Creep-Fatigue Interaction of Modified 9Cr-1Mo Steel in a Very High Vacuum Environment and Experimental Analysis of Overstress

Creep-fatigue tests were conducted with Modified 9Cr-lMo steel at 600°C in a very high vacuum of 0.1 μPa in order to investigate a pure creep-fatigue behavior which is free from the environmental effect of the air. On the whole, the time-dependent life reduction occurs when the duration of tension is longer than that of compression. A good correlation was found between the creep-fatigue life and the fracture mode. When the fracture mode is transgranular, no time-dependent life reduction occurs. In contrast, when it is predominantly intergranular, a significant life reduction is observed. The overstress was experimentally analyzed to evaluate the creep-fatigue damage. The relationship between the overstress and the inelastic strain rate can be fitted, being independent of the strain range.