Hydrogen-assisted cracking in a precipitation-hardened stainless steel: effects of heat treatment and displacement rate

The hydrogen embrittlement susceptibility of PH 13-8 Mo stainless steel was evaluated using non-linear fracture mechanics methods. The initiation toughness, Ji, and the resistance to stable crack growth, dJ/da, were measured using precracked compact specimens. Specimens were electrochemically charged with hydrogen prior to fracture testing in air. After charging, a monotonically increasing load-line displacement was applied to produce the J-integral curve for stable crack growth. Crack length was monitored by the direct-current electric potential method; current switching eliminated thermal voltage contributions. The fracture properties of PH 13-8 Mo stainless steel were severely degraded by hydrogen. Ji for material in the hydrogen-charged condition was degraded by as much as 98% compared to the uncharged condition. The fracture mode exhibited dramatic transitions from microvoid coalescence in uncharged specimens to intergranular cracking in charged specimens tested at slow displacement rate. Toughness tests performed at higher displacement rates exhibited less susceptibility and failed by a mixed mode of transgranular facets and microvoid coalescence. The rate dependence is an indication that the higher displacement rates limit hydrogen transport to the crack-tip process zone. The contributions of hydrogen transport by diffusion and by dislocation transport to the crack-tip process zone are discussed. The displacement rate is related to a local controlling crack-tip parameter, or crack-tip strain rate.