primarily due to restricted expansion when the heat of welding is applied. With preplanning, distortion can be kept to a minimum. Design factors of distortion control include:

Joint Efficiency

(1) Minimize welding by using standard shapes. (2) Use partial-penetration joints when design permits. (3) Balance welding around the neutral axis. Use double “V” or “U” joints so that welds can be balanced by welding alternately on either side of the plate. (4) Weld toward restraint. (5) Do not use excessive force to align joints for fit up. (6) Preplace or preposition weld joints to accommodate weld joint shrinkage. (7) Prestress weld joints to off set weld shrinkage. (8) Peen welds. (9) Use processes that use the minimum total heat input to fabricate the joint. (10) Use the least amount of total weld metal consistent with adequate design. (11) Use subassemblies to accommodate distortion shrinkage.

Joint efficiency is the ability of the weld joint to transfer stresses between the members being welded and the weldment. Weld joint efficiency requirements will, to a large extent, determine what type of weld joints are needed. Welds joints can be put into two groups, full penetration and partial penetration. A full-penetration joint has fusion through the entire cross section of the weld. A partial-penetration joint allows an unfused area within the joint cross section, and the weld does not completely penetrate the joint thickness. Keep in mind that joint efficiency is not totally equated with weld size, but is largely dependent on factors including weld metal properties and stress transfer.

Fatigue Resistance When designing a weld joint, fatigue resistance must be considered. Any abrupt change in cross section or direction along the path of stress flow will reduce the fatigue life. To increase fatigue life, sharp corners should be replaced with smooth transitions, simple butt joints should be used instead of lap or “T” joints, and changes in cross section should include a minimum 4:1 taper into or out of a weld joint. Smoothly ground weld reinforcement will also increase the fatigue life. Weld discontinuities such as excessive reinforcement, undercut, overlap,

Accessibility Joint accessibility for the welder needs to be taken into consideration when issuing design instructions and drawings for assembly in the shop or field. Is the job new 59

SECTION 9—WELD JOINT DESIGN CONSIDERATIONS

(4) What physical and chemical properties need to be considered? (5) What material types and thicknesses are available? (6) Are dissimilar metals to be joined? (Some filler metals are extremely crack sensitive depending on the base metal/filler metal combination.)

construction or repair? Can component subassemblies be accomplished in the shop? Shop assembly increases accessibility by permitting plates to be turned, and piping to be rolled. To assist in making sound design/planning decisions, the design engineer should visit and review the job site prior to issuing work instructions.

Fabricated components, throughout their life span, may have to be weld repaired. The designer should keep this in mind for weldability and welding access in the field. Components that will be weld repaired or replaced often should be designed for ease of welder access and ease of weldability (i.e., process to be used, base material type, and joint design).

Environmental Conditions A job site and work condition review will assist in selection of weld process and associated joint design. Adverse weather conditions and contaminates such as water, paint, and oil in or near an existing weld joint are factors to consider when designing new weld joints, or specifying repairs and welding processes. Windy conditions require additional sheltering if a gas-shielded welding process is selected.

Pipe Welding Considerations Pipe Joint Design Design criteria for welded pipe joints are set forth in specifications. These criteria are dependent upon the piping system. The designer is guided in selection of a particular joint design by one or more of the following factors:

Temperature Control Heat transfer from the welding process into the weld and the adjacent base metal can cause damage to components. When welding a component with finished machined surfaces control of temperatures before, during and after welding has to be addressed so as to keep distortion of those surfaces to an acceptable level. There may be paint that needs to be protected on the inaccessible backside of a component that requires weld repair. Material thickness and maximum temperature the paint can withstand have to be known before welding. A mock-up of the job may have to be done. O-ring seating surfaces located near weld repair areas require heat input control for distortion protection. Some considerations for controlling heat to a desired area can be done by preheat and interpass temperature restrictions, limiting length of weld bead, joint design, size of welding electrode, number of weld passes, and forced cooling (air or water).

(1) Contractual requirements. (2) System application (pressure, temperature, corrosion, erosion, etc.). (3) Piping material. (4) Piping wall thickness. (5) Specifications. (6) Classification of joint (i.e., consumable insert, backing ring, socket, or square butt). (7) Availability of fittings. (8) Welding accessibility for butt or socket joints vary with location. Butt welds usually require greater working space. (9) Butt weld joints are more difficult and generally more expensive to make than socket welds. (10) Butt weld fittings may be joined directly to one another, requiring only one joint. (11) A length of pipe between two socket fittings requires two weld joints.

Base/Filler Material

A piping designer should take into consideration that any joint in any piping system could be subject to failure under a given combination of adverse circumstances, and must therefore be accessible for repair. Designers must also consider that repairs may be necessary in the field and not under ideal conditions. Consequently, whenever a welded joint can be eliminated in design, it automatically eliminates a source of possible future failure. This could be done by designing bends instead of welded joints into a given piping system.

Base metal and filler material selection can determine what weld joints can be used. When choosing base/filler materials the designer has to be aware of conditions that may affect the final selection such as: (1) Is the project new construction, rework, or repair? (2) What is the form of the base material (wrought, forged, or cast)? (3) What service environment will the material be subjected to? 60

SECTION 9—WELD JOINT DESIGN CONSIDERATIONS

Summary

Bibliography/Recommended Reading List

When designing weldments, the design engineer needs to know the properties of the materials selected, the service requirements of the weldments, and the fundamentals of the fabrication processes.

American Welding Society. Standard Symbols for Welding, Brazing, and Nondestructive Examination (A2.4). Miami, Fla.: American Welding Society. ———. Welding Handbook, 8th ed., vol. 1, Welding Technology (WHB-1.8). Miami, Fla.: American Welding Society.

Engineers designing welded components need to know basic mathematical formulas for calculating forces and their effects. The calculations need direct application to the specific weld joint design that is selected for joining component parts.

Blodgett, O. W., Design of Weldments. The James F. Lincoln Arc Welding Foundation, Cleveland, Ohio. Cary, H. 1997. Modern Welding Technology. Englewood Cliffs: Prentice-Hall, Inc. (Available through AWS.)

Weldment design should always be based on the thinner member being joined. The minimum amount of weld metal should be used while maintaining weld joint strength requirements.

Metals Handbook, 9th ed. vol. 6, Welding, Brazing, and Soldering. American Society for Metals, Metals Park, Ohio.

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Weld Distortion and Control Contents Introduction ..................................................................................................................................................................... 64 Mechanisms of Weld Distortion ..................................................................................................................................... 64 Theoretical Shortening under Ideal Conditions ........................................................................................................... 65 Metal Properties Affecting Distortion ........................................................................................................................... 65 Considerations for Minimization of Weld Distortion................................................................................................... 66 Bibliography/Recommended Reading List ................................................................................................................... 69

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SECTION 10—WELD DISTORTION AND CONTROL

Section 10—Weld Distortion and Control

Introduction UNWELDED PLATE

Control of distortion during fabrication is essential. Weld distortion is evident on practically all weldments. It is readily visible on thin plate and sheet metal when welded. In addition to lowering stress-carrying capabilities, distortion can make it extremely difficult to fit subassembly parts together to complete a fabrication. Weld distortion can be extremely expensive to correct and it is easier to prevent than to correct for critical applications. Fabrication distortion is caused by:

ARC IS STRUCK

HEATED PORTION EXPANDS

• Volumetric shrinkage of molten metal solidifying to ambient temperature. • Thermal shortening of metal adjacent to a weld. • Filler metal with higher yield strengths than their base metals.

ARC IS EXTINGUISHED

The metallurgical and mechanical properties of the base metal greatly influence the degree of distortion experienced. Because of their different mechanical and metallurgical properties, different base metals exhibit varying amounts of distortion for the same amount of heat input. The following base metals are listed in order of increasing distortion problems: HY-100, HY-80, highstrength steel, mild steel, Monel®, copper-nickel and 300 Series stainless steel.

CONTRACTING SHAPE WITH COOLING

RESOLIDIFIED WITH RESIDUAL STRESS

Mechanisms of Weld Distortion Figure 10.1—Shrinkage in a Weld Caused by Expansion and Contraction

We l d M e t a l S o l i d i fi c a t i o n S h r i n k a g e (Shortening) Figure 10.1 exhibits the primary distortion mechanism involved when a weld is deposited in a grooved plate. The weld that is deposited in the plate will decrease in volume about two percent during solidification and cooling to ambient temperature. If the solidified weld metal could be removed from the base metal it would have a shorter length than the base metal length it was deposited upon. Since the yield strength of the weld deposit is invariably higher than the base metal, the weld

will cause the adjacent base material to distort and to have high residual stress at ambient temperature.

Distortion of Metal Adjacent to a Weld Due to Heat Input Another mechanism by which weld distortion operates is that the metal (which includes previously depos64

SECTION 10—WELD DISTORTION AND CONTROL

ited weld metal) adjacent to the weld is heated under restricted expansion (compression). The metal is only free to thermally expand and plastically deform in the through thickness dimension of the base metal (perpendicular to the weld surface). The end result is that the nonuniformly heated area adjacent to a weld is generally thicker, narrower in width, and shorter in length than base metal outside the heated area (see Figure 10.1).

cation can result in distortion. It should be noted that as the temperature increases, the strain to cause yielding (permanent deformation) decreases. As one inch of base metal is heated, it must thermally expand (volumetrically) to accommodate this increased energy; and since it cannot thermally expand in its length because of restraint (ideal), the metal experiences compressive strains. At low temperatures these strains are elastic (no permanent deformation). As the temperature is increased, the compressive strains become so high that they reach the compressive yield strain of the base metal, and any increased temperature results in plastic deformation. By subtracting the yield strain curve from the thermal expansion curve, the resulting curve represents the permanent deformation (shortening) of the one-inch length of base metal after it has been subjected to temperatures higher than approximately 180°F. By developing these curves for different base metals from published data and then comparing them, increased information can be gained as to what temperatures are necessary to initiate deformation under ideal conditions with single axial restraint. Under ideal conditions very small changes of temperature are required to reach base metal yield points and to cause plastic flow.

Restricted Expansion during Heating Cycle When base metal is heated in a plate by a single pass of weld metal, the base metal being heated adjacent to the weld thermally expands as the temperature rises while the base metal a short distance away from this heated area is much cooler. The cooler base metal restrains thermal expansion in the plane of the plate. This places the heated area adjacent to the weld under high compressive strain. The yield strain of the base metal being heated decreases with increasing temperature while that of the adjacent cooler base metal remains stronger due to its lower temperature. As the adjacent metal continues to be heated to higher temperatures as a result of thermal conduction from the deposited weld, it reaches yield point. The heated area adjacent to the weld yields (or plastically flows) perpendicular to the plate or weld surfaces (see Figure 10.1).

Metal Properties Affecting Distortion

Cooling Cycle

Each component brings its own manufacturing history of residual stress to the fabrication assembly. Castings have solidification stresses, and plates are stressed by rolling and cooling operations. Each metal’s properties (thermal expansion, thermal conductivity, elasticity, yield strength, its structural shape) influence the severity of manufacturing stresses and resultant weld distortion.

As the heated area adjacent to the weld cools off, the area that is plastically deformed is always at a lower temperature than the center of the weld. This causes retained increased thickening or deformation that was formed during the heating cycle in this peripheral weld area. During the cooling cycle, the periphery area (this may be previously deposited weld metal or the base metal) has a higher yield strength than the weld, due to its lower temperature. The average thickness at ambient temperature of this plastically flowed area is greater than it was prior to heating. This thickening can only be accommodated by shortening of dimensions in the plane of the base metal. In other words, the thermally upset base and weld metal now have a shorter length and shorter width in the plane of the plate. The end result of this shortening in width and length and increased thickness is high elastic and plastic deformation of the weld and the base metal adjacent to the weld metal. This results in weld distortion and high residual stress.

Thermal Expansion The coefficient of thermal expansion is used to express the amount of expansion and contraction of metal with temperature change. The higher the coefficient, the more expansion and contraction is predicted for a given temperature change. 300 Series stainless steel, coppernickel, and Monel® all have high thermal expansion rates and this, in conjunction with relatively low yield strains, makes them subject to considerable distortion.

Thermal Conductivity Metals with low thermal conductivity will experience increased distortion due to a greater temperature gradient between the weld and base metal. The reason for this is that when weld metal is deposited, the base metal cannot conduct the heat of welding away rapidly and a large difference in temperature is created in a narrow zone,

Theoretical Shortening under Ideal Conditions The nonuniform heating and cooling of the weld metal and the surrounding base metal during weld fabri65

SECTION 10—WELD DISTORTION AND CONTROL

• A double “V” groove requires less welding than a single “V” in the same plate thickness and will help balance angular distortion. • Use partial penetration joints whenever possible. • Use smallest weld size possible to meet the strength requirements. Oversized welds increase distortion. • Use flanged corners instead of welded corners. • Use intermittent welds to reduce welding where practical (see Figure 10.2).

causing large thermal expansion in this localized area. Plastic flow occurs in this restricted expansion area, resulting in distortion. Stainless steel has much lower thermal conductivity than mild steel and this is partially why it is much more susceptible to distortion than mild steel.

Modulus of Elasticity Modulus of elasticity is a measure of the stiffness of a metal. A material with a higher modulus is more resistant to distortion, since increased stress is required to strain or stretch the material. However, it should be noted that the mild steel and most alloy steels have nearly the same modulus of elasticity.

Balance Welding Weld in a sequence that offsets one shrinkage stress against a previously deposited weld on the opposite side (see Figure 10.3). However, the resultant shrinkage of the second weld will not equal the distortion of the first weld. Distortion is best controlled by welding both sides simultaneously such as double fillet welds where practical.

Yield Strength/Strain Yield strength or yield strain is an indication of the metal’s resistance to permanent deformation. A lower yield strength filler metal accommodates some of the dynamic weld joint stresses by stretching rather than creating distortion. Conversely, base metals with high yield strain are less susceptible to distortion due to their increased strength.

Joint Design Joint designs should have minimum root openings and bevel angles, yet provide adequate joint access for good fusion for the welding process used. Joint designs need to be selected that will balance welding (see Figure 10.4).

Manufacturing Residual Stresses Components have their own residual stress patterns that can affect the resultant distortion of the finished product after welding. Because of different cooling patterns, castings, plate, and shapes have varying residual stress patterns that directly affect distortion. For example, the flanges of rolled I-beams are typically under tension, while the webs are under compression, because during manufacture the thinner cross section of the webs cool first followed by the flanges. The cooling and shortening of the flanges place the webs under compression.

Restraint Reduce distortion by restraining assemblies in fixtures or to other assemblies. When practical weld toward maximum restraint; weld away from edges; weld toward the centroid or the center of gravity when practical.

Neutral Axis Position weld joints at a neutral axis to provide less leverage for shrinkage forces, or balance welding around the neutral axis.

Considerations for Minimization of Weld Distortion

Structural Tacks

Minimize Welding

Structural tacks help force the welds to deform rather than distorting the base metal. Structural tacks are usually a minimum of two inches long and are often two layers thick. These tacks will restrain the root opening from closing during welding and will help force the root weld pass to deform instead of pulling the plates together. It should be kept in mind that any restraint that can be placed on hot weld metal will be very effective, due to its low yield strength at high temperatures. The structural tacks need to be thick enough so that they cannot be fused through by subsequent passes, as this will reduce some of their effectiveness.

Avoid welding when practical. Use standard shapes or rolled sections, use castings or break/roll material into different shapes rather than welding several pieces together. • Minimize root openings and bevel angles of full penetration joints. However, adequate access for the welding process and operator must be provided. • A “J” or “U” groove requires less welding than a bevel or “V” groove and will result in less distortion of thicker metals. 66

SECTION 10—WELD DISTORTION AND CONTROL

Figure 10.2—Fillet Weld Applications 67

SECTION 10—WELD DISTORTION AND CONTROL

welded. This will counteract weld and base-metal shortening and reduce weld distortion. The two interference fit examples show how it may be used to maintain critical inside dimensions and circularity when adding length to or repair welding a finished machine part.

Preset

Figure 10.3—Sequence Welds

Alignment of the joint is set up out of position to allow for anticipated shrinkage during welding to bring the assembly to the desired position (see Figure 10.6). Use mockup test assemblies to estimate amount of preset necessary as the amount of preset will vary with each welding process and the parameters used.

α

Overwelding Weld convexity increases shrinkage but does not increase strength. Use intermittent welds or partial penetration welds where practical. Do not oversize fillet welds. Excessive distortion can result from oversizing small fillet welds by one size on thin material.

β

Weld Process Selection

Figure 10.4—Joint Designs to Minimize Distortion

The greater the total heat input by conventional welding processes the greater the distortion. Weld process selection should consider weld deposition, amperage, and travel speed. Use of largest suitable electrode is frequently recommended as transverse weld stresses are somewhat cumulative and distortion can be increased by multiple passes.

Fit Up Proper edge preparation with joint alignment and minimum root openings decrease welding and result in less weld distortion. Do not use excessive force to align joints for welding. High residual stress will be induced into the weld joint resulting in weld distortion.

Prefabrication Welds made in the flat position or with joint rotation allow use of larger electrodes and deeper-penetrating weld processes with less joint preparation. The result is less weld distortion.

Backstep Welding Subassemblies

Backstep welding uses the restraint of a previously deposited weld to minimize the tendency of the two plates from pulling together as the welding progresses.

Fabrication of subassemblies provides additional distortion control, since each subassembly can be adjusted for distortion before being matched to another subassembly.

Controlled Wandering (Backstep) Weld Peening

A backstep weld is used in wandering sequence. Welds made in short segments, distributed along the length of the joint, cause less distortion than continuous welding (see Figure 10.5).

