ISSUE 3/2010

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Mouquet, O.
Contribution to the study of a transformation of an ausferritic SG cast iron in the CCT and TTT modes

With thick castings, or when the quenching medium is not drastic, the cooling path of the casting may include a relatively slow CCT mode and a TTT mode. This "thermal path" results in the formation of intermediate pearlite or ausferrite structures before the isothermal plateau. This study has shown that two types of ausferrite are deposited, at two different times. It has been shown that such a structure responded differently during the TTT plateau, in particular at the moment of activation of stage II, which can become very early. With such a structure, the metallographic examination cannot readily distinguish upper ausferrites. If pearlite appears, the diagnostic of an inappropriate heat treatment or of deficient hardenability of the cast iron is readily established.       CCT type upper region ausferrite is less stable during the TTT hold. And it is very likely to be deposited during rather slow cooling. If this happens, the TTT hold must be reduced or at least weighted according to the kinetics at the skin, since otherwise the elongation and impact strength of the cast iron may be impaired.

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