Predicting the Ms temperature of steels with a thermodynamic based model including the effect of the prior austenite grain size

A thermodynamic based model for predicting the martensite-start temperature (M-s) of steels has been developed that accounts for variations in the austenite grain size (D-gamma). This is achieved by introducing two additional energy terms in the physical expression of the critical driving force proposed by Ghosh and Olson. Since grain refinement leads to stronger austenite, a higher driving force and thus a lower M-s is required to initiate the shear transformation. This first mechanism is described by a Hall Petch strengthening term. Secondly, the aspect ratio of martensitic units, i.e. laths, increases when D-gamma becomes lower than a critical diameter D-c. This implies more stored energy and therefore a higher driving force is required which also contributes to a decrease of M-s. Model calculations show a good agreement with experimental dependencies between M-s and D-gamma. Although the developed Ms model has a strong thermodynamic basis, predictions can be made without having access to thermodynamic calculation software and databases. Instead, for a certain critical driving force the corresponding Ms can be calculated using a simple composition dependent relationship that has been adequately validated with thermodynamic calculations. Model parameters have been optimized by fitting against experimental data of more than 100 alloys. Benchmarking of model predictions against calculations made with various empirical models from literature demonstrates a significant improvement in accuracy. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.