• Author / Uploaded
• chaitanyachegg

Citation preview

Chapter 4

Metal Alloys: Structure and Strengthening by Heat Treatment QUALITATIVE PROBLEMS 4.16 You may have seen some technical literature on products stating that certain parts in those products are “heat treated.” Describe briefly your understanding of this term and why the manufacturer includes it. Heat treating, in general, subjects the alloys to controlled heating and cooling cycles to produce a microstructure that improves the mechanical properties of the alloy. Manufacturers mention heat treating because it generally implies an improvement in the properties of the parts, particularly strength, hardness, and wear resistance, although the process is usually accompanied by an increase in cost. 4.17 Describe the engineering significance of the existence of a eutectic point in phase diagrams. A eutecic point corresponds to a composition that has the lowest melting temperature for that alloy system. The low melting temperature associated with a eutectic point is an important aspect of soldering, and also helps in controlling thermal damage to parts being joined. 4.18 What is the difference between hardness and hardenability? Hardness represents the material’s resistance to permanent indentation (Section 2.6 starting on p. 67), whereas hardenability is the material’s capability to be hardened by heat treatment processes. 4.19 Referring to Table 4.1, explain why the items listed under typical applications are suitable for surface hardening. Surface hardening is useful in increasing wear resistance, fatigue resistance, or indentation resistance, without producing a part that is hard and brittle throughout (which would result in low toughness). The parts listed under typical applications would either be exposed to high wear conditions (tools, dies, and gears), cyclic loading (rotating shafts and cams), or where surface damage would render the parts useless (bolts, gears, cams). 51 © 2014 Pearson Education, Inc. Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction ,storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to : Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

Metal Alloys: Structure and Strengthening by Heat Treatment

52

4.20 It generally is not desirable to use steels in their as-quenched condition. Explain why. Steels are rarely used in their as-quenched condition because they are very brittle and thus lack toughness. These detrimental conditions are overcome by tempering the steel, which restores toughness. 4.21 Describe the differences between case hardening and through hardening, insofar as engineering applications of metals are concerned. Case hardening is a treatment process that hardens only the outer surface of a part; the bulk retains its toughness, which allows for blunting of surface cracks as they propagate to the core. Case hardening generally induces a residual compressive stress on the workpiece surface which, in turn, helps retard fatigue crack initiation. Through-hardened parts have a high hardness level across the whole part; consequently, a crack could propagate easily through the cross-section of the part, causing failure. 4.22 Describe the characteristics of (a) an alloy, (b) pearlite, (c) austenite, (d) martensite, and (e) cementite. i. Alloy: Composed of two or more elements, at least one of which is a metal. The alloy may be a solid solution or it may form intermetallic compounds. ii. Pearlite: A two-phase aggregate consisting of alternate lamellae of ferrite and cementite. The closer the pearlite spacing of the lamellae, the harder the steel will be. iii. Austenite: Called gamma iron, it has a face-centered cubic structure. The fcc structure allows for higher solubility of carbon in the crystal lattice. This structure also possesses a high level of ductility, which increases the steel’s formability. iv. Martensite: Forms by quenching austenite. It has a body-centered tetragonal (bct) structure, and carbon atoms in interstitial positions impart high strength to the structure. It is very brittle and hard. v. Cementite, also known as iron carbide (Fe3 C). Cementite is a hard and brittle phase. 4.23 Explain why carbon, among all elements, is so effective in imparting strength to iron in the form of steel. The size of the carbon atom allows it to have high solubility in the high-temperature fcc phase of iron (austenite). At low temperatures, the structure is bcc and has very low solubility of carbon atoms. Upon quenching, the austenitic structure transforms to bct martensite, which produces a large amount of distortion in the crystal lattice, enough to allow the solubility of carbon, but not other larger atoms. 4.24 How does the shape of graphite in cast iron affect its properties? The shape of graphite in cast iron has the following basic forms: i. Flakes: Have sharp edges which act as stress raisers. The shape makes cast iron low in tensile strength and ductility, but it still has good compressive strength. The flakes also act as vibration dampers. ii. Nodules: Spheroids formed by graphite when magnesium or cerium is added to the melt. This form has increased ductility, strength, and shock resistance over flakes, but the damping capacity is reduced.

