Stress Relaxation during Dwells for Creep and Fatigue Cycling of Type 316H Stainless Steel at 550°C

Creep-fatigue tests are used to simulate the thermally induced straining relevant to power plant applications. Experiments were conducted 316H stainless steel, with fatigue and creep fatigue tests conducted at 550 degrees C on a test specimen extracted from an ex-service component. Dwells of 50 hours duration were introduced at a number of points in the cycles to investigate the effect of dwell position and elastic follow-up on stress relaxation behaviour. This paper explores the effectiveness of using forward creep deformation laws to predict the observed relaxation behaviour with and without elastic follow-up. The predicted creep behaviour was based on using different selected creep equations including standard power law equations (e.g. Norton equation), power law creep with a back stress component introduced and also primary creep equations. It is shown that significant modifications to these equations are required to provide accurate descriptions of stress relaxation during the short duration dwells.