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 temperered marten‐site ferritic steel at 873 K in air. The stress ratio R=σ_min/σ_max ranged from‐1 (“pure” fatigue) to 1 (“pure” creep). The maximum stress σ_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 and the lifetime are controlled by mean stress σ_massof the stress cycle. In the stress ratio interval — 1 < R < 0.2 the lifetime is controlled by the stress amplitude na. 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.