Formulation of air environmental effect on creep-fatigue interaction

A series of creep-fatigue experiments has been conducted with 304 stainless steel at 650 C, 2/ (1/)//4/Cr-1Mo steel at 550 C and modified 9Cr-1Mo steel at 600 C in air and in a very high vacuum environment of 0.1μPa. A damage model based on the overstress concept was employed to evaluate the ''pure'' creep-fatigue life observed in this high vacuum environment which is completely free from the environmental effect of air. This damage model for the vacuum data was also applied to data obtained in air in order to evaluate the environmental effect of air on the creep-fatigue interaction. It was found that the fatigue damage is highly affected by the air environment, resulting in a time-rate-dependent life reduction. This life reduction is mainly controlled by the strain rate or the time duration of the compression loading stage. The environmental effect of air on the creep damage is complicated. The analyses based on this damage model suggest that the air environment accelerates the creep damage in 304 stainless steel, gives no effect in modified 9Cr-1Mo steel and reduces the creep damage in 2/ (1/)//4/ Cr-1Mo steel. ((orig.))