Transformation plasticity of retained austenite in stage-I tempered martensitic steels

Abstract

In the present study, controlled experiments were designed to isolate the role of retained austenite and investigate the associated transformation plasticity effects on shear instability, which is the precursor to fracture in these ultrahigh-strength steels. The stability of the retained austenite is the most important parameter controlling its transformation plasticity and toughening effects. The M/sub s//sup {sigma}/ temperature is adopted here as a quantitative characterization of the stability of retained austenite. It is the temperature at which the transformation occurs at the yield stress of the parent phase. Below M/sub s//sup {sigma}/, the transformation is stress-assisted and nucleation occurs at the same pre-existing nucleation sites responsible for the transformation on cooling. Above M/sub s//sup {sigma}/, yielding of the austenite by slip generates new nucleation sites and the transformation is strain-induced. The M/sub s//sup {sigma}/ temperature is a function of stress state due to the interaction of the transformation volume change with stress triaxiality. Recent experiments in metastable fully-austenitic steels have demonstrated that transformation plasticity can substantially enhance uniform ductility and toughness when fracture is controlled by shear instability. These property improvements appeared near the corresponding M/sub s//sup {sigma}/ temperatures for the uniaxial tension and crack-tip stress states. It is, therefore,more » necessary to determine the stress-state dependence of the M/sub s//sup {sigma}/ temperature before assessing the effects of transformation plasticity on mechanical properties.« less