Phase transition kinetics in Austempered Ductile Iron (ADI)

A wide range of properties in Austempered Ductile Iron (ADI) components are related to changes in the proportions and morphology of the phases present in the heat-treated microstructure. The final microstructure (and thus the properties) strongly depends on composition, as-cast microstructure and phase transformation kinetics during austempering. An optimum combination of high carbon austenite with bainitic ferrite confers excellent mechanical properties to such cast iron. The phase changes with chemical composition and heat treatment, make it possible to produce a whole family of ADI. This in turn allows a wide range of applications with ADI competing favorably against steel forgings and aluminum alloys in terms of mechanical properties, manufacturing cost, physical properties and weight saving. In this study, ADI samples were austempered (heat treated) and the phase transitions were analyzed in situ during the process and ex-situ after interrupted austempering. The phase fractions (austenite, ferrite, martensite, cementite, graphite, etc.) and their relation to bulk properties, like electrical resistivity, magnetic properties and mechanical properties (e. g. strength), and others were measured using SEM and TEM, XRD, neutron diffraction and Mössbauer spectroscopy, dilatometer, and nanoindentation. This combination of complementary techniques allows the correlation of the phase transition kinetics with the resulting properties. This will finally be used to optimize the heat treatment conditions, namely the heat treatment window (HTW).     «

A wide range of properties in Austempered Ductile Iron (ADI) components are related to changes in the proportions and morphology of the phases present in the heat-treated microstructure. The final microstructure (and thus the properties) strongly depends on composition, as-cast microstructure and phase transformation kinetics during austempering. An optimum combination of high carbon austenite with bainitic ferrite confers excellent mechanical properties to such cast iron. The phase changes with...     »