The fatigue lives of in phase thermo mechanical fatigue (TMF) and isothermal fatigue (IF) were carried out in cast hot
work die steel in stress control in this paper. It is found that the lives obtained under in phase thermomechanical fatigue condition (at a
temperature interval of 250℃～550℃) are lower than those obtained under isothermal fatigue condition (at 550℃) at the same stress amplitude. It can be concluded that the TMF of Cr Ni Mo cast hot work die steel brings more damage than IF at the same stress amplitude. Therefore, if temperature varies, it is not reliable to design on the basis of dada from the isothermal fatigue at maximum temperature. Under IF conditions, the path of the crack initiation and growth are mainly transgranular. The intergranular cracks dominate in TMF tests under the lower stress amplitude and the feature of transgranular propagation occurs with the increment of the stress amplitude. A model of life
prediction, based on Chaboche law, was discussed. Damage coefficients that are functions of the maximum temperature and the variation of temperature are introduced in the model so as to evaluate TMF lives in load control. Considering the additional stress caused by the varying temperature under the thermo mechanical fatigue, the damage coefficients per cycle can be described by a function with not only maximum stress and mean stress, but also maximum temperature and temperature range. According to the equivalent temperature method in accumulated damage, the maximum temperature was taken as the equivalent temperature in this paper. It is considered that the additional damage produced by the variation of temperature may be simulated by exponential coefficient. With this method, the lifetime prediction gives results corresponding well to experimental data.