Transformation of residual austenite under thermomechanical variations

The austenite to martensite transformation seems to have an influence on the resistance to micro-pitting. Then, the residual austenite transformation was studied for different thermal and thermomechanical conditions. Three homogeneous steels (80, 50 and 27MnCr5) exhibiting three γrés fractions at room temperature were studied since their composition correspond respectively to the surface layer, the Hertz zone and the bulk material of a carburized part. In this paper we only present the 80MnCr5 results. Tempering under tensile stress tests and compression fatigue tests were performed on samples initially treated according to four heat treating procedures (in order to get different residual austenite amounts and stabilities). The compression fatigue tests results revealed the influence of the initial γrés amount, the applied stress and the internal stress state on the γrés transformation kinetics. The tempering under tensile stress experiments showed that the start temperature of transformation as well as the transition carbides precipitation kinetics, are not modified by the applied stress, whereas they do in the case of the austenite transformation. Moreover they allowed us to establish a destabilisation criterion : residual austenite transforms when the applied stress is close to the yield stress of the austenite (depending on the deformation rate)