Propagation of Surface Cracks under Creep and Fatigue Conditions at Elevated Temperatures

Abstract

Propagation of surface cracks under creep-fatigue conditions at elevated temperatures was studied with a SUS 304 steel and a 21/4 Cr-1Mo steel. The shape of cracks was semi-circular in most cases and the propagation rate was the same in the surface direction as in the depth direction. The creep crack propagation rates of surface cracks under constant stresses agreed with those of through-thickness cracks in the J diagram. Microscopic fracture morphology was predominantly intergranular creep type. In the cases of repeated stresses, similarly to the case of through-thickness crack previously reported, crack propagation was controlled by ΔK in the low stress region and by J in the high stress region, microscopic fracture morphology being fatigue striation type and creep dimple type, respectively. This is true, however, only for large cracks and, in the case of small cracks, crack propagation was controlled by J even in the low stress region, microscopic fracture morphology being still fatigue striation type.