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Technical Information 26 Fading of Nodularity in Ductile Iron When properly treated and inoculated ductile iron is held for prolonged times, it is common to observe deterioration in the nodule shape of the graphite. This is often referred to as fading of nodularity. Fading of nodularity is typically related to one of two possible phenomena, either fading of magnesium or fading of inoculation. It is important that the correct type of fading is pinpointed, as possible cures to avoid poor nodularity during time will differ greatly between the two fading phenomena. The micrograph below shows a good ductile iron microstructure immediately after magnesium treatment and post inoculation. The two lower micrographs show examples of microstructures for the two principal fading mechanisms.

Magnesium fading

Inoculant fading

Compacted graphite from magnesium fading

Irregular shaped graphite from inoculant fading

Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway

Office address Hoffsveien 65B Oslo Norway

© Copyright Elkem ASA Telephone +47 22 45 01 00 Telefax +47 22 45 01 52

Web www.foundry.elkem.com Org. no. NO 911 382 008 MVA

Revision No. 2.1 20.03.2004

Technical Information 26

2

Magnesium fading effects: When magnesium is lost to the liquid metal surroundings (e.g. slag, refractory, evaporation) it may cause a loss in nodularizing power over time. This is typically related to a loss of residual magnesium content in the iron, or even a pick-up of sulphur or oxygen from the surroundings, causing a gradual consumption of the residual magnesium. When residual magnesium becomes insufficient, poor nodules referred to as compacted graphite or vermicular graphite will occur. An example of this is given in the left micrograph. Residual magnesium and effects of Mg-fading is described in more detail in Elkem Technical Information Sheet 7. Inoculant fading effects: When the metal is held for prolonged times after addition of post inoculant, the effects of the inoculant will gradually fade. Depending on the type of inoculant and the addition rate, the ductile iron will gradually lose its nodule count and the nodules will also lose their spherical shape. Typically irregular shaped nodules are resulting, associated with a general drop in nodule count. An example of this is given in the right micrograph. Fading of inoculation is described in more detail in Elkem Technical Information Sheet 6. When discovering a poor nodularity condition, it is important for the trained foundry operator to clearly distinguish between the two types described. Cures for the two types of poor nodularity may in some instances actually be converse to each other. Possible cures for Mg-fading: Magnesium fading causing compacted graphite shapes can be cured by the following actions: •

Increase residual magnesium and/or rare earth’s by adding more nodulariser.

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Avoid high metal temperatures and long holding times.

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Reduce base iron sulphur content by using purer charge materials and additives.

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Improve slag skimming operations to avoid resulphurising of the iron.

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Avoid metal exposure to air causing oxidation of residual magnesium.

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Using a RE-containing post inoculant to compensate for Mg-losses.

Possible cures for inoculant fading: Inoculant fading causing irregular shaped graphite and a low nodule count can be cured by the following actions: •

Use a more powerful, fade resistant post-inoculant or add more inoculant

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Avoid high holding temperatures and long holding times

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Use a second, late addition of a powerful specialty post-inoculant material

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Sometimes irregular shaped nodules and low nodule counts may be a result of excessive magnesium or nodulariser addition, – i.e. reduced Mg-addition may improve the nodularity.