Weld shrinkage may be counteracted by peening, which stretches the weld in the area peened. Peening is most effective at higher temperatures. A blunt tool should be used to plastically deform (stretch) the weld after slag is removed. However, a root bead should only be peened lightly or not at all because of the danger of causing a root crack. Peening marks should be removed from the final weld surface if they are expected to inter-

Prestressing and Interference Fits Prestressing surfaces to be welded by bending or by interference fits develops tensile stresses on surfaces to be 68

SECTION 10—WELD DISTORTION AND CONTROL

N OF DIRECTIO

OGRESS 3

WELD PR

4

2 1

Figure 10.5—Controlled Wandering (Backstep) Weld

added to the load stress may exceed the material’s yield strength. Stress relief after welding reduces residual stress of fabrication. Stress relief, properly accomplished, is essential for components that are to receive intricate machining to close tolerances. Without stress relief, the component will “walk” or move during the machining of the weld deposit (removing the weld deposit changes the residual stress pattern) and the part may be lost as scrap when finished.

Figure 10.6—Presetting for Fillet and Butt Welds

Bibliography/Recommended Reading List fere with final inspection. Peening marks may act as stress risers if peening is not accomplished properly.

Stress Relief

American Welding Society. Welding Handbook, 8th ed., vol. 1, Welding Technology (WHB-1.8). Miami, Fla.: American Welding Society.

The shrinkage of a cooling weld applies stress to the weld metal and surrounding base metal. These stresses

———. Welding Processes and Practices (WPP). Miami, Fla.: American Welding Society.

69

SECTION

11

Checklist for Sound Welding Decisions Contents The Intended Service Requirements.............................................................................................................................. 72 Base and Weld Metal Properties .................................................................................................................................... 72 Fabrication Conditions Imposed .................................................................................................................................... 73

71

SECTION 11—CHECKLIST FOR SOUND WELDING DECISIONS

Section 11—Checklist for Sound Welding Decisions

❒ Tensile and compressive stresses and strains (axial/ bending). ❒ Corrosion resistance. ❒ Erosion resistance. ❒ Abrasion resistance. ❒ Impact resistance (stress and temperature dependent). ❒ Creep resistance. ❒ Temperature (high- or low-temperature physical properties). ❒ Fatigue resistance (thermal and mechanical). ❒ Aesthetics (color match, anodizing, and appearance).

No item has a greater effect on weldability than the common everyday decisions made by the designer, welding engineer, shop supervisor, and the journeyman mechanic. Before making a welding decision, there are three basic areas that should receive consideration: • The intended service requirements. • The base metal and weld metal properties. • The fabrication conditions imposed. As an aid, to make sure you have checked items that can affect weldability, a Sound Welding Decisions Checklist is provided in this section. This list is not all inclusive and may be added to depending upon the individual’s specific experience. However, it is a list of specific areas that deserve attention in avoiding the majority of serious welding problems. Once a problem or potential problem area is identified, it should be referred to or coordinated with the person having technical cognizance, i.e., the designer, welding engineer, or the production supervisor. It should be noted that:

Base and Weld Metal Properties Welding Engineering Cognizance The effects on the HAZ and weld metal properties due to the maximum welding temperature reached and the rate of change of temperature are of primary importance. They can result in weld defects, metallurgical notches and drastic changes in base metal properties.

• The design engineer is most knowledgeable about the intended service of requirements. • The welding engineer is most knowledgeable about the weld metal. • The production supervisor and mechanic are most knowledgeable about the imposed fabrication conditions (environment).

❒ ❒ ❒ ❒ ❒ ❒ ❒ ❒ ❒

The Intended Service Requirements

❒

Design Cognizance

❒ ❒ ❒ ❒ ❒

Will the weld and heat-affected zone (HAZ) provide the desired service? Consider only those service requirements that make the weldment fit for use.

72

Base metal chemical composition. Diluted weld metal properties. Tensile/yield strength. Elongation/ductility. Quench hardenability properties. Work hardening properties. Age-hardening properties. Susceptibility to hot cracking. Solubility of gases (porosity, embrittlement, and cracking). Homogeneity/soundness (coring, loose structure, and lamellar tearing). Sensitization to corrosion and corrosion cracking. Grain size. Refractory oxide formation. Conductivity (thermal). Viscosity or fluidity.

SECTION 11—CHECKLIST FOR SOUND WELDING DECISIONS

❒ Number of welds. ❒ Weld shrinkage. ❒ Distortion control, prestress, structural tacks, back setting, and peening. ❒ Weldment design (joint design, weldment geometry, material selection, weld sizes, and equipment maintenance). ❒ Cleanliness. ❒ Weld location (accessibility, restrictions, position, shop vs. field). ❒ Weld sequence (shop controlled). ❒ Weather or atmosphere. ❒ Purging requirements. ❒ Thermal stress developed Restraint Safety of men and equipment. ❒ Quality control evaluations (defects vs. failures and cost to meet NDE). ❒ Weld procedure (various welding parameters).

❒ Thermal expansion (yield strength vs. temperature and restraint). ❒ Machinability.

Fabrication Conditions Imposed Production Supervisor’s Cognizance The actual welding environment and welding resources will often determine the success or failure of a welding project. ❒ Skilled, qualified mechanics (experience and good morale). ❒ Schedule. ❒ Available equipment, materials, and facilities. ❒ Welding process characteristics.

73

SECTION

12

Defects and Discontinuities of Welding Contents Introduction ..................................................................................................................................................................... 76 Weld Acceptance Standards ........................................................................................................................................... 76 The Cause of Welding Defects ........................................................................................................................................ 76 Weld Discontinuities ........................................................................................................................................................ 77 Cracking ........................................................................................................................................................................... 77 Delayed Cracking—Hydrogen Embrittlement ............................................................................................................. 83 Techniques for Allowing Hydrogen to Escape from Welds.......................................................................................... 84 Minimizing the Available Amount of Hydrogen to the Weld Deposit......................................................................... 84 Welding Processes and their Available Hydrogen ........................................................................................................ 84 Summary of Rules to Minimize Hydrogen Embrittlement Cracking ........................................................................ 85 Slag Inclusions ................................................................................................................................................................. 85 Lack of Fusion or Incomplete Fusion ............................................................................................................................ 85 Lack of Penetration ......................................................................................................................................................... 86 Porosity ............................................................................................................................................................................. 87 Undercut (Including Root Undercut) ............................................................................................................................ 88 Tungsten Inclusions ......................................................................................................................................................... 88 Arc Strikes........................................................................................................................................................................ 89 Spatter .............................................................................................................................................................................. 90 Crater Pit.......................................................................................................................................................................... 90 Burn-Through .................................................................................................................................................................. 91 Incomplete Insert Melting .............................................................................................................................................. 91 Root Centerline Crease ................................................................................................................................................... 91 Melt-Through................................................................................................................................................................... 91 Oxidation .......................................................................................................................................................................... 91 Root Concavity................................................................................................................................................................. 91 Root Convexity................................................................................................................................................................. 92 Bibliography/Recommended Reading List ................................................................................................................... 92 75

SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING

Section 12—Defects and Discontinuities of Welding

Introduction

Personnel Skill and Experience

Weld discontinuities are unintentional conditions that occur during the welding process and result in undesirable conditions in the weld or the adjacent base metal. If weld discontinuities are expected to be detrimental to the service, they must be evaluated. All unintentional weld conditions that could possibly affect the weld’s service, and that are caused by the welding process, are called discontinuities. Those discontinuities that do not meet the acceptance standards imposed by the owner/engineer or are reasonably expected to have detrimental effects on service are call defects.

The operator’s manual skill and knowledge with specific materials and processes can greatly reduce weld reject rates.

Welding Process Characteristics Each welding process has specific characteristics that make it more susceptible to a certain type of weld discontinuity such as: • • • •

Weld Acceptance Standards

SAW—Solidification centerline cracking. GMAW—(short arc)—Lack of fusion. SMAW—Starting porosity. GTAW—Tungsten inclusions.

Weld acceptance standards are usually written for the worst case. They may not be reasonable in all cases. Engineering evaluation and judgement may be necessary to prevent unnecessary expenditures of resources to repair a defect that would not affect the service of a weldment. The designer/owner that imposes acceptance standards should always keep in mind that in some cases a weld repair just to meet acceptance standards may cause a component to become less useful, or in some cases useless. A pore of porosity that does not meet the acceptance standard and is deep within a casting wall would frequently be better left as found rather than attempting repair. The reason for this is that when the casting is repaired, it will have: a nonuniform metallurgical structure (cast metal has been replaced with weld metal); high residual stress; lower fatigue performance; micro shrinkage in the adjacent cast metal may develop into micro casting tears; serious distortion problems may develop; and frequently, corrosion resistance properties are lowered.

Base Metal Selection and Properties

The Cause of Welding Defects

Welding Environment

There are many specific causes of welding discontinuities, however they can be generally categorized in the following groups:

The welding environment probably has a greater influence on the total number of weld rejects than any other condition. The greatest influence is whether the

The specific properties of a base metal can make it much more prone than other base metals to certain weld defects such as: • Quench and tempered and heat-treatable steels are very susceptible to hydrogen embrittlement and delayed cracking of the weld and the heat-affected zone (HAZ). • Tin bronzes are susceptible to intergranular cracking (hot shortness). • Copper-nickel alloys are prone to incomplete fusion and porosity. • Inconel ® alloys are subject to oxide inclusions, and centerline cracking on concave bead shapes. • Aluminum alloys are susceptible to porosity and incomplete fusion. • Mild steel is very susceptible to lamellar tearing in heavy sections with high through thickness stresses.

76

SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING

Cause

weld is made in the field or the shop. Some field environmental conditions that increase weld rejects are: • • • • •

Cracking occurs when the metal under stress exhausts its ductility and does not have the ability to elastically or plastically flow without fracturing.

Out of position welding. Restricted access welds. Mirror welds. Wind, rain, cold, etc. Wet systems to purge.

Service Impact Cracking is one on the most serious discontinuities, since it usually requires the least amount of energy to progress into a failure. Any cracking that exists transverse to primary stress in fatigue or is subject to impact loading must be considered hazardous, unless the base material is specifically designed for energy absorption under these types of conditions. However, because a crack is present or exists does not mean that the weldment is unsuitable for service. When cracks are left in service they must be evaluated for suitability based upon orientation, location, loading and the base metal properties.

Weld Discontinuities The common discontinuities found on completed welds are: • • • • • • • • • • • • • •

Crater cracks. Face cracks. Heat-affected zone cracks. Lamellar tears. Longitudinal cracks. Root cracks. Root surface cracks. Throat cracks. Toe cracks. Transverse cracks. Underbead cracks. Weld interface cracks. Weld metal cracks. Lamination.

Hot Cracking This cracking occurs at elevated temperatures and generally travels in grain boundaries between the grains of crystalline metal. If fracture surfaces are exposed to air during hot cracking they frequently will exhibit temper colors. Hot cracking is most common in bronze and brass alloys, copper-nickels and base metals that are high in elements such as lead, tin, zinc, phosphorous, and sulfur. Welds that experience galvanize (zinc) contamination or are made on free machining metals are also subject to hot cracking.

For the location of the above-listed discontinuities in weld joints and the associated base metal, see Table 12.1 and Figures 12.1–12.6.

Solidification Cracking This cracking mechanism is commonly witnessed in deep penetrating welds which may have long solidifying surfaces that have a chevron shape as is often seen with submerged arc, flux cored arc and electroslag welding. It is generally caused by insufficient liquid weld metal necessary to accommodate a restrained, contracting volume of solidifying weld metal. When grain growths from each side of a weld are nearly parallel to each other, as in deep penetrating welds, they close off a finite amount of liquid metal to accommodate weld metal shrinkage or contraction (see Figure 12.8). The last metal to freeze will most likely be higher than the rest of the weld in low melting constituents like sulfur, phosphorous, tin, lead, or zinc, which also increases its crack susceptibility.

Cracking Definition A linear rupture of base and/or weld metal. It has a high length-to-width ratio and is characterized by a sharp tip.

Shape Usually a tight linear separation of metal that can be very short (microscopic) to very long indications extending the full length of a weld.

Location Crater Cracking

Cracking takes place in the weld, the weld-base metal fusion line or the weld heat-affected zone (HAZ). Weldments of age hardened or brittle metal may sometimes crack several inches away from the weld. Figure 12.7 exhibits potential crack locations.

The craters that appear at weld bead termination sites are common areas of cracking (see Figure 12.9). This weld metal is the most rapidly cooled metal of the entire weld. It is being cooled in all directions after the welding 77

SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING

Table 12.1—Fusion Weld Discontinuity Types Type of Discontinuity Porosity Uniformly scattered

Discontinuity Identification

Appearing in Figure Number

Location*

Remarks

1a

12.2, 12.6

W

Weld only as discussed herein

Cluster

1b

12.1, 12.2, 12.3, 12.5, 12.6

W

Weld only as discussed herein

Linear

1c

12.2, 12.6

W

Weld only as discussed herein

Piping

1d

12.1, 12.2, 12.6

W

Weld only as discussed herein

Inclusions Slag

2a

12.1, 12.2, 12.3, 12.4, 12.5, 12.6

W

Incomplete fusion

3

12.1, 12.2, 12.3, 12.4, 12.5

W

At joint boundaries or between passes

Inadequate joint penetration

4

12.1, 12.2, 12.3, 12.4, 12.5

W

Root of weld preparation

Undercut

5

12.1, 12.2, 12.3, 12.4, 12.5, 12.6

Underfill

6

12.1, 12.2, 12.3, 12.5

Overlap

7

Laminations Delamination

HAZ

Junction of weld and base metal at surface

W

Outer surface of joint preparation

12.1, 12.2, 12.3, 12.4, 12.5, 12.6

W/HAZ

Junction of weld and base metal at surface

8

12.1, 12.2, 12.3, 12.4, 12.5, 12.6

BM

Base metal, generally near midthickness of section

9

12.1, 12.2, 12.3, 12.4, 12.5, 12.6

BM

Base metal, generally near midthickness of section

Seams and laps

10

12.1, 12.3, 12.4, 12.5, 12.6

BM

Base metal surface, almost always longitudinal

Lamellar tears

11

12.3, 12.5

BM

Base metal, near weld HAZ

Cracks (includes hot cracks and cold cracks) Longitudinal

12a

12.1, 12.2, 12.3, 12.4, 12.5, 12.6

W, HAZ, BM

Weld or base metal adjacent to weld fusion boundary

Transverse

12b

12.1, 12.2, 12.3, 12.4, 12.5, 12.6

W, HAZ, BM

Weld may propagate into HAZ and base metal

Crater

12c

12.1, 12.2, 12.3, 12.4, 12.5, 12.6

W

Weld, at point where arc is terminated

Throat

12d

12.1, 12.2, 12.3, 12.4, 12.5, 12.6

W

Weld axis

Toe

12e

12.1, 12.4, 12.5, 12.6

Root

12f

12.1, 12.2, 12.3, 12.4, 12.5, 12.6

Underbead and heat-affected zone

12g

12.1, 12.2, 12.4, 12.5, 12.6

Fissures

HAZ W

Weld metal, at root

HAZ

Base metal, in HAZ

W

*W—weld, BM—base metal, HAZ—weld heat-affected zone

78

Junction between face of weld and base metal

Weld metal

SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING

Figure 12.1—Double-V-Groove Weld in Butt Joint (See Table 12.1)

Figure 12.2—Single-Bevel-Groove Weld in Butt Joint (See Table 12.1) 79

SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING

Figure 12.3—Welds in Corner Joint (See Table 12.1)

Figure 12.4—Double Fillet Weld in Lap Joint (See Table 12.1) 80

SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING

Figure 12.5—Combined Groove and Fillet Welds in T-Joint (See Table 12.1)

Figure 12.6—Single Pass Fillet Welds in T-Joint (See Table 12.1) 81

SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING

LEGEND

2

2 1

1

1

1

1 C ATE C AC 2 FACE C AC E ATA FFECTED O NE C AC LAE LLA TEA LONGITUDINAL C AC O OT C AC

O OT SUF ACE C AC T O AT C AC TOE C AC 1 T ANS E SE C AC 11 UNDE EAD C AC 12 WELD INTEF ACE C AC 1 WELD E TAL C AC

1 11

12 1

2

1

1

Figure 12.7—Potential Weld Crack Locations

arc is withdrawn and does not have the arc as a source of heat to slow its cooling rate. The weld crater solidifies from its perimeter toward its center, making all of its shrinkage strains away from the crater center. Weld bead termination sites are usually concave in shape (a high stress profile) and are very prone to cracking. Metals most likely to crater crack have wide solidification ranges, low ductility at high temperatures, or are subject to low melting temperature segregates. Aluminum, copper-nickel, Inconel® , Monel® , AISI 4130, and AISI 4140 are quite susceptible to crater cracks, which is a form of hot cracking. For Gas Tungsten Arc Welding (GTAW), welding current decay should be used to help reduce the shrinkage stress while continuing to fill the crater to create a flat or slightly convex bead termination site. By reducing (decaying) the current before extinguishing the arc the actual crater size that must solidify is much reduced in diameter. Thin, concave welds (like weld craters) have high susceptibility to centerline cracking (see Figure 12.10).

Figure 12.8—Solidification Cracking Mechanism

Figure 12.10—Typical Centerline Cracks Due to Thin Concave Bead Shapes

Figure 12.9—Typical Crater Cracks 82

SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING

Cold Cracking Cold cracking generally occurs below 400°F. Martensitic base metals are subject to high stresses and have a microstructure that is susceptible to low-temperature cracking. Cold cracks are predominately transgranular, occurring across metal grains, while hot cracking usually occurs between grains or interdendritically.

