Transformation of residual austenite under thermomechanical variations

The austenite to martensite transformation seems to have an influence on th e resistance to micro-pitting. Then, the residual austenite transformation was studied for different thermal and thermomechanical conditions. Three ho mogeneous steels (80, 50 and 27MnCr5) exhibiting three gamma(r epsilon s) f ractions at room temperature were studied since their composition correspon d respectively to the surface layer, the Hertz zone and the bulk material o f a carburized part. In this paper we only present the 80MnCr5 results. Tem pering under tensile stress tests and compression fatigue tests were perfor med on samples initially treated according to four heat treating procedures (in order to get different residual austenite amounts and stabilities). Th e compression fatigue tests results revealed the influence of the initial g amma(r epsilon s) amount, the applied stress and the internal stress state on the gamma(r epsilon s) transformation kinetics. The tempering under tens ile 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 transf ormation. Moreover they allowed us to establish a destabilisation criterion : residual austenite transforms when the applied stress is close to the yi eld stress of the austenite (depending on the deformation rate).