INTERACTION OF HIGH-CYCLE FATIGUE AND CREEP IN 9-PERCENT-CR-1-PERCENT-MO STEEL AT ELEVATED-TEMPERATURE

High-cycle-fatigue/creep experiments were performed on a 9%Cr-1%Mo tem perered martensite ferritic steel at 873 K in air. The stress ratio R = sigma(min)/sigma(max) ranged from -1 (''pure'' fatigue) to 1 (''pure '' creep). The maximum stress sigma(max) was kept constant at 240 MPa. The lifetime depends on the stress ratio R in a non-monotonic way. In the stress ratio interval 0.6 < R < 1.0 both the creep strain rate an d the lifetime are controlled by mean stress sigma(mean) of the stress cycle. In the stress ratio interval -1 < R < 0.2 the lifetime is cont rolled by the stress amplitude sigma(a). The fatigue/creep interaction occurs in between these intervals. The fatigue/creep loading induces transformation of the tempered martensite ferritic structure into an e quiaxed subgrain structure. The resulting subgrain size depends strong ly on the stress ratio.