Lamellar Tearing Some mild steel base metals are subject to cracking adjacent to welds due to their inability to withstand high through-thickness stresses, because their through-thickness ductility is low. This low ductility is the result of non-metallic inclusions that developed during the initial pouring of the ingot to make the material. These inclusions are in the form of oxides, silicates, sulfides, etc. When the plates are rolled, some of these inclusions are rolled out into thin, lamellar plate inclusions lying parallel to the plate surface at different depths throughout the thickness of the material. When high through-thickness stresses are placed on the plate by welding, cracking occurs by shearing from one inclusion in the plate to another inclusion within the base metal. This cracking pattern has a stair-step appearance in the base metal (see Figure 12.11). Corner welds that involve welding across the end of both members and butt welded joints, minimize through-thickness strains and are most desirable because in some applications they place the metal under through-thickness compression. TEE welds are examples of unavoidable through-thickness stresses. In these cases, the most successful remedy is clad welding the plate in the area of the groove’s reinforcement fillet or fillet weld or peen each weld pass. Recommendations to prevent lamellar tearing are: (1) Select weld joints that minimize through-thickness stresses. (2) Peen all weld layers. (3) Keep weld sizes down. (4) Use low-strength filler metals. (5) Use thin plate (less than one half inch). (6) Provide for slight root openings under “T” welds. (7) Select fine-grain steels with stabilized inclusions than cannot be rolled out into lamellar inclusions.

Figure 12.11—Examples of Lamellar Tearing

Delayed Cracking—Hydrogen Embrittlement

presence and the effects of hydrogen during welding can cause cracking of the base metal (under-bead cracking) or weld metal immediately or several days after the weld was completed. Thus comes the term delayed cracking.

The presence of hydrogen can develop tremendous stress when present in welds and their heat-affected zones in low-alloy steels (heat treatable) such as HY-80, HY-100, AISI-4130, AISI-8630, and AISI-4340. The 83

SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING

200°F minimum preheats and interpass temperatures are not suitable for all base metals.

The presence of hydrogen causes a drastically localized loss of ductility in ferritic welds and their heat-affected zones. Classical cracks are transverse across the weld deposit. Hydrogen may also contribute to other types of cracks such as longitudinal cracks and localized cluster cracks depending on the restraint and environmental conditions. Techniques and procedures that a welder must use to minimize cracks are:

Hold the Weld at Interpass Temperature after Welding The weld and the adjacent base metal should be held near the maximum interpass temperature for at least one hour after welding; and on heavy sections, 24 hours is highly desirable to permit hydrogen to diffuse from the weld without developing cracks.

(1) Minimize the amount of hydrogen available to the molten weld metal. (2) Allow hydrogen to escape from the weld and the HAZ.

Minimizing the Available Amount of Hydrogen to the Weld Deposit

Techniques for Allowing Hydrogen to Escape from Welds

Welds should be made with the lowest hydrogen content materials available. Weld electrodes must be kept clean, dry and properly stored in a heated, vented oven when not in use. Welds should be made only on clean and dry base metals. All sources of hydrogen should be minimized.

Deposit Thin Beads Thin weld passes allow hydrogen to escape much more rapidly than thick weld beads, because the hydrogen has less distance to travel. The time required for hydrogen gas diffusion is a square relationship of the weld pass thickness. Therefore doubling the weld pass thickness quadruples the time for hydrogen to escape from the weld.

(1) Water, rain water, condensed water from torches, and the atmosphere are all sources of hydrogen. (2) Combined water in electrode coatings and fluxes are sources of hydrogen. Only baking will drive off combined water. Many coatings are hydroscopic and will automatically absorb moisture out of the air.

Time Delay between Deposition of Successive Weld Passes

(3) Paint, grease pencils, oils, wood, paper, shop dirt, and tape are all potential sources of hydrogen.

Hydrogen escapes most rapidly immediately after a weld is deposited, probably because of the amount of hydrogen present and the elevated temperature of the weld deposit. It is much more effective to provide hydrogen diffusion time between each weld pass than to deposit several layers of weld metal and then allow time for hydrogen to escape. Hydrogen escapes much slower from heavy weld buildups.

Welding Processes and their Available Hydrogen Of the commonly used welding processes, gas metal arc welding (GMAW) has a lower amount of hydrogen available than do the shielded metal arc, flux cored arc or submerged arc welding. Since SMAW consumes its hydroscopic coating during the welding process, higher amounts of hydrogen are available to the molten weld puddle. Flux cored arc welding, like SMAW, has a flux that is consumed in the welding process that may introduce hydrogen to the molten weld puddle. Of the arc welding processes that utilize flux, submerged arc welding requires the greatest amount of concern because of its high flux usage. SAW consumes approximately a pound and a half of flux for each pound of weld metal deposited. Also, its weld passes are generally thicker than the other processes, which makes it more difficult for hydrogen to escape.

Closely Control the Preheat and Interpass Temperature Maintaining the preheat and interpass temperatures increases the diffusion of hydrogen from the weld. It is essential that the preheat soaks completely through the base metal thickness. Rapid cooling of the weld deposit makes it much more susceptible to cracking. It has been reported that at temperatures above 200°F hydrogen embrittlement cracking is unlikely to occur. This means that a minimum preheat temperature of 200°F during welding will minimize the possibility of hydrogen-induced cracking and also accelerate the removal of hydrogen. However, 84

SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING

Summary of Rules to Minimize Hydrogen Embrittlement Cracking

ing, paper, wood, grease, and paint will all contribute to slag formation if they are allowed to contaminate the weld joint.

(1) Good housekeeping and proper handling of welding materials. (2) Use thin weld beads. (3) Allow time for hydrogen to escape between weld passes. (4) Maintain interpass temperature for up to 24 hours after welding. (5) Use welding processes that will minimize the amount of available hydrogen.

Cause By far the most common reason for slag inclusions is incomplete de-slagging of the previous weld layer, leaving slag at sharp re-entrant angles and at the edges of weld beads. When an SMAW deposit is made, slag may tightly adhere to both the start and stop areas, leading to slag inclusions. Slag entrapments can be caused by a number of reasons: lack of operator skill; amperage too low; improperly prepared weld joints; improper back gouging; erratic progression; weave too wide; electrode too large; and improper bead placement. There is also a common misconception by welders that tightly adhering slag, which is difficult to remove, can be burned out by the next pass by turning up the amperage. This practice is extremely risky, unreliable, and should not be attempted.

Slag Inclusions Definition A slag inclusion is a non-metallic solid inclusion trapped within weld metal, between weld passes, or between the weld metal and the base metal.

Service Impact

Shape

Most acceptance standards allow some slag to be contained within weld deposits but it is limited in size and frequency. It is not considered nearly as severe a defect as a crack, since slag is usually globular in shape. It is more closely allied to porosity in its effects on service, except that slag, when its area becomes significant or its shape is not globular, will probably affect fatigue performance to a greater degree than porosity.

Slag inclusions are usually elongated globular shapes. However, slag can be found in stringers and in thin or very thick layers. Slag inclusions can be microscopic in size or very much larger, extending through the entire thickness of a weld or across a complete layer of weld (as can be found in submerged arc welds).

Location The most common area for slag entrapment is in the first layer and between the root layer and the second layer. Slag is also found at sharp re-entrant angles between weld passes and between weld passes and the base metal. Overhead welding is most prone to slag inclusions, due to the quick freezing slag of smaller weld beads.

Lack of Fusion or Incomplete Fusion Definition This condition exists when there is incomplete fusion between the weld metal and a preceding weld deposit or the weld metal and base metal (fusion that is less than complete). Frequently, incomplete fusion appears as metal-to-metal contact with a thin line between beads with no metallurgical bonding.

Sources of Slag The major source of slag is the flux and electrode coating used in SMAW, FCAW, and SAW processes. However, GMAW and GTAW processes may contain slag inclusions due to metal and silicon oxides. As the amount of oxygen increases in GMAW shielding gases, the amount of slag produced increases. Slag can also occur with GTAW process, when the molten end of the filler metal is removed from the shielding gas and oxides are formed. The oxides are introduced into the weld puddle when the filler metal is added. Another source of slag is the base metals themselves, since nonmetallic oxides and nitrides are available to form slag from these materials. Shop dirt in the form of oxides of burning and goug-

Shape On weld cross sections, lack of fusion is usually a curved line. On a radiographic image, lack of fusion will frequently appear to have a straight portion with a curved end (a tail).

Location Lack of fusion occurs between weld passes, or between weld passes and the base metal (see Figure 12.12). 85

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INCO LETE FUSION

INCO INCO

INCO

LETE FUSION

LETE FUSION

LETE FUSION

INCO

LETE FUSION

C

Figure 12.12—Lack of Fusion Locations Cause

Table 12.2—Melting Temperature Comparison Chart—Base Metal vs. Refractory Oxide

The most common cause of incomplete fusion is inadequate welding heat (amperage/current). The heat input from welding must be great enough to melt the base metal and to allow sufficient time for thin surface oxides to be removed. Other causes of incomplete fusion are improper travel speeds (too fast or too slow), insufficient electrode size, improper joint design selection, improper electrode or torch manipulation, improper surface cleaning, and improper removal of refractory oxides. Refractory oxides melt at much higher temperatures than their base metals and are a major cause of incomplete fusion, particularly in high-nickel or aluminum alloys. The only oxide that melts at a lower temperature than its metal constituent is iron oxide. Also some filler metals are alloyed with elements that form refractory oxides, such as: titanium, aluminum, nickel, magnesium and chromium. Base metals that readily form refractory oxides are: aluminum, Inconel®, copper-nickel, Monel®, and K-Monel® (see Table 12.2).

Melting Temperature (°F)

Oxide

Melting Temperature (°F)

Aluminum

1100

Al2O3

3659

Monel®

2400

NiO

3614

Inconel®

2525

Cr2O3

4109

Titanium

3137

TiO2

3344

Metal

any lack of fusion. Lack of fusion is considered a serious defect when it is open to the surface of a joint. In general, lack of fusion is considered a higher stress concentration factor than slag, due to its linear shape and sharper edges.

Removal of Oxides Wire brushing has little effect on the removal of refractory oxides for most metals except aluminum. In most instances grinding, pickling, or machining is used to remove oxides. When welding K-Monel® light grinding is frequently required between each layer or every other layer of weld metal.

Lack of Penetration

Service Impact

Shape and Location

Some acceptance standards permit lack of fusion in welds. However, other acceptance standards do not permit

Lack of penetration is the unfused or unpenetrated portion of a weld joint (see Figure 12.13).

Definition Unintentional, incomplete penetration of a weld through the thickness of a weld joint.

86

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INCO

INCO

LETE O INT E NET ATION

LETE O INT E NET ATION

Figure 12.13—Incomplete Penetration

Cause Lack of penetration is most commonly caused by improper weld joint selection for the weld process used, or the use of improper welding parameters such as: welding amperage too low, improper travel speeds, improper polarity, improper filler metal selection, improper filler metal manipulation, or improper backgouging. Another error is sometimes just forgetting to weld the opposite side of a weld joint. Weld joint designs must be properly selected for the base metal, filler metal, and welding process used. The welding arc penetration and the weld puddle fluidity depend on these conditions. Unlike mild steel, low-alloy steels, and stainless steels, high-nickel and copper alloys do not spread or wet easily.

Figure 12.14—Locations of Porosity

Service Impact Few codes permit lack of penetration when a full penetration weld is required. Lack of penetration is a very serious discontinuity when fatigue is involved (particularly when the axis of the discontinuity is transverse to primary stress). Lack of penetration has resulted in a large percentage of service failures involving ship hulls, piping systems, pressure vessels, and machinery components. Lack of fusion that is totally enclosed within a weld is not nearly as hazardous as lack of penetration open to a joint surface (i.e., a one-sided joint weld). However, when properly selected for service, partial penetration joints welded from one side are desirable joint designs.

Location Porosity is always located within the weld deposit. Lack of visual porosity on a weld surface is not an indication of a porosity-free weld.

Cause Porosity is caused when there is not enough time for the gas bubble to escape the molten weld metal before the puddle solidifies. Porosity can be formed by any volatile material being trapped within molten metal; gas evolving out of the weld from elements that were in solution at a higher temperature; the reduction of metal oxides or nitrides; chemical reactions that create sulfur dioxide or carbon monoxide gas; and entrapment of shielding gas. In some metals like aluminum, gaseous elements (most likely hydrogen) will come out of solution near the solidifying dendrites growing into the liquid metal. The liquid metal cannot keep the element in solution as the puddle cools and solidifies, so the element collects into a gas pocket. Hydrogen is 20 times more soluble in molten aluminum than in solid aluminum.

Porosity Definition Porosity is a void or gas pocket contained within a weld. The gas is entrapped within the weld during the solidification process (see Figure 12.14).

Shape The most common shape is spherical; however, porosity can be elongated and extend from one weld pass into another. 87

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Sources of porosity include: • • • • • • • • • • •

Moisture in electrode coatings or fluxes. Shop dirt contamination of the weld joint. Paint. Oil. Rust (oxides and nitrides). Free machining constituents of metals such as sulfur or selenium. Excessive welding arc lengths that aspirate atmospheric gas. Inadequate purging of equipment that uses external shielding gases. Excessive amperage that will disrupt the shielding gas. Poorly maintained equipment. Travel speeds that are too high or too low, etc.

UNDERCUT UNDERCUT UNDERCUT OVERLAP

OVERLAP

Figure 12.15—Undercut

Porosity can come from so many sources that it is sometimes difficult to identify the contributing source and resolve the problem. There is a common misconception among welders that porosity can be burned out by the following pass, however, this technique rarely works.

that undercut would contribute to a weld-associated failure other than for critical fatigue applications. The thinner the base metal, the more likely it is that undercut may affect the service of the weldment.

Service Impact

Tungsten Inclusions

Nearly every acceptance standard permits some porosity to be present within a weld. Although welds have been removed from service with extensive amounts of porosity, it is very rare to find porosity leading to service failures, particularly if the amount of porosity is within acceptance standard limits.

Definition Particles of tungsten in a weld or in the adjacent base metal surface deposited from the electrode used in the gas tungsten arc welding process (see Figure 12.16).

Shape

Undercut (Including Root Undercut)

The tungsten inclusions may appear as globular shapes or as splintered or flaked pieces. The inclusions may occur in clusters or widely distributed particles. The size of the inclusions may range from several thousandths of an inch to pieces larger than the tungsten electrode diameter.

Definition, Shape and Location Undercut is an intermittent or continuous narrow groove, immediately adjacent to a weld toe, that has been melted into the base metal and not subsequently filled by weld metal. Undercut is always a groove that is left below the adjacent base metal surface. Undercut may be on either or both faces of a weld, even if the weld was made from only one side (see Figure 12.15).

Location Inclusions can be located completely within the weld deposit, on the weld surface, or on the adjacent base metal surface. They most frequently occur at arc starts.

Service Impact Cause

Undercut has a direct effect on weld performance in fatigue loading of critical applications. Acceptance standards typically specify that undercut shall not exceed 1/64 in.; however, some standards allow undercut to 1/32 in. deep or 10% of the thickness of the base metal, whichever is less. Other codes may state only that the welds are to be reasonably free from undercut. It is rare

Tungsten inclusions may be caused by excessive amperage for the size of tungsten, dipping the tungsten in the weld puddle, touching the tungsten with the filler metal, improperly ground tungsten, linear ruptures in the tungsten as received from the manufacturer, tungsten stickout too long, or inadequate shielding gas flow. 88

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Figure 12.16—Tungsten Inclusions

Alternating current (ac) requires larger tungsten diameter than does direct current (dc) straight polarity because of higher electrode heat input. A tungsten diameter increase of 0.001 in. is recommended for every 1.25 amps of alternating current. Usually one size larger tungsten is recommended for a-c welding than is used for the same amperage range with dc.

Shape

Note: Tungsten inclusions cannot be burned out of a weld by increasing welding current and rewelding over the area. Tungsten melts at 6120°F, which is more than 3000°F higher than the melting temperatures of most common metals. Tungsten inclusions must be removed by grinding or machining.

Location

It appears as a localized disturbance of the metal surface and exists as a single spot or multiple spots, usually with some alignment (see Figure 12.17). Convexity and undercutting may exist with the arc strikes. Small fissures (cracks) and porosity may also exist.

Arc strikes may be found on the finished weld or base metal surface. Arc strikes made in weld joints or on interior weld passes that will be welded over are usually not

Service Impact Tungsten is considered to have a similar effect on service as porosity. Service failures associated with tungsten are rare, even where the amounts on tungsten exceeded the acceptance standard. Tungsten inclusions are usually accepted to the same standard as porosity.

Arc Strikes Definition An arc strike is caused by unintentional melting or heating of a finished weld or base metal surface by an electric current. It results in a localized weld and heataffected zone (HAZ).

Figure 12.17—Arc Strikes 89

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of concern. However, if the arc strikes are extensive, light grinding of the surface may be advisable.

Cause Arc strikes may be caused by improper manipulation of the electrode, loose ground clamps, and low amperage that causes sticking and dragging of the welding electrode. Even magnetic particle inspection prods can cause arc strikes, if improperly used.

Service Impact Arc strikes are critical on heat-treatable steels. An arc strike area creates a hardened weld and HAZ due to the extremely rapid quenching caused by the surrounding metal. Arc strikes should not be permitted on high-speed rotating shafts particularly if they are made of quenchhardenable steel. Some standards may require that all arc strikes and associated HAZ be removed by grinding. Arc strike protection of the base metal adjacent to the weld joint is a good practice and should be maintained for critical applications.

Figure 12.18—Spatter

Service Impact Excessive spatter could mask a defect during weld inspections and in those areas it must be removed. Some standards exclude the presence of weld spatter. Spatter should be considered similar to arc strikes for highly hardenable heat-treatable metals that are subject to severe fatigue loads, such as high-speed rotating shafts.

Spatter Definition Globular metal particles that are ejected from the welding arc area. Weld spatter does not form an intentional part of the weld.

Crater Pit Shape Definition, Shape, and Location

Spatter is usually spherical or globular in shape.

