ON THE THERMODYNAMIC STABILITY OF RETAINED AUSTENITE IN 4340-STEEL

The effect of retained austenite on the mechanical behavior of quenche d and tempered steels depends on its thermodynamic stability against m echanically-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 austen ite occurs during tempering for both, high and low temperature, austen itizing treatments employed. In addition, it appears that the assumpti on of a fully-biased rather than a fully-random distribution of nuclea tion-site potencies is in better agreement with the observed experimen tal behavior. Finally, the stress state dependence of the retained aus tenite stability could account for the mechanical behavior variation r eported in literature.