Model for the maximum fraction of retained austenite in austempered ductile cast iron

Austempered ductile irons usually contain a large quantity of retained austenite that can help to optimise their mechanical properties. A generic model has been created that enables the estimation of the maximum fraction of retained austenite as a function of the chemical composition and heat treatment. It is found that the maximum fraction of retained austenite depends on two competing effects. The first is that austenite is stabilised by transforming to bainite at lower temperatures, since, consistent with the T0 curve, the limiting carbon concentration of the austenite increases as the transformation temperature is reduced. The counteracting effect is that the formation of bainitic ferrite itself consumes austenite, so less is available for retention on cooling to ambient temperature. The model has been validated by comparison with experiments on homogenised samples of cast iron.