A crater pit is usually a circular-shaped depression. It is a cavity that extends down into a weld at its termination site (weld stop).

Location Weld spatter may or may not attach or weld itself to the existing weld or the adjacent base metal (see Figure 12.18).

Cause Crater pits are caused by a volumetric contraction of molten metal during solidification and lack of filler metal to fill the void. It is usually the result of abruptly removing the welding arc or not using current decay with GTAW. It may be minimized by gradually reducing the weld current (current decay) at the weld stop, increasing the travel speed, gradually withdrawing the welding arc, or adding additional filler metal at the termination site of GTAW welds.

Cause The most frequent causes of weld spatter include: excessive welding current; arc length too long; or short circuiting the welding arc causing a high surge of welding current that will expel weld spatter. Some spatter is normal with most welding processes except submerged arc welding and gas tungsten arc welding. 90

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Service Impact

Root Centerline Crease

Unless crater pits have cracks or other associated weld defects, they are not expected to cause in-service failures. Crater pits are not addressed by most acceptance standards.

An intermittent or continuous shallow linear groove (centerline crease) concavity formed by distortion or upsetting of the root surface by subsequent weld passes.

Melt-Through

Burn-Through A void or hole fused and extending into the surface on the back side of a consumable insert weld, a backing strap weld, or open root weld. Droplets of metal may stick out from this burn-through area.

A root surface or base metal irregularity on a consumable insert or closed root full penetration joint resulting in fusion completely through a localized region without development of a void or opening. The melt-through may or may not be associated with the weld root.

Incomplete Insert Melting

Oxidation

Incomplete melting of the consumable insert either with or without the fusion between the insert and the base metal along one or both sides of the consumable insert (see Figure 12.19).

Root surface oxidation results from partial or complete lack of purge of atmospheric gases and moisture from heated or molten weld metal on the root surface of a weld joint (see Figure 12.20). This condition may be in the form of a slight oxidation, a heavy black scale, or an extremely rough crinkled appearance. Similar conditions result on the weld face when drafts of air displace the arc shielding gas and permit oxidation.

Root Concavity A root surface depression or concavity that may be due to gravity (weld segment in the overhead position), surface tension, or too high of an internal purge pressure on the molten metal. Also, distortion caused by subsequent weld passes can cause concavity of the root pass. It is controlled by using the proper joint design, the correct

ORIGINAL INSERT NOT MELTED

INSERT NOT COMPLETELY MELTED

WELD JOINT INSERT COMPLETELY MELTED

Figure 12.20—Oxidation

Figure 12.19—Incomplete Melting 91

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size consumable insert, and proper welding parameters (i.e., amperage, travel speed, and purge pressure).

Root Convexity Root weld reinforcement (convexity) that is beyond the base metal surface which may be due to gravity (weld segment in the flat position), surface tension, or low internal purge pressure effects on molten metal (see Figure 12.21). Also, distortion caused by subsequent weld passes can cause convexity of the root pass. It is controlled by using the proper joint design, the correct size consumable insert, and proper welding parameters (i.e., amperage, travel speed, and purge pressure).

Figure 12.21—Convexity

References/Recommended Reading List American Welding Society. Guide for Nondestructive Inspection of Welds (B1.10). Miami, Fla.: American Welding Society.

———. The Everyday Pocket Handbook for Visual Inspection and Weld Discontinuities (PHB-2). Miami, Fla.: American Welding Society.

———. Guide for Visual Inspection of Welds (B1.11). Miami, Fla.: American Welding Society.

———. Welding Handbook, 8th ed., vol. 1, Welding Technology (WHB-1.8). Miami, Fla.: American Welding Society.

———. Practical Reference Guide for Radiographic Inspection Acceptance Criteria (PRG). Miami, Fla.: American Welding Society.

———. Welding Inspection (WI-80). Miami, Fla.: American Welding Society. ———. Welding Inspection Technology (WIT). Miami, Fla.: American Welding Society.

———. Standard Methods for Mechanical Testing of Welds (B4.0). Miami, Fla.: American Welding Society.

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Nondestructive Examination Contents Introduction ..................................................................................................................................................................... 94 When Should NDE Be Performed?................................................................................................................................ 94 Nondestructive Examination Methods .......................................................................................................................... 94 Bibliography/Recommended Reading List ................................................................................................................. 102

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Section 13—Nondestructive Examination

Introduction

repair, base metal defect repair, and other repair welding should be inspected prior to welding. A nondestructive test of the repair area and surrounding material can be used to determine the extent of defects and ensure part soundness before repair welding begins.

The field of Nondestructive Examination (NDE) provides methods to measure or detect soundness criteria placed on material by design requirements. NDE gathers this information through testing, in such a way as to not adversely affect the serviceability of the material; hence, the term nondestructive testing. Quality cannot be inspected into a material; however, NDE can ensure that the material has acquired quality attributes. The proper selection and application of NDE to key phases of fabrication or assembly can save time, material, and money, as well as provide assurance that design requirements are met.

Finished Product Inspection With the proper selection and application of various NDE methods, NDE can provide overall quality assurance of the finished product.

In-Service Inspection Maintenance inspection can be scheduled to meet the service needs of a product. Monitoring of known and suspected wear and stress areas with NDE can determine the frequency of service required.

When Should NDE be Performed? When to perform an inspection can be as important as selecting an NDE method. After a designer has completed the selection of materials and determined an assembly design, it becomes the fabricator’s responsibility to meet those requirements. To do this, inspection mile markers are essential.

Nondestructive Test Methods There is a wide variety of NDE methods available today. More than one may be used on the same weldment. The various test methods do not necessarily compete with each other. For detailed military requirements see MIL-STD-271 and MIL-STD-278. The following sections present a brief overview of major NDE methods.

Receipt Inspection Raw materials that meet specification requirements are essential for high-quality fabrication. Various NDE methods often provide a quick and relatively inexpensive way to verify the material’s properties or identification. When material problems or discrepancies are caught at this stage, rather than during fabrication or in service, savings of time and money can be immense.

Visual Inspection Testing (VT) VT provides important information about weld conformance and practical quality control. VT uses trained observation and attention to requirements as the primary inspection criteria. Although visual inspection is exclusively a surface application, experience and training can reveal discontinuities such as surface porosity and some types of surface cracks, weld bead contour and roughness, bead placement, residual slag and oxide films, undercut, joint misalignment, and other indicators of potential inferior quality. Adequate lighting, cleaning of inspection area, and specific inspection standards are required to effectively ensure quality. See Table 13.1.

In-Process Inspection During fabrication, visual inspection of the joint designs, weld preparations, and the deposited weld metal provides evidence of a quality product. In-process inspection ensures that quality is built into an assembly and can reduce final inspection requirements. Welding applications that involve weld buildup for corrosion or wear 94

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developer is applied, the dye that bleeds out of the discontinuities provides a visible indication of the flaw. PT is typically used to inspect forgings, castings, extrusions, and weldments of most solid, nonporous materials. The surface being inspected must be sufficiently smooth to allow removal of excess dye without over cleaning the test surface. If open to the inspection surface, all weld and base metal defects can be detected including cracks, lack of bond, cold shuts, edge lamination, shrinkage areas, laps, and open porosity. The portability of PT materials make them an ideal tool for field inspection. See Table 13.2.

Table 13.1 Visual Inspection Testing Advantages Best method for detecting weld bead contour and roughness, weld bead placement, residual slag, undercut, and joint misalignment. Minimal equipment required. Very fast and economical to use.

Disadvantages Limited to surface and alignment attributes. No easily recordable permanent record. Cannot reliably detect some types of surface cracks.

Results are obtained immediately.

Precleaning

Does not interfere with other work in the area.

The test surface must be cleaned (usually with isopropyl alcohol) to remove contaminants to allow the penetrant to penetrate surface discontinuities (see Figure 13.1).

Results are obtained immediately.

VT inspection acceptance criteria must first be defined. VT provides a simple method of examination with diverse applications and can be used in all phases of fabrication. Welders and fitters should continuously compare their product to the fabrication requirements. Inprocess observations should include joint alignment, the presence of cracks and surface porosity, the condition of the weld crater, weld bead contour and placement, and weld size. In addition to a clean inspection area, lighting and surface finish are important factors for visual inspection. Background lighting should be supplemented with a portable light directed at the inspection area. The light should be moved as a pointer to focus attention as the inspection progresses. Areas that are questionable and need additional attention should be marked and inspected again. Poor surface finish may mask surface discontinuities. It is recommended that paint be removed and the inspection preparation be extended a minimum of 1 in. beyond the weld, or as required by the fabrication standard. The following tests are often helpful aids to VT:

Table 13.2 Penetrant Testing Advantages

Disadvantages

Well suited to nonmagnetic Detection is limited to discontinuities open to the test materials that cannot be surface. inspected with MT. Inspection materials/ equipment are relatively low cost, easy to use, and highly portable.

Temperatures are restricted (typically 50 to 100°F) but hightemperature penetrants, up to 500°F, are available.

Test results are easy to interpret.

Post cleaning of penetrant and developer is required.

Results are obtained immediately.

Some penetrants are flammable and some require special disposal.

Complex equipment not required.

Some inspection materials can be corrosive to sensitive base metals.

Personnel are easy to train. Inspection process is time consuming (typically one hour per inspection). No practical limit to specimen size. Coatings such as paint and scale Capable of detecting very must be removed prior to inspection. small (0.003) linear and rounded indications. No easily recordable permanent record of actual surface Does not interfere with indications. other work in the area.

(1) Visual. Color and written codes help identify material composition and characteristics. (2) Magnification. It is often beneficial to use magnification as an aid to VT. However, it should be used for acceptance only where specified in the inspection requirements.

Penetrant Testing (PT)

Rough and porous surfaces can hide actual indications and create false indications.

A liquid penetrant (dye) is applied to the surface being inspected, which penetrates any discontinuities open to the surface. When the excess penetrant is removed and a 95

SECTION 13—NONDESTRUCTIVE EXAMINATION

Figure 13.1—Cleaned Test Surface

Figure 13.2—Penetrant on Test Surface and in Crack

Applying Penetrant After the cleaners have evaporated, a liquid penetrant is applied and allowed time to penetrate discontinuities by capillary action (see Figure 13.2). Both color contrast dye, which can be seen with visible light, and fluorescent dye, which can only be seen with ultraviolet light, are commonly used, depending on the specific application.

Removing Excess Penetrant The excess penetrant not remaining in the discontinuities is removed (see Figure 13.3). There are three basic categories of penetrants based on the method of their removal: post emulsifier, which becomes water soluble after the emulsifier has been applied; solvent washable, which requires solvents to remove the penetrant; and washable, which is removed with water.

Figure 13.3—Excess Penetrant Removed

Applying Developer The developer is evenly applied on the test surface. Adequate time is allowed for it to act as a “blotter” for the discontinuity (see Figure 13.4). The test surface is then examined with visible light, when color contrast dyes are used, or with ultraviolet light, when fluorescent dyes are used.

Postcleaning After the test surface has been evaluated, it is cleaned (usually with isopropyl alcohol) to remove the developer and any remaining penetrant.

Figure 13.4—Visible Indication after Application of Developer 96

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Magnetic Particle Testing (MT) A magnetic field (flux) is applied to the inspection area (see Figure 13.5). The field is maintained while an iron oxide powder is evenly applied to the surface. If a discontinuity causes a sufficiently strong disruption (flux leakage) of the magnetic field, it will attract the iron particles. The stronger the flux leakage, the stronger the forces of magnetic attraction (see Figure 13.6). The greatest flux leakage occurs when the major dimension of the discontinuity is perpendicular to the direction of the magnetic lines of force. Conversely, where there is no disruption of the field, the particles are easily removed by blowing them off with regulated air (when using dry MT method) or by letting them flow off with a liquid medium (when using the wet MT method) (see Figure 13.7). When the field is oriented at 45 degrees or greater from perpendicular, the flux leakage is undependable for discontinuity detection. For that reason, the field must be oriented in at least two directions (angled at from 30 to 45 degrees from each other) to ensure adequate flux leakage. The MT method is limited to the inspection of ferromagnetic materials and can detect surface and (under certain conditions) near-surface discontinuities. MT is typically used to inspect ferromagnetic castings, weldments, extrusions, forgings, and components subject to cracking. See Table 13.3.

Figure 13.5—U-Shaped Magnet in Contact with a Ferromagnetic Material Containing a Discontinuity

Precleaning Usually grinding or “needle gunning” to remove paint, slag, and scale is sufficient precleaning. The iron particles must be allowed to migrate freely across the inspection surface.

Figure 13.6—Throat Crack in a Fillet Weld Root

Applying the Magnetic Particles The iron oxide particles are applied to the surface of the test area while the electrical current is still producing the magnetic field. There are two widely used methods of doing this: (1) Dry Method—in which a thin, even coat of dry iron oxide particles are sprinkled or dusted onto the surface. There are a choice of colors available to provide a color contrast on the test surface. (2) Wet Method—in which iron oxide particles are suspended in water, kerosene or a petroleum distillate. The liquid is allowed to flow onto the test surface. Florescent coatings are usually added to the particles to enhance their visibility.

Establishing the Magnetic Field Figure 13.7—Direct Method (Longitudinal Magnetism)

There are four primary methods used to establish the magnetic field within the part being inspected. The 97

SECTION 13—NONDESTRUCTIVE EXAMINATION

Table 13.3 Magnetic Particle Testing Advantages Inspection materials and equipment are relatively low cost, easy to use, and highly portable. Test results are easy to interpret. Results are obtained immediately. Personnel are easy and economical to train. Inspection time is relatively short. No practical limit to specimen size. Other work in the area is not disturbed.

Disadvantages Limited to ferromagnetic materials. An electrical power source is required. No easily recordable permanent record. Multiple inspections (angled 30 to 45 degrees from each other) may be required in order to detect all linear discontinuities.

Figure 13.8—Central Conductor (Circular Magnetism)

Degaussing may be required (especially for machinery applications).

are most easily detected. Defects oriented perpendicular to the length of the central conductor cannot be detected with this method since the direction of the magnetic field cannot be maneuvered beyond that as shown.

Does not reliably detect rounded discontinuities. Inaccurate interpretation of nonrelevant indications may result from changes in: • Sectional geometry. • Heat treatment. • Magnetic permeability. • Magnetic writing.

(3) Wrapped Coil Method. A magnetic field can be established by placing a wire coil around a longitudinal part. As the current flows through the coil, a longitudinal field is set up around the conductor and within the part. Linear discontinuities lying perpendicular to the axis of the part are most easily detected. Defects oriented in line with the length of the part cannot be detected with this method, since the direction of the magnetic field cannot be maneuvered beyond that as shown.

geometry of the part and defect orientation usually determine which method or methods are used to inspect a given part.

(4) Yoke Method. A magnetic field can be established as shown in Figure 13.9 by placing a yoke in contact with the flat surface of the part. When coupled with the part, the magnetic field in the yoke induces a corresponding field in the part. No current from the yoke flows into the part. The magnetic flux lines within the part flow between the two ends of the yoke. Linear discontinuities lying perpendicular to the line connecting the yoke ends are most easily detected.

(1) Direct Method. A magnetic field can be established as shown in Figure 13.8 by passing an a-c or d-c current directly into the test area with electrodes placed in contact with the part. The magnetic flux lines within the part run perpendicular to the current path that flows between the two electrodes. Linear discontinuities lying parallel with a line connecting the two electrodes are most easily detected. This method is best suited to flat surfaces. Care must be taken when testing certain materials with high hardenability to avoid moving the electrodes while the current is flowing. This can result in arc strikes which result in very small, and potentially very hard, heat-affected zones.

Removing the Excess Magnetic Particles While the electrical current is still producing the magnetic field, the excess particles are blown off with lowpressure air (when the dry method is used) or they are allowed to run off (when the wet method is used). If done properly, only the particles attracted by the flux leakage will remain. After the test surface has been evaluated, it is easily cleaned, usually by brushing or wiping, to remove any remaining particles.

(2) Central Conductor Method. A magnetic field can be established by placing a central conductor within a hollow cylindrical part. As the current flows through the conductor, a cylindrical magnetic field is set up around the conductor and within the part. Linear discontinuities lying perpendicular with the central conductor 98

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Figure 13.10—Sound Deflection from a Discontinuity

tered by internal conditions and the geometric shape of the test piece. A rapid succession of pulses (creating a beam) are received and displayed on a cathode ray tube (CRT) (for thickness measurements, a digital display may be used) as spikes (blips). The blips (or lack of blips) are evaluated as to their horizontal distance on the CRT, their height, and their shape. These CRT presentations in turn are compared to known physical standards. Like miniature sonar, UT can be used to detect, locate, and in some cases describe the shape of discontinuities. See Table 13.4.

Figure 13.9—Yoke Method

Discontinuity Detection MT may be used to detect many surface and nearsurface discontinuities in ferromagnetic materials. These include all types of cracks, voids, inclusions and laminations. As these discontinuities become closer to the surface and sharper (less rounded) they become easier to detect.

Applications In addition to being able to detect the discontinuities mentioned below, UT can be used to accurately measure thickness, loss of thickness, determine soundness, investigate attachment location, determine bond quality, and determine liquid levels. It can do so by passing through plastic, metal, glass, ceramic, and other materials from one side only.

Ultrasonic Testing (UT) Pulses of ultrahigh-frequency sound are produced when a pulsed electrical current is used to “shock” a piezoelectric transducer (search unit) (see Figure 13.10). The pulses are dissipated rapidly in air; therefore, they must be coupled onto the test piece through a liquid, couplant medium. The transducer is then electrically switched to receive (or another search unit can be designated to receive) the reflected pulses which have been al-

Ultrasonic Wave Types Both longitudinal and shear waves are commonly used for ultrasonic inspection. Longitudinal waves are propagated into the test material at an angle normal to its 99

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Eddy Current Testing (ET)

Table 13.4 Ultrasonic Testing Advantages Modern equipment is highly sensitive, programmable and portable. Results are obtained immediately. Internal discontinuities in the proper orientation are easily detected.