© 2014 Pearson Education, Inc. Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction ,storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to : Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

Metal Alloys: Structure and Strengthening by Heat Treatment

53

iii. Clusters: Much like nodules, except they form from the breakdown of white cast iron upon annealing. Clusters have properties similar to flakes. iv. Compacted flakes: Short thick flakes with rounded edges. This form has properties that are between nodular and flake graphite. 4.25 In Section 4.8.2, several fluids are listed in terms of their cooling capacity in quenching. Which physical properties of these fluids influence their cooling capacity? The main physical properties of the fluids that influence their cooling capacity are thermal conductivity and specific heat. Agitation (rapid movement of the quenching medium) is an effective way of increasing the cooling capacity of the quenching medium. 4.26 Why is it important to know the characteristics of heat-treating furnaces? Explain. The size, shape, and heating media of heat-treating furnaces make them useful in various applications. For example • Batch: Usually large furnaces that allow a large number of parts to be treated simultaneously. Batch furnaces are important for parts such as bolts or cams that are produced in large but finite quantities. • Continuous: Offers close control over heating cycles. Some parts have complex heating cycles, requiring controlled heating and cooling rates to develop desired microstructures, and this can be best achieved in a continuous furnace. • Gas-fired: Can be used for gas carburization of parts. Carburization is a valuable hardening process that can be used for gears, cams, etc. • Electric: Offers closest control over furnace atmospheres. Sometimes it is important to exclude oxygen or nitrogen to avoid oxidation or the formation of nitrides during heat treating. 4.27 Explain why, in the abscissa of Fig. 4.16c, the percentage of pearlite begins to decrease after 0.8% carbon content is reached. Pearlite is a eutectoid transformation of steel that occurs at 0.77 weight percent carbon. Its microstructure consists of about 88% ferrite and 12% cementite. As the carbon content is increased, more than 12% of cementite is formed. The microstructure consists of pearlite and excess cementite, and the excess cementite reduces the percentage of pearlite. 4.28 What is the significance of decarburization? Give some examples. Decarburization results in a loss of carbon from the surface layers of a part. The lower carbon at the surface consequently results in lower strength and hardness. Fatigue life and wear resistance are also reduced. If a bolt or screw is decarburized during heat treatment, the tendency to strip the threads will be increased. Decarburization is especially harmful in the heat treatment of tool and die steels, since the softer surface would have less wear resistance. 4.29 Explain your understanding of size distortion and shape distortion in heattreated parts, and describe their causes.

© 2014 Pearson Education, Inc. Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction ,storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to : Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

Metal Alloys: Structure and Strengthening by Heat Treatment

54

Because of microstructural changes during heat treatment and variations in the rate of heating and cooling in different regions of a part, heat treatment can cause distortions. Size distortion involves changes in the dimensions of the part without a change in shape, whereas shape distortion involves bending, twisting, and similar nonsymmetrical dimensional changes. These are illustrated below; the red image shows a size distortion, although greatly exaggerated compared to shrinkage from heat treating or casting, for example. The green image shows a shape that has distorted, but the nominal size of the part has remained constant. Size distortion:

Shape distortion:

4.30 Comment on your observations regarding Fig. 4.18b. Several observations can be made: (a) Hardness decreases with increasing distance from the quenched end, due to the slower cooling rate. (b) For plain-carbon steels, hardness increases with increasing carbon content, as shown in Fig. 4.18a on p. 116. (c) The hardness is higher for nickel- and chrome-alloy steels (see Table 5.3 on p. 138 and the discussion of austenite and ferrite formers on p. 109), with nickel having a greater effect on hardenability.

