ISSUE 4/2014

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Druschitz, A. P., Ostrander, M.; Aristizabal, R. The science of intercritically austempered ductile iron (IADI)

Intercritically austempered ductile irons (IADI) have a desirable combination of mechanical and physical properties due to transformation induced plasticity (TRIP). The microstructure of IADI consists of well-formed graphite nodules, a continuous matrix of ferrite and isolated islands of austenite. Globally, a number of researchers have worked on IADI with varying degrees of success. In this paper, the underlying science of IADI is discussed in detail. The production of IADI is dependent upon chemistry and heat treatment. Improper chemistry and/or heat treatment will result in poor hardenability (the IADI microstructure will not be obtained throughout the component thickness). The choice of chemistry is also important since the chemistry and morphology of the retained austenite phase determines the characteristics of the austenite to martensite transformation (the TRIP effect). Manganese is often used to produce retained austenite in TRIP steels. However, segregation of manganese to the last areas to solidify in ductile iron produces iron-manganese carbides, porosity and poor tensile elongation. Manganese also promotes the formation of retained austenite with a blocky morphology that transforms to martensite at low applied strains and does not produce a strong TRIP effect. On the other hand, nickel promotes the formation of retained austenite with an acicular morphology that transforms to martensite at high applied strains and produces a strong TRIP effect. Molybdenum has a strong effect on hardenability but should be kept below 0.20 wt % to prevent the formation of an excessive amount of molybdenum carbides, which decreases ductility. The heat treatment temperatures for the two steps of intercritical austempering have the opposite effect compared to conventional austempering. For IADI, the austenitizing temperature determines the relative amounts of pro-eutectoid ferrite and austenite. A pearlitic starting microstructure is required since this provides a ready source of carbon for the austenite since, at the intercritical austenitizing temperatures, dissolution of pearlite can be slow and dissolution of graphite nodules is very slow if copper is present. Since the austenitizing temperature determines the maximum possible amount of retained austenite, it determines the maximum tensile strength. Since austenite nucleates on the prior ferrite-pearlite grain boundaries during austenitizing, the size of the retained austenite is controlled by the intercritical austenitizing time and temperature. Quenching to the austempering temperature must be fast enough to prevent the formation of pearlite and water saturated salt is recommended. The austempering temperature does not play a critical role in determining the tensile properties of IADI but it does play a role in the overall ability to through harden.

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