| Effect of hold time on the low cycle fatigue life of 12%Cr rotor steel was studied at 873K in air atmosphere. In order to investigate the effect of creep-fatigue interaction, hold time of 10 min was applied at tensile, compressive and tensile-compressive peak strain, respectively. The experimental results show that the life of creep-fatigue cycling is shorter than that of continuous cycling. For all the tests except tensile hold time cycling, the transgranular mode fracture was observed and no grain boundary creep cavites were shown. In case of tensile hold time cycling, mixed mode(inter and transgranular) fracture was observed and creep cavities were shown at grain boundaries and lath martensite boundaries. However, regardless of hold mode, the hold time cycling 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. Especially, in the case of tensile hold time cycling, due to the fatigue crack interactions with creep cavities, the fatigue life is significantly decreased. |
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