Disadvantages High initial cost for equipment and training. Equipment is complex and fragile. Considerable operator skill and experience is required to interpret the results.

Accurate location of the discontinuity.

The test surface must physically conform to the search unit contact surface, grinding and paint removal may be necessary.

Access is required from one side only.

Poor surface and near-surface resolution.

Excellent sensitivity.

Linear discontinuities lying parallel to the plate surface are readily detected.

Process can be automated for rapid inspection of plate and bar stock. Linear discontinuities lying perpendicular to the plate surface are readily detected.

ET is a NDE method whereby a coil, energized with high-frequency alternating current, produces an alternating magnetic field which is introduced into the test material (see Figure 13.11). The field induces alternating currents (eddy currents) in the metal. The eddy currents, in turn, produce a weaker, alternating magnetic field which opposes the primary field. A second coil, used as a reference, is electrically “bridged” (i.e., inserted into a commercial Wheatstone induction bridge configuration) with the primary coil. As the primary coil is moved across the test material, the bridge electrically “compares” both coils. If the eddy current field moves within range of a discontinuity or change in material properties, the magnitude and direction of the eddy currents are altered. This change of eddy current density changes the inductive reactance of the primary coil. These variations are registered on an output device and, depending on the sensitivity of the electrical instrumentation, can be used to locate discontinuities or variations in the test material. Due to the “skin effect” of both the eddy currents and the primary current, the coil is limited to detecting surface and near-surface parameters. The depth of effective penetration is determined by the primary coil power output and frequency. See Table 13.5.

No easily recordable permanent record. Large grain metal castings (i.e., 300 series stainless) can be difficult to inspect.

surface. They are mainly used for thickness and soundness examination. Shear waves are introduced into the test material at an angle offset from normal to the surface. Shear wave frequency is much greater than that of longitudinal frequencies. For that reason, their resolving power is also much greater. Discontinuities whose major dimension is oriented parallel to the ultrasound path are difficult to detect with both wave types. Very thin materials are also difficult to examine with longitudinal and shear waves. Ultrasonic shear waves are able to detect: • • • • •

Internal cracks and surface cracks on far side only. Cold shuts. Porosity. Nonmetallic inclusions. Voids.

Ultrasonic longitudinal waves are able to detect: • • • •

Laminations. Porosity. Nonmetallic inclusions. Voids.

Figure 13.11—Induced Eddy Currents in Test Object 100

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practical depth of detection, which is typically no more than 1/4 in.

Table 13.5 Eddy Current Testing Advantages A permanent record can be produced with special computer techniques. Equipment is low to moderate cost and portable. Some techniques are easily automated.

Radiographic Testing (RT)

Disadvantages

RT uses the penetrating power of X-rays or gamma rays to produce an image of the interior conditions of the test material on film (see Figure 13.12). The material is positioned between the source of radiation and a piece of unexposed film. After going through the test material, the rays expose the film. The film is developed, revealing any change in thickness of the material by the variation of shades of gray (film density) of the film. The pattern and degree of variation in film density is used to determine the type, size, and position of the discontinuity, and ultimately determine its acceptability. See Table 13.6. RT is capable of detecting all of the discontinuities detectable with VT, MT, PT, and also most subsurface discontinuities, some of which may not be detectable with UT. RT is most sensitive to discontinuities whose major dimension is aligned with the source beam, whereas UT is most sensitive to discontinuities whose major dimension is perpendicular to the sound beam.

The depth of the inspection area is limited to about 1/4 inch (depending on the frequency of the primary coil). Some types of equipment are expensive and complex. Some techniques require expensive training.

Some techniques/ procedures allow the paint to remain on the test surface.

The final evaluation is dependent on inspector interpretation.

Some techniques allow the inspection surface to be wet, oily, or greasy if the primary coil is waterproof.

The surface must be sufficiently smooth and clean to keep the test coil at a consistent distance from the surface.

Limitations

Applications

The application of RT for weld inspection is largely dependent on the location of the joint in the weldment, the joint configuration, and section thickness. These factors may combine to limit the best use of RT and favor another method. One of the primary limitations of RT is its difficulty in detecting cracks or other closed discontinuities which are not aligned with the source beam. Multiple shots at various angles can be used to detect most

ET can be used to identify and classify materials by comparing them to a standard. Thickness measurements of thin sheet materials and nonconductive coatings are also common. Surface and subsurface discontinuities can also be detected which may be related to welding or other fabrication processes.

Advantages The primary advantage of ET over other methods is that it can be completely automated and produce accurate results at high speeds without ever contacting the part being tested. ET is, therefore, especially well suited to continuous production lines. Another advantage is that the signal generated by ET is often proportional to the size of the discontinuity detected.

Limitations Surface cleanliness is important for accurate results, since any magnetic particles on the surface may cause nonrelevant indications. The major limitation of ET is that the primary coil design must be compatible with the part geometry and defect or attribute type being investigated. Care must also be used when calibrating the ET equipment with a reference standard as this will determine the accuracy of the results. Another limitation is the

Figure 13.12—Orientation of Radiation Source, Test Plate, and Radiographic Film 101

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cially true for field work. For these reasons, RT should only be required where absolutely necessary. When properly trained personnel use well-maintained equipment according to approved procedures, RT is a safe and viable NDE method that plays a valuable role in assuring that the required quality is obtained.

Table 13.6 Radiographic Testing Advantages

Disadvantages

Permanent records easily obtained.

High initial cost for equipment and training.

Internal discontinuities in the proper orientation are easily detected.

An electrical power source is required (except gamma).

References/Recommended Reading List

High hazard potential. Excellent sensitivity. Wide variety of materials may be tested. Linear discontinuities lying perpendicular to the plate surface are easily detected.

American Welding Society. Guide for Nondestructive Inspection of Welds (B1.10). Miami Fla.: American Welding Society.

Laminations and other linear discontinuities lying parallel to the plate surface are difficult to detect.

———. Guide for Visual Inspection of Welds (B1.11). Miami, Fla.: American Welding Society.

Considerable time required for setup, exposure, and interpretation of results.

———. Practical Reference Guide for Radiographic Inspection Acceptance Criteria (PRG). Miami, Fla.: American Welding Society. ———. Standard Methods for Mechanical Testing of Welds (B4.0). Miami, Fla.: American Welding Society.

discontinuities; however, due to limited accessibility, multiple shots are often not possible. When cracks are suspected to occur in a critical applications, another NDE method such as UT may be a better choice.

———. The Everyday Pocket Handbook for Visual Inspection and Weld Discontinuities—Causes and Remedies (PHB-2). Miami, Fla.: American Welding Society. ———. Welding Handbook, 8th ed., vol. 1, Welding Technology (WHB-1.8). Miami, Fla.: American Welding Society.

Radiation Hazard One unique disadvantage with RT is that radiation exposure to humans can result in permanent injury and death, without the individual ever knowing they were exposed. The necessary precautions that must be taken to protect the health of workers in the area accounts for much of the expense associated with RT. This is espe-

———. Welding Inspection (WI-80). Miami, Fla.: American Welding Society. ———. Welding Inspection Technology (WIT). Miami, Fla.: American Welding Society.

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Information for the Welder Contents Introduction ................................................................................................................................................................... 104 Material .......................................................................................................................................................................... 104 Filler Material ................................................................................................................................................................ 104 Welding Process ............................................................................................................................................................. 104 Quality Control and Quality Assurance ...................................................................................................................... 105 Weld Size and Penetration ............................................................................................................................................ 105 Distortion Control.......................................................................................................................................................... 105 Standards and Specifications........................................................................................................................................ 105 Welding Department Required Information Facilitated by the Design Engineer/Technical Support .................. 105 Bibliography/Recommended Reading List ................................................................................................................. 107

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Section 14—Information for the Welder

Introduction

welding on metals strengthened by cold rolling or age hardening will cause substantial loss in strength in the HAZ. The chemistry determines the electrode type and heat treatment required to achieve the desired mechanical properties. Although specific information is usually provided by a welding engineer, many machine-shop weld repairs require material background by shop supervision beyond that of the average welder. This additional information is helpful in making proper weld repairs. If the base material is low-alloy steel and is being welded in the final heat-treated condition, care must be taken so that post-weld heat treatment (PWHT) does not exceed the tempering temperature of the base material. If the item is being heat treated after welding, a welding material that will respond similarly to the base material must be selected. Improper heat treatment can cause loss of mechanical and corrosion-resistant properties.

For the welding department to join components into a product that will perform in service, the engineer must communicate to the welder the exact requirements of the design and quality control necessary to ensure that the part is built as designed. To perform this function, the engineer may call on other experts to assist him. The welding department would provide information on equipment and welding skills available to perform the work. The metallurgist would provide information on material selections and their behavior, and provide procedures for heat treating to achieve the desired mechanical properties. The welding engineer would provide information on welding and provide the welding procedures. Welding procedures can be written to cover a variety of situations and base material grades. Because of this, details of the procedure should be clarified to the welding department by the design engineer. Documentation requirements should be detailed on the engineering instruction with clear direction to access forms or with copies of forms to be filled out and attached. In most organizations, the quality assurance requirements are buried deep in corporate procedures and difficult for the welder to uncover. The procedures should be interpreted by the engineer, and details provided to the welder. The term “welder” as used in this section refers to the actual production welder or his immediate supervisor.

Filler Material The filler material should be specified by the welding procedure or the engineering instruction. The welding procedure should be reviewed to ensure that it is clear in selecting the correct filler material. Some welding procedures are written to cover a variety of materials and situations and need to be clarified for the welder at time of use.

Material

Welding Process

A welder must know the base material to determine a correct welding procedure. Providing material specifications to the welder ensures that he has this information. In some special cases, the production welding supervisor or welder must know the base material chemistry, along with the mechanical requirements. Some specifications only specify physical requirements, i.e., tensile and yield strengths. These types of specifications allow a variety of materials to be used for the end product. In other cases,

Each organization assigns the responsibility of process selection to different groups at different levels of management. In today’s world of CAD/CAM, productivity improvements, and quality demands, the welding equipment may be preselected by engineering and specified by the work document. In other cases, the selection is left to the production shop foreman or the welder performing the work. Quality, weld appearance, productivity, qualifications, welder skills, equipment, and 104

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dimension required. When hardfacing materials or corrosion-resistance materials are being applied to improve service performance, the minimum material thickness should be specified.

available electrode, all can influence the process selection and determine the end product cost. The job may require cleanliness levels to the point that flux and other leftover items cannot be tolerated, e.g., work inside a nuclear plant. A high-quality, but slower welding process can sometimes be more productive than the faster welding process that requires rework. The welding instruction should specify the welding process or specify that the process is the choice of the welding department.

Distortion Control All welding creates distortion, which is caused by shrinkage of the weld nugget during solidification. Welding shrinkage cannot be eliminated, but it can be controlled, through design and fabrication methods. The assembly must be designed to allow for the weld shrinkage that will occur. Weld sequencing and restraints can be used to control the shrinkage in a desired direction. Different welding processes will produce a different amount of shrinkage. In general, the faster the welding process and the more weld produced per pass, the less shrinkage and distortion will occur. Also a smaller joint design cross section reduces shrinkage. A square butt joint, with no bevel, and welded with the submerged arc process would produce the least amount of shrinkage.

Quality Control and Quality Assurance Quality control and quality assurance can be confusing to the welder. The quality assurance requirements are usually specified in some corporate document that is referenced on the engineering instructions. Because of this, the welder gathers quality control information mostly by word of mouth. It is rare that a welder will spend his time reviewing documents to determine the quality control and quality assurance requirements. The welder will usually seek the information from a person that he knows is familiar with the work. Surprisingly, this system works quite well. However, the process can be improved a great deal by providing this information via the work instruction. Information as to what inspections are required and when, and what documents need to be completed will help the job progress and avoid costly rework. For quality control and quality assurance, the more detailed the information to the specific task and the easier it is for a welder to understand, the better the job will progress. Preferably, the welder is given a sheet of specific instructions to accomplish the desired work. The welder should also know whether he is qualified to the appropriate standards to perform the work required.

Standards and Specifications Specifying standards, specifications, and applicable revisions on the engineering instructions can be helpful in solving problems and answering unanticipated questions; also, providing phone numbers of persons or departments that have technical authority helps to resolve problems quickly. Most standards cover a variety of work, so it is wise to spell out the category the work fits, i.e., high pressure and temperature piping or low temperature and pressure piping, such as pressure vessel category. See Figure 14.1.

Welding Department Required Information Facilitated by the Design Engineer/Technical Support

Weld Size and Penetration Joint configuration, buildup thickness, and/or weld efficiency (i.e., full penetration) should be clearly specified to the welder. Fillet and partial penetration welds are the most economical and are frequently used. In most structural fabrication shops, the routine of using partial penetration welds can cause errors when full penetration welds are required and not clearly identified. This error has caused serious rework in many instances. Proper and consistent use of welding symbols is imperative and AWS A2.4, Standard Symbols for Welding, Brazing, and Nondestructive Examination, is a national standard. Weld buildup, as in machinery repair work, should be specified in final finish dimensions. The welder should determine how much metal to apply to achieve the finish

Coordinate Decisions with Fabrication Shop Prior to Issuing Instructions (1) (2) (3) (4) (5) (6) (7) (8) (9) 105

Description of work. Drawing numbers. Volume of work. Is process preselected? By whom? Details of selected process/equipment. Welding procedure. Status of welding procedure? Qualified? Base material specification. Base material chemistry (special cases).

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Figure 14.1—Typical WPS 106

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(10) Base material mechanical requirements (special cases). (11) Base material post weld heat treat requirements. (12) Filler material. (13) Joint type and weld penetration requirement. (14) Inspection requirements and when inspection is to be performed. (15) Acceptance criteria. (16) Documentation (record) requirements, and who is required to record and/or certify. (17) Specify the responsible office of the design engineer, welding engineer, and job specialist. (18) Standards applicable to this job. (19) Whom to contact when questions arise.

———. Suggested Filler Materials for Welding Structural Steels (chart). Miami, Fla.: American Welding Society. ———. Suggested Preheat Temperatures for Welded Structural Steel Materials (chart). Miami, Fla.: American Welding Society. ———. The Everyday Pocket Handbook for Arc Welding Steel (PHB-1). Miami, Fla.: American Welding Society. ———. The Everyday Pocket Handbook for Gas Metal Arc Welding (GMAW) and Flux Cored Arc Welding (FCAW) (PHB-4). Miami, Fla.: American Welding Society. ———. The Everyday Pocket Handbook for Gas Metal Arc Welding (GMAW) of Aluminum (PHB-8). Miami, Fla.: American Welding Society.

Bibliography/Recommended Reading List

———. The Everyday Pocket Handbook for Shielded Metal Arc Welding (SMAW) (PHB-7). Miami, Fla.: American Welding Society.

American Welding Society. WELDPERFECT: The Easy Guide to Perfect Welding (WPERF). Miami, Fla.: American Welding Society.

———. The Everyday Pocket Handbook for Visual Inspection and Weld Discontinuities—Causes and Remedies (PB-2). Miami, Fla.: American Welding Society.

———. Joint Weld Terminology and Standard Welding Symbol Interpretation (textbook). Miami, Fla.: American Welding Society.

———. The Everyday Pocket Handbook for Visual Inspection of AWS D1.1, Structural Welding Code— Steel, Fabrication and Welding Requirements (PHB-6). Miami, Fla.: American Welding Society.

———. Lens Shade Selector (F2.2). Miami, Fla.: American Welding Society.

———. The Everyday Pocket Handbook on Metric Practices for the Welding Industry (PHB-5). Miami, Fla.: American Welding Society.

———. Prequalified Joint Details in AWS D1.1, Structural Welding Code—Steel (chart). Miami, Fla.: American Welding Society.

———. The Everyday Pocket Handbook on Welded Joint Details for Structural Applications (PHB-3). Miami, Fla.: American Welding Society.

———. Standard for Welding Procedure and Performance Qualification (B2.1). Miami, Fla.: American Welding Society.

———. Welding Symbols (chart). Miami, Fla.: American Welding Society.

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Fitting Aids Contents Background .................................................................................................................................................................... 110 Current Problems in Fabrication................................................................................................................................. 110 Fitting Aid Categories ................................................................................................................................................... 111 Bibliography/Recommended Reading List ................................................................................................................. 117

109

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Section 15—Fitting Aids

Background

problems during the fabrication process. Edges and surface conditions are not always as they should be and need to be checked when materials enter the plant. Mill tolerances are often excessive and can add up during fabrication, if not found before the material is used. The cumulative effect of this may result in excessive localized stress and distortion.

The use of fitting aids has been and will continue to be an integral part of fabrication. Fitting aids were in use when the ancients built the first wooden ships. Since then, the methods used for fitting and aligning have changed. Over the past one hundred years, materials and fabrication methods have changed dramatically. Currently, building materials are mainly ferrous alloys. Non-ferrous metals, composites, and wood are used in a small portion of the total fabrication. All of these materials have flaws, inaccuracies, and stresses that will manifest themselves during the fabrication process as distortions of one sort or another. This, and the need to hold and align components, create the basic need for fitting aids. It may be said that a fitting aid is a method or device used to hold or align (or both) two or more parts in a predetermined location. They may also be used to correct distortion while assembling components of structures, and the structures themselves. The ideal situation would be to use the minimum number of aids possible, but unfortunately, inherent flaws in the materials and other influencing factors seem to prohibit this. It is important to have a knowledge of the available aids so that an intelligent choice may be made for any particular application. Current technology, such as line heating, can dramatically increase the accuracy of parts, assemblies, and overall fabrication. Use of such technology should decrease the need for fitting, while increasing productivity.

