Creep Fracture Behavior and Metallurgical Factors affecting Rupture Ductility of 0.5 Mo Steel

Abstract

Creep fracture behavior and metallurgical factors affecting rupture ductility have been studied for the improvement of rupture ductility of 0.5Mo steel. The heats of the steel exhibit minimum rupture ductilities of 310% in elongation and 413% in reduction of area when tested at 450° and 500°C.
The decreases in ductilities correspond with the transition in fracture modes from transgranular to intergranular creep fractures. Intergranular creep fracture is caused by grain boundary cracking such as wedge-type one under higher stresses and by formation of cavities associated with grain boundary carbide particles under lower stresses. The effect of surface cracks on rupture properties is relatively insignificant although the mean length of the cracks is much longer than those of the grain boundary cracks and cavities.
Main factors affecting the grain boundary cracking and the cavitation in 0.5Mo steel are grain size, amounts of impurities, and size and distribution of grain boundary carbide particles. It was confirmed that higher ductility than that of 1Cr-0.5Mo steel is easily attained without reduction in rupture strength by means of controlling these factors.