The cyclic response behavior, failure mechanism, and life prediction model of 9% Cr‐based steel under different strain ratios in the low cyclic fatigue regime at 430°C

During steam turbine rotor operation, the startup and showdown processes can cause asymmetric loading stress, leading to fatigue damage. To research the cyclic deformation behavior and fatigue failure mechanisms of 9% Cr‐based steel under asymmetric loading conditions, the strain ratios including −1, 0, and 0.1 are introduced into the low cyclic fatigue tests at 430°C. The results show the cyclic softening behavior is observed under three strain ratios at 430°C. As the strain ratio increases, the crack initiation sources, plastic strain, and consumed energy present an increasing trend. Comparing the microstructure morphology at different strain ratios, the coarsening lath and carbides affect fatigue crack initiation. Besides, the grain rotation and dynamic recrystallization can be observed, resulting in the cyclic softening behavior during the fatigue test. Finally, a new fatigue life prediction model is proposed at 430°C. The low cycle fatigue tests are conducted under R = −1, 0, and 0.1 at 430°C. The dynamic recrystallization is observed, resulting in the cyclic softening behavior. A fatigue life prediction model considering the strain ratio is proposed at 430°C.