Cyclic hardening and softening of high-strength steels

316L and P91 steels, commonly used for high-temperature applications in power plants, experience low-cycle fatigue. In this work, the cyclic stress-strain behaviour of 316L and P91 materials was analyzed. Experimental data from the material's hysteresis loops and cyclic stress response were used to derive material parameters for numerical simulation of the cyclic behaviour. The displacement-controlled model with strain amplitudes of 0.4% and 0.6% and a constant strain rate of 0.001 s-1 was used in the simulation. The simulation results show that 316L stainless steel exhibits cyclic hardening, with the maximum stress increasing from 311.3 MPa in the first cycle to 354.7 MPa at the half-cycle, corresponding to approximately 14% cyclic hardening. In contrast, P91 steel exhibits cyclic softening of about 8.3%, as the stress decreases from 497.9 MPa in the first cycle to 456.7 MPa at the half cycle. The findings also demonstrated that tension and compression loadings with larger strain amplitudes produced higher maximum stress. It also demonstrates that higher strain amplitudes result in a greater decrease in stress near the half cycle because hardening/softening is more pronounced at higher strain amplitudes.