Effect of hold time on low cycle fatigue behavior of 12%Cr rotor steel at 873K was studied in air. Total strain amplitude was controlled to be 1, 1.5, 2, and 2.5% with zero mean strain. In order to investigate the effect of creep-fatigue interaction, hold time of 10 min. at tensile, tensile-compressive, and compressive peak strain was added.
The experimental results show that the life of hold time fatigue is shorter than that of continuous cycling. For all the tests except tensile hold time fatigue, the transgranular mode fracture was observed and no grain boundary creep cavites were shown. In the case of tensile hold time fatigue mixed mode (inter and transgranular) fracture was observed and creep cavities were shown. at grain boundaries and lath martensite boundaries.
But, regardless of hold mode, the hold time decreased the fatigue endurance.
From the experimental results, the decrease in the fatigue life with hold time can be explained as follows : Oxidation during hold time decreases the number of cycles for crack initiation. Since the recovery during hold time reduces the flow stress, the crack tip opening displacement is enlarged. By that way, the crack growth rate is increased. Regardless of hold mode, two effects mentioned above reduced the fatigue life. Moreover, in the case of tensile hold time fatigue, since the fatigue crack interacts with cavities, the fatigue life is significantly decreased.
