Cyclic constitutive modeling of 316LN stainless steel considering thermal aging mechanism

Tensile properties and low cycle fatigue behavior of 316LN austenitic stainless steel were investigated after varied thermal aging durations at 773 K up to 30000 h. After thermal aging for 30000 h, the material exhibits remarkable degradations in both the yield stress and ultimate tensile strength at room temperature and 623 K, and there is a significant decrease in cyclic hardening level at 623 K. These facts indicate that the long-term thermal aging treatment induces softening of this material, which results in the decrease of plastic strain energy density under low cycle fatigue test and the prolongation of fatigue life. From the observation of microstructures, it is found that in the aged material, there existed differences in dislocation structure, the increase of grain size, the transformation of second phase distribution, and the decrease of grain boundaries, which are the significant reasons for the decreasing of cyclic hardening. By introducing the evolution of grain size and thermal aging effect, a modified visco-plasticity constitutive model based on the Ohno-Wang II kinematic hardening rule is proposed and successfully used to predict the cyclic behavior of virgin and thermal aged material at both room and elevated temperatures.