Kinetics of martensite formation in plain carbon steels: critical assessment of possible influence of austenite grain boundaries and autocatalysis

Abstract

The kinetics of the martensitic transformation in Fe–0·80C has been determined from dilatometry data and shows no significant variation when the cooling rate is changed by two orders of magnitude. All kinetic data can be adequately simulated by the Koistinen and Marburger (KM) equation using a specific start temperature T_KM and rate parameter α_m. This finding supports the suggestion that the transformation is athermal, and moreover, the absence of a time dependence strongly indicates that autocatalytic nucleation does not contribute to the transformation kinetics in plain carbon steels on measurable time scales. Furthermore, dilatometry experiments with different austenitising conditions were conducted to examine the effect of the prior austenite grain size on the overall kinetics of martensite formation. The present results indicate that the progress of martensite formation beyond a fraction f?=?0·15 is independent of the prior austenitising treatment. It is therefore concluded that austenite–austenite grain boundaries have no significant effect on the overall nucleation and growth of athermal martensite, which is consistent with a model proposed by Ansell and co-workers.