Mechanisms of Intergranular Fracture During Fatigue Crack Growth in a Quenched and Tempered Steel

Abstract

Intergranular fracture during the transition from intermediate to near threshold crack growth rates has been shown to occur in a quenched and tempered steel. The inception of intergranular fracture occurs at a level of stress intensity range(ΔK) at which the cyclic plastic zone size at the crack tip becomes equal to the prior austenitic grain size. It appears that for such a situation in a low strain hardening microstructure a transgranular planar slip is required in order to transport crack tip atomic hydrogen to the grain boundary. It has also been shown that even in the absence of any similarity in the dimensions of cyclic plastic zone and the prior austenitic grains, intergranular fracture can occur by a combined action of embrittlement and environment. The threshold stress intensity range ΔKth decreases with an increase in the percentage of intergranular fracture at threshold.