QUANTITATIVE PROBLEMS 4.31 Design a heat-treating cycle for carbon steel, including temperature and exposure times, to produce (a) pearlite–martensite steels and (b) bainite– martensite steels. The heat-treat cycle for these conditions can be obtained from Fig. 4.17c. For part (a), it is desired to produce a pearlite-martensite steel, so it is important that the cooling rate be maintained between 140◦ and 35◦ C/s when cooling the material from the eutectoid temperature. Such a cooling rate can be achieved with a salt or oil quench, where the bath temperature will determine the cooling rate and the ultimate percentage of pearlite and martensite. For part (b), it is desired to have bainite, which forms under very rapid cooling (see the discussion on p. 112). Thus the two heat-treat cycles desired can be sketched as shown below.

© 2014 Pearson Education, Inc. Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction ,storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to : Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

Metal Alloys: Structure and Strengthening by Heat Treatment

55

800 1400

Eutectoid temperature

700

1200

TTT diagram to produce martensite and pearlite

500

35

°F

TTT for bainite

/s

300

1000 800

s °C/

400

140 °C

Temperature (°C)

600

600 400

200 100 Martensite 0 10-1

1

Martensite + pearlite 10

102

200 Pearlite 103

104

105

Time (s)

4.32 Using Fig. 4.4, estimate the following quantities for a 75% Cu–25% Ni alloy: (a) the liquidus temperature, (b) the solidus temperature, (c) the percentage of nickel in the liquid at 1150◦ C (2102◦ F), (d) the major phase at 1150◦ C, and (e) the ratio of solid to liquid at 1150◦ C. i. Liquidus temperature: 1400 ◦ C (2550 ◦ F). ii. Solidus temperature: 1372 ◦ C (2500 ◦ F). iii. At 1400 ◦ C (2550 ◦ F) the alloy is still all liquid, thus the nickel composition is 80% . iv. The major phase at 1400 ◦ C is liquid, with no solids present since the alloy is not below the liquidus temperature. v. The ratio is zero, since no solid is present. 4.33 Extrapolating the curves in Fig. 4.14, estimate the time that it would take for 1080 steel to soften to 40 HRC at (a) 300◦ C and (b) 400◦ C. From the graph of hardness of tempered martensite, for 200 ◦ C the time is 107 s and for 300 ◦ C it is 104 s. 4.34 A typical steel for tubing is AISI 1040, and one for music wire is 1085. Considering their applications, explain the reason for the difference in carbon content. Music wire is formed by wire drawing processes (see Sections 15.7 through 15.10), and the combination of high carbon content and large amount of work hardening (that accompanies the drawing process) gives the wire a very high yield stress. The high yield strength is necessary to allow the strings to be pulled in tension to obtain the proper pitch. Tubing requires higher ductility for subsequent forming operations (such as bending, flanging, and bulging) where it undergoes more rigorous deformation. The lower carbon content gives the steel the required ductility for processing.

© 2014 Pearson Education, Inc. Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction ,storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to : Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