Storage and Handling Storage and handling of such materials as plate, pipe, shapes, and castings can have a deleterious effect on surface condition and dimensional accuracy if done improperly. Problems occur when material is handled roughly or misused while in storage.

Burning and Cutting Burning or cutting operations can greatly increase the accuracy of assemblies if performed correctly. The heat of cutting may cause shrinkage and other distortions that must be monitored and offset. Edge condition must be held to specifications or fitting and welding time will be increased.

Forming and Shaping When material is mechanically formed on rolls or similar equipment, deformation results. This causes stresses that will, in turn, cause distortion when stresses are relieved by the heat of welding or cutting, removal of portions of the material and any heat treatments. Other than forming parts by line heating, there is little that can be done to eliminate or reduce stresses set in by mechanical forming. All operations must be closely watched to ensure accuracy.

Current Problems in Fabrication

Fitting and Welding

The problem of inaccuracy is many-faceted and begins before the material enters the plant.

Stresses induced by fitting restraints, large gaps, and welding are well known. These stresses result in distortion and deviation from design. These problems can be reduced by obtaining precise fitting, reducing the amount of welding and sequencing welding to minimize distortion.

Foundry and Mill Problems start with the way many materials are made. Stresses from rolling mills and heat treatments can cause 110

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Fitting Aid Categories The fitting aid devices documented herein are classified by their primary method of mechanical advantage in their design. Some of the aids shown use a combination of methods for force application. • • • • • • • • •

Wedge devices (see Figures 15.1–15.4). Threaded devices (see Figures 15.5–15.8). Hydraulic devices (see Figures 15.9–15.10). Pneumatic devices (see Figure 15.11). Gear-pulley devices (see Figures 15.12–15.13). Magnetic devices (see Figures 15.14–15.15). Strongbacks (see Figure 15.16). Padeyes, stays, and cables (see Figures 15.17–15.19). Jigs, mocks, and fixtures (see Figures 15.20–15.22).

Figure 15.2—Weld-On Saddle

Wedge Devices (Figures 15.1–15.4) Wedge, a piece of hard material, as wood or metal, tapering from a thick board to a thin edge that can be driven or forced into a narrow opening. Configurations are usually made from one-inch-thick steel plate and are typically 12 in. and 17 in. in length.

Figure 15.3—Yoke and Pin

Step-Cut Dog, also known as dog, a metal device used for holding or backing the force applied by a wedge or other tool. This device is attached by welding. Weld-On Saddle, also known as “U” dog yoke and hairpin, a “U”- or “L”-shaped metal device used in conjunction with a wedge to straddle and hold one part to another. Yoke and Pin, also known as clip and wedge, a flat metal plate with an oblong hole, used in conjunction with wedges or bull pins to align edges of plate. Pulldown, a metal device welded or mechanically fastened to the part at one end and slotted at the other, used in conjunction with a wedge and anchor clip to pull one part toward another.

Figure 15.4—Pulldown

Threaded Devices (Figures 15.5–15.8) Push-Pull Jack, also known as steamboat jack and ratchet jack, a device having a ratcheting sleeve with opposite internal threads at each end or with an internal thread at one end and a swivel at the other. The effective length of the device can be changed by rotating the sleeve or swivel.

Figure 15.1—Wedge and Step-Cut Dog 111

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Figure 15.5—Push-Pull Jack

Turnbuckle, a device having a metal loop or sleeve with opposite internal threads at each end or with an internal thread at one end and a swivel at the other. The effective length of the device can be changed by the sleeve or swivel. Jacking Clamp, any number of devices which are booked or welded having a screw at one end to apply force for aligning. Clip and Bolt, a device consisting of an angle support and a headless bolt, and used to pull parts toward each other. The angle support and bolt can be welded or mechanically fastened.

Figure 15.6—Turnbuckle

Portable Hydraulic Ram, also known as Portapower and Enerpack, a hydraulic device having an oil reservoir and a single- or double-acting cylindrical piston, used for lifting, pushing and holding parts together. Used where short reach or stroke is required and in conjunction with other devices for fitting.

Hydraulic Devices (Figures 15.9–15.10) Hydraulic Jack, also known as buda jack and bottle jack, a hydraulic and geared device having a singleor double-acting cylindrical piston used for hoisting or lifting.

Figure 15.7—Jacking Clamp 112

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Figure 15.8—Clip and Bolt

Figure 15.10—Portable Hydraulic Ram

Figure 15.11—Vacuum Saddle Gear-Pulley Devices (Figures 15.12–15.13) Chainfall, also known as chain hoist, a device having gears and pulley(s) and operated by chain to obtain mechanical advantage in lifting or pulling.

Figure 15.9—Hydraulic Jack

Come-Along, a device having a ratcheting gearpulley arrangement to change the effective length of a chain for lifting or pulling.

Pneumatic Devices (Figure 15.11) Vacuum Saddle, also known as vacuum jacking clamp, an air-operated device having suction pads for gripping relatively smooth surfaces and a “U”- or “L”shaped metal structure for straddling and holding parts together. This device is used in conjunction with a screw and thread or hydraulic ram for applying pushing force.

Magnetic Devices (Figures 15.14–15.15) Magnetic Saddle, also known as magnetic jacking clamp, a device employing an electrically induced or permanent magnetic field(s) and a “U”- or “L”-shaped metal 113

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Figure 15.12—Chainfall

Figure 15.14—Magnetic Saddle

Figure 15.13—Come-Along

Figure 15.15—Fitting Magnet

Strongbacks (Figure 15.16)

structure for straddling and holding ferrous metal parts together. This device is used in conjunction with a screw and thread or hydraulic ram for applying pushing force.

Strongback, any number of devices used to restrain applied forces and/or hold alignment. These devices may be welded or mechanically fastened and are used with many other tools for applying forces to parts.

Fitting Magnet, a device employing an electrically induced magnetic field(s) for drawing ferrous metals together. 114

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Figure 15.16—Strongback

Padeyes, Stays, and Cables (Figures 15.17–15.19) Padeye, also known as a doughnut, a metal device for use as an anchor, support and/or connector for lifting and applying force. This device can be welded, clamped or mechanically fastened. Stay, a strip of stiffening material used to hold, prop, and/or support parts. This device can be welded or mechanically fastened. Cable, a wire bundle or rope with means for attaching ends, used for lifting, pulling and holding parts. This device is normally attached by mechanical means to other fitting aids.

Mocks, Fixtures, and Jigs (Figures 15.20–15.22) Mock, a device which imitates the shape of an object for reference or support. Fixture, a device used to hold, position, and/or align a workpiece for an operation or process. Jig, a device used to guide a tool; a template. Jigs are often incorporated into fixtures.

Figure 15.17—Padeye 115

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Figure 15.18—Stay

Figure 15.19—Cable Figure 15.21—Fixture

Figure 15.22—Jig

Figure 15.20—Mock 116

SECTION 15—FITTING AIDS

Specialized Devices

Panel Line with One-Side Butt Welding Station, stiffener attachment and welding station, web frame attachment and welding station, and bulkhead attachment and final welding station.

One-Side Welding System, a system that consists of a heavy structural frame hydraulic magnetic or pneumatic clamping rams, plate positioning and/or feeding conveyors, weld backing bar and welding equipment.

Panel Line with Tacking Station, two-side butt welding station, stiffener attachment and welding station, web frame attachment and welding station, and bulkhead attachment and final welding station.

Stiffener Positioning System, a system that consists of a heavy structural beak and/or gantry, hydraulic, magnetic and/or pneumatic stiffener clamping and positioning equipment and tack welding equipment.

Bibliography/Recommended Reading List

Web Positioning System, a system that consists of a heavy structural beam and/or gantry, hydraulic magnetic, pneumatic, and/or gear-pulley equipment and tack welding equipment.

American Welding Society. Welding Handbook, 8th ed., vol. 1, Welding Processes (WHB-1.8). Miami, Fla.: American Welding Society.

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Welding Metallurgy: Practical Aspects Contents Introduction ................................................................................................................................................................... 120 Primary Base Metal Groups ......................................................................................................................................... 120 Bibliography/Recommended Reading List ................................................................................................................. 127

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Section 16—Welding Metallurgy: Practical Aspects

Introduction

(4) Low-alloy steel hardened by quenching and tempering (heat treatment after welding) (i.e., AISI-4130, 4140, 8620, 8630, 4340). (5) 300 Series austenitic stainless steel (304, 316, 321, 347). (6) Copper-nickel (90Cu:10Ni, 70Cu:30Ni). (7) Nickel-copper (70Ni:30Cu). (8) Inconel® (NiCrFe, 600 and 625). (9) Aluminum. (10) Metals strengthened by cold work (mild steel and Monel®). (11) Metals strengthened by age hardening (K-Monel®, HSLA, 17-4 ph stainless steel, aluminum 6061-T-6, 356T6).

Designers should have an understanding of the primary concepts of welding metallurgy as it relates to the service of a weldment. This should include how metals obtain and maintain their mechanical and corrosionresistant properties. There should also be an awareness of the effect that welding and filler metal composition have on the metallurgy and service life of the weldment. The designer should be aware of: (1) The specification requirements that are invoked. (2) How metals obtain their strength and other mechanical properties. (3) How metals obtain and maintain their corrosion resistance. (4) The effect welding has on base metal mechanical properties. (5) The effect welding has on base metal corrosion resistance. (6) How filler metals obtain their strength and corrosion resistance. (7) Special treatments which are necessary to maintain base and weld metal properties. (8) Detrimental elements that reduce weldability.

From this list the first seven groups will be reviewed as to: • How these metals develop their mechanical and corrosion-resistant properties. • How welding affects these properties. • What special processes are required to maintain desired properties. • What metallurgical effects put the weldability of the metal at risk.

Mild Steel and High-Strength Low-Alloy Structural Steel (OS, HS, ASTM A 36, A 242, A 441, A 588, A 572, and Grade 50)

Primary Base Metal Groups The following list covers the majority of base metals and their strengthening mechanisms used by designers in their day-to-day work.

1. Base Metal Strengthening Mechanisms of Mild Steel

(1) Mild steel (ferrous alloys not strengthened by heat treatment) [i.e., ordinary steel (OS), ASTM A 36]. (2) High-strength, low-alloy, structural steel (normalized) (i.e., HS of MIL-S-22698, ASTM A 242, A 441, A 572, and A 588 Grade 50). (3) Low-alloy steel with quench hardening and tempering heat treatment before welding (i.e., T-l, HY-80, HY-100, HY-130, ASTM A 514).

Mild steel, with typically well less than 0.29% carbon, has a relatively low hardenability and is not typically strengthened by heat treatment. Its primary yield and tensile strength properties are primarily developed from its microstructure of pearlite and ferrite. Mild steel’s chemistry is basically carbon up to approximately 0.30%, manganese up to approximately 1.5%, and a small amount of silicon, with residuals of phosphorous and sulfur. 120

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6. What Special Treatments or Actions are Necessary to Maintain Base and Weld Metal Properties

High-strength low-alloy steels have similar microstructures and chemistries as mild steel and they are strengthened by a fine-grained (small) crystal structure created from small additions of elements such as vanadium, nickel, chromium, or molybdenum and frequently receive normalizing heat treatment. This process consists of heating the steel to approximately 1550°F and air cooling.

There are no special routine treatments of these steels other than when heavy sections are cut and welded. Due to the rapid cooling rate of heavy sections, preheating in the range of 150°F to 200°F is sometimes advisable. These steels are typically only stress relieved for machining dimensional stability or when required by fabrication codes for pressure vessels or other designs where cyclic fatigue is a major concern.

Mild steels such A-36 or OS typically will have yield strengths below 50 ksi, and the high-strength steels such as HS, those steels that contain a grain-refining element such as vanadium will have yield strengths of 50 ksi or more. Typical applications are ships, bridges, buildings, towers, automobiles, tanks, boilers, pressure vessels, home utensils, appliances, etc.

7. Detrimental Elements that Reduce Weldability Some mild steels have lead or sulfur additions to improve machining operations. These steels are frequently called free machining steels. These additions may cause porosity and/or develop low melting temperature constituents that cause hot short cracking. Welding is not recommended and should be avoided if at all practical, but if welding must be accomplished, use of low-hydrogen shielded metal arc filler metals may reduce cracking and porosity.

2. Corrosion Resistance of Mild Steel All of these steels have relatively poor resistance to corrosion unless they are thermal sprayed with aluminum or zinc, or are painted or galvanized. Sometimes these steels are alloyed with a small amount of copper which causes a tightly adhering oxide scale that retards the normal rate of corrosion. Steels alloyed with copper may be used without painting.

Low-Alloy Steel with Quench Hardening and Tempering Heat Treatment before Welding (High-Strength Steels T-l, HY-80, HY-100, HY-130, and ASTM A 514)

3. Welding Effects on Mild Steels Base Metal Mechanical Properties

1. Base Metal Strengthening Quenched and Tempered Steel

Welding has very little effect on the mechanical properties of OS and HS base metals. There is increased hardness in the heat-affected zone (HAZ) but the depth of hardening is not found to the extent that is present in the quenched and tempered low-alloy steels. The HAZ’s maximum hardness is limited by its carbon content. Mild steel is the most readily welded of all the steels.

Mechanisms

of

Quenched and tempered steels such as HY-80 have moderately high hardenability. They are alloyed to produce weldments with high yield strengths with good ductility and high toughness at low temperatures without post-weld heat treatment. Postheat treatment is rarely accomplished after welding. When postheat is accomplished, it is usually for dimensional stability and requires special procedures. These steels receive their primary yield and tensile strength from the microstructure of tempered bainite and/or martensite, which are typically created by quenching the material from approximately 1550°F and tempering (reheating) at 1100°F or higher. The carbon content is kept low to improve weldability and eliminate the need for post-weld heat treatment. Depending on the alloy, maximum carbon contents are usually in the range of 0.12 to 0.23%. Alloying elements are typically less than 6 percent and may be comprised of elements such as manganese, nickel, chromium, molybdenum, silicon, vanadium and boron. The function of these elements is to increase strength, hardenability, and toughness at low levels of carbon content (improved weldability). Each base metal specification has well defined mechanical properties. Typical applications

4. Welding Effects on Base Metal Corrosion Resistance There is no loss of corrosion resistance of the base metal due to welding. 5. How Filler Metals Obtain Metal Strengthening and Corrosion Resistance Recommended weld metals have higher tensile and yield strengths than mild steel or high-strength structural base metals. This is primarily due to the presence of manganese, silicon and other deoxidizers in the filler metals which are used to improve the welding characteristics and weld quality. These weld metals do not have significantly better corrosion resistance compared to base metals. They require similar protection as the base metal (usually galvanizing, thermal spraying, or painting). 121

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similar to those of the base metal. Filler metal carbon contents are typically less than 0.12%.

are pressure vessels, bridges, ballistic protection, ships, mining, penstocks, and buildings. The designer should keep in mind that if mild steel or grain-refined mild steel will meet the service requirements, they should be used since they are easier and more economical to fabricate.

6. What Special Treatments or Actions are Necessary to Maintain Base and Weld Metal Properties? Strict adherence to the manufacturer’s recommendations or code requirements is essential if the desired properties of these base metals are to be preserved. Preheat and interpass temperatures that are too low during the welding process can result in base metal and weld metal cracking. Probably the most frequent cause of cracking of these metals while welding is lack of adequate preheat. When preheats are required they should be soaking preheats. Preheat is frequently required to prevent restraint cracking on heavier sections. Soaking preheats can be verified by measuring the preheat on the opposite (back side) of the weld joint. Preheats that are too high will produce undesirable nonuniform microstructure. Preheat and interpass temperatures that are too high will result in low impact properties. This can cause a drastic loss in the ability to arrest crack propagation during impact loading particularly at low temperatures. Heat input that is too high is detrimental for good impact resistance. Excessively high preheats, high interpass temperatures, high amperage, and slow travel speeds can cause a retarded cooling rate. All in-process welding conditions causing excessive heat input result in the same outcome: a retarded cooling rate from peak welding temperatures. Retarded cooling rates cause a heterogeneous microstructure that is more susceptible to fracture on impact loading.

2. Base Metal Corrosion Resistance of Quenched and Tempered Steels before Welding The quenched and tempered steels have only slightly better corrosion resistance than mild steel. They are routinely painted for corrosion protection. Improved corrosion resistance of these steels comes from the small amount of alloying elements present. They are rarely galvanized probably due to their susceptibility to hydrogen embrittlement. 3. Welding Effects on the Base Metal Properties of Steels that are Quenched and Tempered before Welding Welding can have drastic effects on the mechanical properties of the base metal and their heat-affected zone (HAZ). The heat-affected zone is the unmelted base metal area next to the weld having any noticeable effects from the heat of welding. These effects may include changes in mechanical properties. These steels are somewhat difficult to weld and special precautions must be observed if the yield strength, impact resistance and ductility are to be maintained. Improper welding techniques can result in hidden cracks and cracking that may not occur until many hours after welding is complete (hydrogen embrittlement cracking). Welding heavy sections without preheat and/or low interpass temperatures can result in extensive HAZ/fusion line cracking. Welding with excessive preheat and interpass temperatures can result in a loss of impact toughness.

