On the thermodynamic stability of retained austenite in 4340 steel

The effect of retained austenite on the mechanical behavior of quenched and tempered steels depends on its thermodynamic stability against mechanically-induced martensitic transformation during loading. In this work, the stability of retained austenite is characterized by the M(s)(sigma) temperature and experimentally determined values of the M(s)(sigma) temperature are compared with model predictions. The comparison suggests that significant carbon stabilization of the retained austenite occurs during tempering for both, high and low temperature, austenitizing treatments employed. In addition, it appears that the assumption of a fully-biased rather than a fully-random distribution of nucleation-site potencies is in better agreement with the observed experimental behavior. Finally, the stress state dependence of the retained austenite stability could account for the mechanical behavior variation reported in literature