Metal Alloys: Structure and Strengthening by Heat Treatment

56

SYNTHESIS, DESIGN AND PROJECTS 4.35 It was stated in this chapter that, in parts design, sharp corners should be avoided in order to reduce the tendency toward cracking during heat treatment. If it is essential for a part to have sharp corners for functional purposes, and it still requires heat treatment, what method would you recommend for manufacturing this part? Cracking could be eliminated by having a sufficiently low cooling rate to avoid thermal shock. A lower cooling rate could be achieved by using a less severe quenching medium, such as air or oil instead of water; die quenching also may be beneficial. Sharp corners can be produced by subsequent machining or grinding of the heat-treated part without any danger of cracking. 4.36 The heat-treatment processes for surface hardening are summarized in Table 4.1. Each of these processes involves different equipment, procedures, and cycle times; as a result, each incurs different costs. Review the available literature, contact various companies, and then make a similar table outlining the costs involved in each process. By the student. Specific costs will vary with location. Costs will also vary with the number and size of parts, specific processing parameters, and the required hardened depth. 4.37 It can be seen that, as a result of heat treatment, parts can undergo size distortion and shape distortion to various degrees. By referring to the Bibliography at the end of this chapter, make a survey of the technical literature and report quantitative data regarding the distortions of parts having different shapes. By the student. This problem appears straightforward, but it is quite challenging because of the call for quantitative data. Distortions will be given as tolerances or deflections, and these need to be translated to particular shapes by the students to gain an appreciation of shape distortions. 4.38 Figure 4.18b shows hardness distributions in end-quench tests, as measured along the length of the round bar. Make a simple qualitative sketch showing the hardness distribution across the diameter of the bar. Would the shape of the curve depend on the bar’s carbon content? Explain. Hardness profiles will be somewhat similar to the curves shown in Fig. 4.20b on p. 117, with the abscissa indicating the distance from the outer diameter, instead of the distance from the quenched end. The shape of the curve will depend on the carbon content since the hardness of martensite increases greatly with increasing carbon content. The magnitude of the hardness will depend on the position along the length of the bar. However, because the radius is smaller than the length, the difference in the cooling rate between the outside radius and the center will not be as high as the differences along the length. An acceptable qualitative curve is as shown below. Note that the curve is increasing, and one expects higher hardness at the outside radius than at the center.

© 2014 Pearson Education, Inc. Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction ,storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to : Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

Metal Alloys: Structure and Strengthening by Heat Treatment

57

70

Hardness (HRC)

60

4340 4140

50 40 30 20 10

12.5 mm

0 0

0.25

0.5 in.

Radius

4.39 Throughout this chapter, you have seen specific examples of the importance and the benefits of heat-treating parts or certain regions of parts. Refer to the Bibliography at the end of this chapter, make a survey of the heattreating literature, and then compile several examples and illustrations of parts that have been heat treated. By the student. There are numerous examples of heat-treated parts; for example, cutlery, gear teeth, nuts and bolts, hand tools, shafts, tools and dies, crankshafts, sprockets, springs, and cams. Most parts that require wear resistance have been heat treated to increase their hardness. In addition, applications where impacts occur and could lead to surface damage often use hardened parts. 4.40 Refer to Fig. 4.24, and think of a variety of other part shapes to be heat treated, and design coils that are appropriate for these shapes. Describe how different your designs would be if the parts have varying shapes along their length (such as from a square at one end to a round shape at the other end). By the student. For constant cross-sections, the coils can closely match the contour of the part, and this represents a fairly straightforward design problem. If the cross-section varies, there are a number of possible solutions, such as: • Using a series of coils that have a contour matching the profile at a given axial location. Thus, the part could be inserted into the coil over the entire length to be heat treated, and it could be treated along the entire surface at one time. • A coil can be used that is compliant, either because of a helix integrated into the coil (like with a spring) or because of geometry as in the support for the coil ends. • Instead of a continuous coil, a series of coil segments can be used, similar to the aperture for a camera. 4.41 Inspect various parts in your car or home, and identify those that are likely to have been case hardened. Explain your reasons. By the student. As discussed in the chapter, parts are through hardened when the mechanical properties through the thickness need to be improved, and in case hardening they are hardened to a certain depth. Case hardening is desirable when the surface

© 2014 Pearson Education, Inc. Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction ,storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to : Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

Metal Alloys: Structure and Strengthening by Heat Treatment

58

should be hard but the substrate should maintain ductility. Examples are gears (as in automobile transmissions), knives, ice skate blades, hammers, screwdriver bits, nuts and bolts and woodworking tools such as drills and saws. Case hardening gives wear resistance while preserving ductility and resistance to stress concentrations.

© 2014 Pearson Education, Inc. Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction ,storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to : Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.