Heat Input (J/in.) = Volts × Amps × 60 Travel Speed (ipm) The maximum heat inputs should be in accordance with the applicable code, standard or qualified welding procedure. A serious loss of the metal’s ability to absorb energy without cracking can result if excessive heat inputs are permitted. Stringer beads (straight-line progression) having higher travel speeds than weaving beads (oscillating progression) are preferred, since this reduces the heat input into the base metal and previously deposited weld metal. It is important to protect the weld metal properties. One of the most limiting factors in welding these quenched and tempered steels is the marginal ability of weld metal to match base metal toughness. Stress relief as previously mentioned is generally not recommended. However, it is sometimes accomplished for dimensional stability of weldments that are to receive close tolerance machining. Stress relief should only be accomplished if permitted by the manufacturer or the governing code or standard. The manufacturer’s or code

4. What Effects Does Welding have on the Base Metal Corrosion Resistance? Welding has little effect on the corrosion resistance of the base metal. 5. How Welds Develop their Mechanical Properties Filler metals typically have chemical compositions similar to base metals except they are not micro alloy hardened by elements such as boron. They are frequently alloyed with elements such as nickel, chromium, molybdenum and manganese. The carbon content is kept much lower than the base metal carbon content. This is because the weld metal goes through an extremely rapid quench and would be susceptible to cracking at carbon contents 122

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improve hardenability (the depth of hardening). These metals often do not have mechanical property requirements in their applicable base metal specification. Varying mechanical properties can be selected by the user and are achieved by varying the heat treatment. These alloys are used for machinery applications such as gears, shafts, and fasteners for increased wear, tensile strength, or sometimes for improved fatigue resistance. The designer should keep in mind that if mild steel, or even the steels that are quenched and tempered before welding (i.e., HY-80/100) will meet the service requirements, they should always be used in lieu of these postweld heat treatable steels.

requirements should be strictly followed. Usually this involves stress relieving the part 50°F below the tempering temperature, if known, and sometimes it requires forced cooling to minimize the loss of fabrication impact properties. Also, it is essential that the weld metal contain less that 0.05% vanadium or the weld will be subject to serious temper embrittlement. Since these alloys are subject to serious embrittlement by improper heat treatment, post-weld stress relief should be under the direction of a welding engineer or a metallurgist. 7. Detrimental Elements that Reduce Weldability The most common element on earth, hydrogen, is also the most hazardous to welding quenched and tempered steels. Hydrogen is commonly produced when water, oil and rust are broken down in the arc. Procedures and techniques to avoid hydrogen embrittlement cracking are discussed in Section 12 of this manual. Other hazardous elements are those such as lead, tin, zinc, phosphorous, sulfur, etc., that will form low melting temperature constituents.

2. Base Metal Corrosion Resistance of Quenched and Tempered Steels after Welding The quenched and tempered steels have only slightly better corrosion resistance than mild steel. They are routinely painted for corrosion protection. Improved corrosion resistance of these steels comes from the small amount of alloying elements. They are not typically galvanized. 3. Welding Effects on the Base Metal Properties of Steels that are Quenched and Tempered after Welding

Low-Alloy Steel with Quench Hardening and Tempering Heat Treatment after Welding (Post Weld Heat Treatable) (AISI-1040, -8620, -8630, -4130, -4140, -4340, etc.)

Welding can drastically affect the mechanical properties of the base metal in the heat-affected zone (HAZ). The heat-affected zone is the unmelted base metal area next to the weld having any noticeable affects from the heat of welding. These effects may include changes in tensile and yield strength, increased hardness, loss of ductility, mechanical and/or corrosion-resistant properties. These steels are typically very difficult to weld and special precautions must be observed or serious cracking may result. Improper welding techniques can result in immediate gross cracking, hidden cracks and cracking that may not occur until many hours after welding is complete (hydrogen embrittlement cracking). Welding heavy sections with no preheat and low interpass temperatures can result in extensive HAZ/fusion line and weld metal cracking.

1. Base Metal Strengthening Mechanisms of Steel Quenched and Tempered after Welding Steels that are quenched and tempered after welding, such as AISI-8620, 4130, or 4340, have high hardenability. They are alloyed to produce high tensile and yield strengths and may have limited ductility and toughness even at room temperatures. These steels are usually selected for their high strengths or hardness for wear resistance. Commonly these metals are welded in a soft condition called annealed. Postheat treatment is accomplished after welding to obtain desired mechanical properties. These steels primarily receive their yield and tensile strength from the microstructure of tempered high-carbon bainite and/or martensite which are created by quenching the material from approximately 1550°F and tempering (reheating) at 1100°F or higher. Desired mechanical properties can be selected by varying the tempering temperature. The carbon contents are typically high which lowers the weldability and requires special procedure requirements. Depending on the alloy, maximum carbon contents are usually in the range of 0.23 to 0.43%. Alloying elements are typically less than 6 percent and may be comprised of elements such as manganese, nickel, chromium, molybdenum, silicon, and vanadium. The primary function of these elements is to

4. What Effects Does Welding have on the Base Metal Corrosion Resistance? Welding typically has no noticeable effect on the corrosion resistance of the base metal. 5. How Welds Develop their Mechanical Properties Filler metals may or may not have similar chemical compositions as their base metals. They are frequently alloyed with elements such as nickel, chrome, molybdenum, and manganese, with the exception that the carbon 123

SECTION 16—WELDING METALLURGY: PRACTICAL ASPECTS

frequently cause unacceptable distortion and scaling of close-tolerance machined components.

content is usually kept much lower than the base metal. This is because the weld metal goes through an extremely rapid quench and is very susceptible to cracking at carbon contents approaching the base metal. Under special circumstances base metal or filler metals that nearly match the base metal are used for welding. It is common to use undermatching filler metal and rarely do welds match the base metal mechanical properties, unless unique and specialized procedures are developed. However, post-weld heat treatment (PWHT) can be selected so that the resultant base metal properties nearly approximate those attainable by the weld. In most cases, designers must take into account that the welds will typically undermatch the base metal properties.

7. Detrimental Elements that Reduce Weldability One of the most common elements on earth, hydrogen, is also the most hazardous to welding these quenched hardenable steels. Procedures and techniques to avoid hydrogen embrittlement cracking are discussed in Section 12 of this manual. Other hazardous elements are those such as lead, tin, zinc, phosphorous, sulfur, etc., that will form low melting temperature constituents.

300 Series Austenitic Stainless Steel (Annealed) [304, 310, 316, 321, 347 CorrosionResistant Steel (CRES)]

6. What Special Treatments or Actions are Necessary to Maintain Base and Weld Metal Properties?

1. Strengthening Mechanisms of Austenitic Stainless Steel Base Metal

Strict adherence to manufacturer’s recommendations, special developed procedures, or code requirements is absolutely essential if the desirable properties of these metals are to be achieved. Preheat and interpass temperatures that are too low during the welding process can result in base metal and weld metal cracks. Preheats as high as 600°F may be required. Typically, these metals may require soaking times after welding at preheat temperatures to minimize cracking or they may require to be immediately stress relieved, if post heat treatment for mechanical properties will not occur until later in the fabrication process. Probably the most frequent cause of cracks while welding these metals is lack of adequate preheat. When preheats are required, they should be soaking preheats. Soaking preheats can be verified by measuring the preheat on the opposite (back side) of the weld joint.

Typical austenitic stainless steels used in industry have 18% or more chromium and 8% or more nickel, with the remaining balance of approximately 50 to 60% iron, and are sometimes called the 18-8 stainless steels. Depending on the alloy type and application, the carbon content will be 0.20% and less. Typically carbon is less than 0.08% for corrosion resistance. Higher carbon content metals (309, 310) are commonly used where hightemperature strength is required. Austenitic stainless steels are not strengthened by quench hardening, since their primary phase and structure do not change with temperature as with the quenched and tempered steel alloys. Their primary yield and tensile strength properties are developed from what is called substitutional and interstitial alloy strengthening. The microstructure is austenitic at all temperatures with small amounts of carbides. Austenitic stainless steels containing nickel, chrome, and manganese with small amounts of silicon are sometimes alloyed with molybdenum, titanium, niobium, or copper with residuals of unwanted sulfur and phosphorous. Austenitic 300 Series stainless steels typically have yield strengths of 35 ksi at room temperature, with very little loss of strength even at relatively high temperatures. Tensile strengths of 75–80 ksi are common with ductility of 30–35%. These steels are typically used for their hightemperature yield strength, oxidation resistance for both high- and low-temperature applications, and excellent toughness. These steels are nonmagnetic.

Generally, there are no welding heat input restrictions on these metals like those required on metals that are quenched and tempered metals before welding. The reason for this is the mechanical properties are achieved through heat treatment after welding and the weld and HAZ are recrystallized during heat treatment. Since these metals are heat treated after welding, they are not usually stress relieved unless they are welded in the heat-treated condition or following manufacturing processes makes it necessary to dimensionally stabilize the metal or to reduce the hardness of the material (as might be needed for machining). The manufacturer’s recommendations or code requirements should be strictly followed. When components are welded in the heattreated condition, stress relieving is usually accomplished 50°F below the tempering temperature. It should always be kept in mind that the temperatures reached during the heat treatment and the quenching process will

Galvanic Series. When any of these 300 series stainless steels are used in combination with a dissimilar metal for sea-water service the galvanic series should be examined. This will help determine if accelerated corrosion can be expected due to the different metal being brought used in the same system. The surface areas of the dissimilar met124

SECTION 16—WELDING METALLURGY: PRACTICAL ASPECTS

carbides, leaving the chrome available to form its protective oxide film. Stressed stainless alloys that have extensive depletion of chrome by sensitization and are in a corrosive halogen product environment, such as salt water, may be subject to stress corrosion cracking. Stress corrosion cracking can be extensive, penetrating completely through the base material thickness in a short period of time.

als can also greatly affect the amount and extent of corrosion. In general austenitic stainless steels are not recommended for sea-water service environments. 2. Corrosion Resistance of Austenitic Stainless Steel Austenitic stainless steels have excellent atmospheric corrosion resistance and resist corrosion very well at high temperatures. When alloyed with copper (320) it has good corrosion and erosion resistance in sea water. The iron in these austenitic stainless steels is protected from corrosion or oxidation by the formation of a protective stable refractory chrome-oxide film. This tenacious film develops when the surface of the metal is exposed to an oxidizing atmosphere and once formed prevents continuing corrosion or oxidation. The continued presence of this oxide film is imperative to maintain corrosion resistance. If service conditions cause the loss of this film, the material will be susceptible to corrosion. Also, the effects of welding heat can cause a loss of the metal’s ability to form these protective oxides. These metals are frequently used without painting.

5. How Welds Develop their Mechanical Properties and Corrosion Resistance Stainless steel filler metals have chemistries similar to the base metals they are intended to weld. They develop their mechanical properties and corrosion resistance due to their similar base metal chemistry and metallurgy. There is no significant improved corrosion resistance of the stainless steel weld metals over the base metal. Some stainless steel filler metals have slight chemistry modifications to permit the development of delta ferrite in the weld deposit. Fully austenitic welds have a tendency to fissure (develop small cracks). When delta ferrite is present in the weld deposit the tendency for fissuring is reduced. Stainless steel filler metals such as 308, 309, 316, all may contain small amounts (usually less than 10%) of delta ferrite when deposited. The stainless filler metal 312 typically has 40% ferrite and is very crack resistant. However, it should not be used for hightemperature applications, as the soft and ductile delta ferrite will change to a hard and brittle phase called sigma ferrite that is extremely crack sensitive.

3. Welding Effects on Stainless Steel Base Metal Mechanical Properties Welding has very little effect on the mechanical properties of austenitic stainless steel base metals. Since austenitic stainless steels are not quench hardenable, the HAZ does not exhibit increased hardness as a result of welding. These steels are readily weldable but do require some special controls to maintain their corrosion resistance. If the stainless steel has been cold worked prior to welding to increase its yield strength, the effects of welding may cause a drastic loss of the base metal’s yield strength in the HAZ to approximately its annealed condition. 4. Welding Effects Resistance

on

Base

Metal

6. What Special Treatments or Actions are Necessary to Maintain Base and Weld Metal Properties Austenitic stainless steel welds are somewhat subject to high-temperature fissuring. They are also subject to reduced corrosion resistance with increased potential of stress corrosion cracking when sensitized. Sensitization by chromium carbide precipitation takes place at 900°F to 1500°F and is time dependent. The more time spent in the sensitizing temperature range the greater the degree of chromium carbide precipitation. To minimize fissuring and sensitization, minimum preheats are used and interpass temperatures are kept low (350°F maximum). By keeping the preheat and interpass temperature low, less time is spent at sensitizing temperatures and the amount of the base metal and degree of sensitization will be reduced. Avoiding high heat input welding processes will also lower the amount of time at sensitizing temperatures and the amount of base metal subject to sensitization. The use of stringer beads is recommended instead of weave beads, which minimizes heat input and sensitization.

Corrosion

The effects of welding can cause a serious loss of corrosion resistance of the base metal adjacent to the weld. For some service applications, the presence of carbon and the heat of welding can result in reduced corrosion resistance of some stainless steel alloys by the preferential formation of chromium carbides in lieu of the protective chrome-oxide films. This depletion of available chromium for oxidation resistance by the formation of chromium carbides is called sensitization. Therefore, for special applications the carbon content may be held to 0.04% or less (extra low carbon 308L or 316L) or additions of titanium (321) or niobium (347) may be made to the base metal. The stabilizing elements of titanium and niobium will form carbides, preferentially chromium 125

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70:30 CuNi (70% copper-30% nickel) wrought metals typically have yield strengths of 18,000–20,000 psi, and tensile strength of 45,000–50,000 psi with elongation of 20–35%. It is considered a hot short material having reduced ductility at temperatures above 1100–1400°F and is subject to high-temperature cracking while under stress. NiCu (70% nickel-30% copper alloy), commonly called Monel®, has the highest tensile and yield strength of any ratio of nickel-to-copper contents. Its minimum tensile strength is 70–85 ksi and its yield strength is 23– 25 ksi with 35% minimum ductility.

7. Detrimental Elements that Reduce Weldability Some stainless steels have selenium, phosphorous, or sulfur additions to improve machining operations. These steels are frequently called free machining steels (303-Se, 316F, 347F). These additions may cause porosity and/or develop low melting temperature constituents that result in hot short cracking during welding. Welding of these alloys is not recommended. The contamination of weld joints by low melting constituents can cause serious weldassociated cracking. Lead contamination from radioactive shielding or from machinists’ hammers, sulfur from sulfurized cutting fluids, lead and tin from soft solder contamination, and cadmium from silver brazing alloys, etc., are examples of potential sources of contamination.

2. Corrosion Resistance of Copper and Nickel Alloys These alloys have excellent atmospheric and corrosion resistance. These metals are frequently used without painting.

Carbon contamination can have a serious effect on the corrosion resistance of stainless steels; therefore, it must be considered a detrimental contaminate. Its potential sources are shop dirt, grease, oil, paint, shellac, antispatter compounds, and carbon arcing slag.

CuNi–90:10 It is typically used for water corrosion resistance with good antifouling resistance. This alloy has a higher iron content than the 70:30CuNi alloys, which improves its strength and corrosion protection. This alloy performs better in stagnant water (pitting/crevice corrosion) than either 70:30 CuNi or Monel® (70%Ni-30%Cu). It is also superior in its antifouling resistance.

Copper-Nickel and Nickel-Copper Base Metals [90:10 CuNi, 70: 30 CuNi , and 70Ni:30Cu (Monel®)] 1. Strengthening Mechanisms of Copper-Nickel and Nickel-Copper Base Metal

CuNi–70:30

These alloys are not strengthened by quench hardening, since their primary phase or crystal structure does not change with temperature as it does with quenched and tempered steel alloys. The primary yield and tensile strength properties are developed from what is called substitutional alloy strengthening (the mixing of copper and nickel atoms in a geometric pattern). Its structure is austenitic at all temperatures. These austenitic copper and nickel alloys have copper and nickel as their major alloying elements, with small additions of elements such iron for yield strength and erosion resistance, manganese to minimize hot cracking, silicon for strength and manufacturing fluidity (high percentages silicon 4% for hardness), and niobium for increased weldability with residuals of unwanted sulfur and phosphorous.

It is used primarily for water corrosion and erosion resistance where increased strength and hardness is required with less antifouling resistance than the higher copper content 90:10 CuNi alloy. It has better erosion resistance in flowing water but has less pitting resistance (crevice corrosion) than 90:10 CuNi. Monel®–70NI:30Cu It is used primarily for water corrosion and erosion resistance where increased strength and hardness is required (with less antifouling resistance) than the higher copper content copper-nickel alloys. It has better performance for erosion resistance than either of the coppernickel alloys, but has inferior performance for crevice corrosion and antifouling resistance.

90:10 CuNi (90% copper-10% nickel) wrought metals typically have minimum yield strengths of 15,000– 18,000 psi, and minimum tensile strengths of 38,000 psi with minimum elongations of 25–35%. It is considered a hot short material having reduced ductility at temperatures above 500°F to 1400°F and subject to cracking while under stress. This metal is also less expensive than the higher nickel content copper-nickel alloys. It is more difficult to weld than 70:30 CuNi and Monel® (70%Ni30%Cu) metals.

Galvanic Series When any of these copper and nickel alloys are used in combination with a dissimilar metal, for water service, the galvanic series should be examined. This will help determine if accelerated corrosion can be expected, due to the different metal being used in the same system. The surface areas of the dissimilar metals can also greatly 126

SECTION 16—WELDING METALLURGY: PRACTICAL ASPECTS

ations. These are frequently called free machining alloys. These additions may cause porosity and/or develop low melting temperature constituents that jeopardize welding due to hot short cracking. Welding of these alloys is not recommended. The contamination of a weld joint by low melting constituents can cause serious weld-associated cracking. Lead contamination from radioactive shielding or from machinists’ hammers, sulfur from sulfurized cutting fluids, lead, tin or zinc from soft solder contamination, and cadmium from silver solder, etc., are all potential sources of contamination.

affect the amount and extent of corrosion. These alloys are frequently recommended and used for water-service environments. 3. Welding Effects on Base Metal Mechanical Properties Welding has very little effect on the mechanical properties of nickel alloys and copper base metals. Since these alloys are not quench hardenable the HAZ does not exhibit increased hardness as a result of welding. These alloys are readily weldable but do require some special controls to minimize cracking and weld porosity. If any of these alloys have been cold worked prior to welding to increase its yield strength, the effects of welding may cause a drastic loss of the base metal’s yield strength in the HAZ, approximating its annealed condition. 4. Welding Effects Resistance

on

Base

Metal

Castings Copper and nickel alloy components that are castings and are not made by weld fabrication should always have suspected poor weldability. Because of the iron contents permitted in these castings and the higher silicon contents to facilitate pouring operations, some of these castings have extremely limited weldability. Unless properly alloyed for weldability, they may experience extensive fusion line cracking. For Monel® castings, iron and silicon contents must be held low, preferably at 1.7%. Niobium is also required to minimize the harmful effects that silicon has on the weldability of Monel®. Niobium-to-silicon ratios of 1.5 to 1.8 are believed to be adequate to minimize weld cracking caused by silicon. A Nb/Si ratio of 1.7 is considered optimum. Iron content above 2% is a major contributor to both weld metal and heat-affected zone cracking. Also, shielded metal arc welding (SMAW) has slight advantages over gas tungsten arc welding (GTAW) for casting repairs.

