Interaction of high cycle fatigue and creep in 9%Cr-1%Mo steel at elevated temperature

High-cycle-fatigue/creep experiments were performed on a 9%Cr-1%Mo tempered martensite ferritic steel at 873 K in air. The stress ratio R = σmin/σmax ranged from -1 (''pure'' fatigue) to 1 (''pure'' creep). The maximum stress σmax 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 and the lifetime are controlled by mean stress σmean of the stress cycle. In the stress ratio interval -1 < R < 0.2 the lifetime is controlled by the stress amplitude σa. The fatigue/creep interaction occurs in between these intervals. The fatigue/creep loading induces transformation of the tempered martensite ferritic structure into an equiaxed subgrain structure. The resulting subgrain size depends strongly on the stress ratio. (author)