Corrosion

The effects of welding have little effect on the corrosion resistance of the adjacent base metal. 5. How Welds Obtain Metal Strengthening and Corrosion Resistance Filler metals for welding alloys of nickel and copper have chemistries/compositions similar to the base metals they are intended to weld. They develop their mechanical properties and corrosion resistance due to their similar base metal metallurgy. There is no significant improved corrosion resistance of weld metals over the base metal. In the case of Monel® filler metal (70% Ni/30% Cu), it is found that under some wet conditions, these filler metals will preferentially corrode over similar composition base metals. Monel ® welds on wetted surfaces of Monel® castings will often preferentially corrode in lieu of the base metal.

Bibliography/Recommended Reading List

6. What Special Treatments or Actions are Necessary to Maintain Base and Weld Metal Properties Since copper and nickel alloys are somewhat hot short and subject to high-temperature fissuring, welding interpass temperatures are maintained relatively low. Typically, welding preheat and interpass temperatures are 350°F maximum.

American Welding Society. Practical Reference Guide for Welding Metallurgy: Fabrication and Repair (PRGM). Miami, Fla.: American Welding Society.

7. Detrimental Elements that Reduce Weldability

———. Proceedings of the U.S.–Japan Symposium on Advances in Welding Metallurgy. Miami, Fla.: American Welding Society.

Some copper and nickel alloys have selenium, phosphorous, or sulfur additions to improve Machining oper-

———. Welding Metallurgy, 4th ed., vol. 1, Fundamentals (WM1.4). Miami, Fla.: American Welding Society.

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Arc Stud Welding Contents Introduction ................................................................................................................................................................... 130 How a Stud is Welded.................................................................................................................................................... 130 Elements of a Stud Weld ............................................................................................................................................... 130 Testing and Judging Welds ........................................................................................................................................... 131 Operating Instructions .................................................................................................................................................. 131 Setting Up to Weld......................................................................................................................................................... 132 How to Handle the Stud Welding Gun ........................................................................................................................ 132

129

SECTION 17— ARC STUD WELDING

Section 17— Arc Stud Welding

Introduction

(1) Welding heat is developed by drawing an electric arc between the stud and the base metal. (2) The two pieces are fused together by the weld.

Arc stud welding is an arc welding process that is used to weld threaded connectors, insulation pins, small brackets, pipe hangers, and concrete shear pins, etc., to other compatible base metals. The process accomplishes fusion at the stud/base metal interface using only the melted electrode as filler metal. It is very fast, economical, is essentially automatically controlled, and requires less operator skill than other welding processes. During stud welding, an arc is drawn between the stud and the base material forming a molten puddle. The stud is then plunged into the puddle and as the molten metal cools a weld is formed. The stud welding equipment is adjusted to control the arc initiation, arc time and the distance the electrode is plunged into the molten metal.

Stud welding differs from manual arc welding, in that the welding cycle is controlled by equipment adjustments.

Elements of a Stud Weld (See Figure 17.1) The stud, inserted in the welding end (chuck) of the gun, is positioned against the work plate (A). The operator then presses the gun to the work plate until the ferrule, which is around the stud, is flat against the plate

Stud welded fasteners are attached with a full penetration weld. They are available in a variety of shapes and sizes. Stud welding may be performed in tight space restrictions and with minimal clearance. Studs can be welded in most positions to carbon and alloy steels, stainless steel, and aluminum alloys. Additional special purpose applications are available for some copper, magnesium, titanium, zirconium, and zinc alloys. Stud welding produces only localized heating and has low heat input. The stud attachment surface must be clean and prepared for arc welding. The stud welding process is limited by the stud size and the stud shape. Only one end of the stud can be welded with this process, and preproduction mock-up testing is usually accomplished for process control. The welded studs may be torque or load tested for production quality assurance.

(a) (A)

(b) (B)

(c) (C)

(d) (D)

(e) (E)

(f) (F)

How a Stud is Welded The process of stud welding has some similarities to the manual arc welding processes. The process consists of two steps:

Figure 17.1—Stud Welding Cycle 130

SECTION 17— ARC STUD WELDING

Operating Instructions

The trigger switch is depressed, starting the welding cycle (B). The stud is drawn away from the work plate, creating an electric arc between the stud and the plate (C). A molten weld pool is created when a portion of the stud and the plate are melted by the arc (D). After the arcing period, the main spring in the gun is automatically released, plunging the stud into the molten weld pool of the plate to form a weld within the ferrule hollow (E). The gun is removed from the completed stud weld and the ferrule is broken away (F).

Gun Accessories (see Figure 17.2) (1) Chuck—The chuck holds or grips the stud in the gun for welding. A different chuck must be used for each diameter of stud. (2) Ferrule Grip—The ferrule grip fits into the foot and holds the ferrule in position for welding. (3) Foot—The foot is used to secure and align the ferrule grip. (4) Legs—The legs are used to hold and provide positioning of the foot and ferrule grip assembly. The leg length must be adjusted for different lengths of studs.

Testing and Judging Welds It is recommended that a few sample welds are made, inspected, and tested whenever new operations start, and at the beginning of each day’s work. Weld a few studs to a piece of metal that is the same composition and thickness as the actual workpiece.

Setting up the Gun (1) Insert a chuck that is of proper size for the stud, and tap lightly with a hammer until it is held firmly in the taper. (2) Put the legs in place and tighten. (3) Fasten the foot to the legs with screws and washers provided. (4) Insert the ferrule grip into the foot and secure by tightening set screws in foot. (5) Insert the stud in the chuck and the ferrule in the ferrule grip.

Inspect the sample stud welds for: (1) Positioning—Perpendicular to the base plate. (2) Weld Quality—Adequate fusion and fillet, weld quality acceptable. (3) Length—Approximately 1/8 in. of the stud length is consumed by weld. Test the sample stud welds to the production requirements. Production studs that require proof testing should be load or torque tested to the design requirements.

Adjustment of Plunge Adjust the legs so the stud extends from 1/8 in. to 3/16 in. beyond the end of the ferrule. When the stud stickout is correct, tighten leg screws so that the assembly is stationary. NOTE: The lift height, which determines the arc length, has been preset at the factory and will automatically compensate for small changes in stud length.

Centering of Ferrule on Stud Adjust the foot so that the stud is centered in the ferrule. This is vital. Improper welds can result if there is any binding or friction between the stud and ferrule during the welding cycle.

Test Action of the Gun Work the lower assembly of the gun in and out to make sure that the stud moves freely through the ferrule. If there is any binding, readjust foot assembly until free action is obtained.

Figure 17.2—Gun Accessories 131

SECTION 17— ARC STUD WELDING

Setting Up to Weld (See Figure 17.3)

proximate and will vary with the type of application. Note that the power supply may not be calibrated to indicate actual welding current.

Turn power OFF when making connections. (1) Position the control unit and the power supply as near the work area as possible. Excessive cable lengths adversely affect stud welding. (2) Connect the ground cable to the terminal of the power supply (positive or negative ground orientation must be determined by the weld procedure test data). Secure the c-clamp to the work plate. Make sure both connections are tight and eradicate any paint or rust at the connection points. (3) Connect the timer input cable to the other terminal of the power supply and to the connector marked “power supply” on the control unit. (4) Connect the control unit ground clamp to the work plate. This completes the electrical path needed to supply power to the control unit. (5) Connect the combination cable (control and welding cable) to the control unit and the gun. (6) Make sure the power supply is set for required polarity, and that the input cable and the ground cable is properly connected. The power supply is typically a constant voltage (CV) type and should have a minimum open circuit voltage of 65 volts dc. (7) Turn on the power supply and the control unit. The light on the control unit should light indicating that power is available and is connected for the required polarity. (8) Adjust the time on the control unit and current setting of the power supply. The settings provided are ap-

How to Handle the Stud Welding Gun (1) Hold the gun firmly with your hand placed so that you can readily press the trigger. (2) Keep your hand off the side cable. (3) Hold the gun square/perpendicular to the work. (4) Be sure the ferrule is seated firmly against the workpiece. (5) Press the gun’s trigger only once and release. Pressing the trigger a second time will damage the chuck or stud threads. (6) Do not move the gun during the welding action. (7) After welding the stud, draw the gun straight back away from the workpiece. Avoid pressing the trigger again.

Bibliography/Recommended Reading List American Welding Society. Recommended Practices for Stud Welding (C5.4). Miami, Fla.: American Welding Society. ———. Welding Handbook, 8th ed., vol. 1, Welding Technology (WHB-1.8). Miami, Fla.: American Welding Society.

Figure 17.3—Stud Welding Setup 132

SECTION

18

Thermal Spray Fundamentals Contents Why Use the Thermal Spray Process? ........................................................................................................................ 134 Corrosion Control Applications ................................................................................................................................... 134 Machinery Component Applications ........................................................................................................................... 134 Fundamentals................................................................................................................................................................. 134 Thermal Spray Processes.............................................................................................................................................. 134 Finishing Thermal Sprayed Coatings .......................................................................................................................... 135 Quality Assurance ......................................................................................................................................................... 137 Future Applications ....................................................................................................................................................... 137 Advantages ..................................................................................................................................................................... 137 Limitations ..................................................................................................................................................................... 137 Bibliography/Recommended Reading List ................................................................................................................. 137

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SECTION 18—THERMAL SPRAY FUNDAMENTALS

Section 18—Thermal Spray Fundamentals

Why Use the Thermal Spray Process?

Fundamentals

The thermal spray process is used primarily for two reasons:

Before a machinery component is repaired (i.e., to build up worn areas) using a thermal spray process, the repair area is undercut. Standard practice is first to clean the component to be thermal sprayed, and then use an aluminum oxide grit blast to create an anchor tooth pattern for the coating to lock into. When repairing machinery components, thermal spray materials are used as bond coats to create a mechanical interaction between the substrate and the coating.

(1) Corrosion control. (2) Machinery component refurbishment.

Corrosion Control Applications More than 20 years of corrosion protection of steel and iron can be obtained by applying thermal sprayed aluminum or zinc coatings. In a wet environment, these coatings can serve as an expendable anode, preventing or minimizing corrosion of steel and iron substrates. Because of the portability of arc and flame spray systems, many components can receive a corrosion-preventing coating either in the shop or in the field. Examples of some of the items that have received this corrosion control method are valves and piping.

Thermal Spray Processes By definition, thermal spray is heating a metallic or a nonmetallic material, in a heating zone to a molten or semi-molten state, and then propelling that material onto a substrate to form a coating. There are four primary spray processes used by fabricators at this time: plasma powder process, arc wire process, flame powder process, and flame wire process.

Plasma Powder

Machinery Component Applications

With the plasma powder process, the heating zone is produced by an arc that is created inside the plasma powder gun between a tungsten electrode and the nozzle. A gas or gas mixture is passed through the arc. This excites the gas into a plasma state, creating temperatures higher than can be obtained with just oxygen fuel flame mixtures. The gas or gas mixture exits the nozzle, forcing the plasma flame outside the gun. A powder is fed into the flame, melted, and propelled to the substrate by the force of the gas or gas mixture (see Figure 18.1). The plasma powder process is used primarily for the refurbishment of machinery components.

The thermal spray process can be a cost-effective method for dimensional restoration. The process is an excellent repair method and should be considered, when appropriate, in lieu of other processes, such as weld repair, chrome plating, or instead of manufacturing of a new component. Thermal spray is also used to enhance the service life of many machinery components. Ceramics can be sprayed on packing areas of valve stems and pump shafts to give those areas a smoother, longer-lasting surface than the substrate material. This will also cause less damage to the packing material and provide better operation of the component. Thermal sprayed metallic coatings that will resist wear, erosion, corrosion, heat, and chemical attack better than the substrate material can be applied to machinery components.

Arc Wire The heating zone of the arc wire process is produced by creating an arc between two continuously fed metallic 134

SECTION 18—THERMAL SPRAY FUNDAMENTALS

Figure 18.1—Plasma Powder Process

Flame Powder

wires. The heating zone melts the wires and the molten material is atomized and propelled to the substrate by compressed air or an inert gas (see Figure 18.2). The arc wire process can be used for corrosion control and machinery refurbishment applications.

The flame powder process uses an oxygen fuel flame to create the heating zone. Powder is fed into the heating zone and melted. The molten material is then atomized and propelled to the substrate by the force of the burning gasses and compressed air (see Figure 18.4). The flame powder process is used primarily for machinery refurbishment applications.

Flame Wire The flame wire process uses an oxygen fuel flame to create the heating zone. A wire is then continuously fed into the heating zone where it is melted and then propelled onto the substrate by the force of the burning gasses and compressed air (see Figure 18.3). The flame wire process can be used for corrosion control and machinery refurbishment applications.

Finishing Thermal Sprayed Coatings The structure of a thermal sprayed coating may be different than the structure of that same material in wrought form. Sometimes this different structure creates

Figure 18.2—Arc Wire Process 135

SECTION 18—THERMAL SPRAY FUNDAMENTALS

4 TO 10 in. (100 TO 250 mm)

Figure 18.3—Flame Wire Process

Figure 18.4—Flame Powder Process 136

SECTION 18—THERMAL SPRAY FUNDAMENTALS

problems in finishing. If personnel are unfamiliar with the finishing of coatings, they should follow the thermal spray equipment and material manufacturer’s recommendations. If thermal sprayed coatings are properly finished and they still separate or flake off during finishing the spraying operation should be reviewed.

with the mating part. This makes it possible to form a precise seal. While the substrate is frequently preheated, the thermal spray process is generally considered a metallurgically cold process. This means in most applications no warpage or heat-affected zone is created.

Quality Assurance

Limitations

To help ensure the quality of thermal sprayed coatings, operators must receive thorough training and be tested to determine whether they have the ability to use all the thermal spray processes required. Tested and/or certified procedures and personnel should be used to spray production components. Facilities must have the personnel and the methods to determine whether their thermal spray equipment and consumables will produce quality coatings. Thermal spray facilities must also have personnel capable of making sound application and coating selection decisions or have an organization that can help them do so.

It should be remembered that when refurbishing machinery components with the thermal spray process, no strength is added to that component. If the undercut depth will reduce the strength of the component enough to make it unusable for service, then thermal spray must not be considered as a repair option. Thermal spraying a highly corrosion-resistant material (Monel®) over a carbon steel will not make that material perform equal to Monel®. Thermal spray coatings are ideally applied with the spray gun perpendicular to the substrate surface; however, most processes can apply high-quality coatings at angles up to 45 degrees from perpendicular. Thermal sprayed coatings should not be used when the area to be repaired would see a point load or line load, such as ball bearings or roller bearings without laboratory service-proven results.

Future Applications There are processes that can accomplish the most critical applications. Vacuum spray systems perform spraying on highly critical components for the airline industry. Then there are the high-velocity oxyfuel system and the high-velocity plasma systems. Throughout the world, industry uses the thermal spray process for numerous applications. These applications include electromagnetic shielding, clearance control (abradable coatings), thermal barrier coatings, and the thermal spraying of babbitt bearings. New processes and equipment that will provide superior coatings for many future applications are also being developed by equipment and thermal spray material manufacturers.

Bibliography/Recommended Reading List American Welding Society. Guide for the Protection of Steel with Thermal Sprayed Coatings of Aluminum and Zinc and Their Alloys and Composites (C2.18). Miami, Fla.: American Welding Society. ———. Guide for Thermal Spray Operator Qualification (C2.16). Miami, Fla.: American Welding Society.

Advantages

———. Thermal Spraying: Practice, Theory, and Application (TSS). Miami, Fla.: American Welding Society.

Thermal sprayed coating properties can be tailored to suit the application. Coatings (metallic and ceramic) can be applied to restore or attain desired dimensions, to provide electrical or thermal shielding (or conduction), or to improve the resistance to abrasion, corrosion, or high temperatures. Thermal spray coatings can also be used for clearance control. Abradable coatings wear when contact is made

———. Thermal Spray Manual (TSM). Miami, Fla.: American Welding Society. Ingham, H., and Shepard, A. Plasma Flame Process. Flame Spray Handbook, vol. III, Metco Inc., Westbury, N.Y.

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Acknowledgment The original Manual was the result of a contract between Ingalls Shipbuilding and the Maritime Administration with support from the U.S. Navy. This project was performed by Puget Sound Naval Shipyard for the Welding Panel, SP-7, of the Ship Production Committee of the Society of Naval Architects and Marine Engineers. Recognized contributors to the original project included the American Welding Society, the Lincoln Electric Foundation, Mr. Omer Blodgett, and Newport News Shipbuilding. In addition, Mr. Frank Gatto, Puget Sound representative to SP-7, was singled out for appreciation. The Manual is meant to serve as a practical guide for engineers, planners, and hands-on professionals to improve scheduling and lessen rework. The Manual was released to the public in 1992.

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