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 stee l was evaluated using non-linear fracture mechanics methods. The initi ation toughness, J(i), and the resistance to stable crack growth, dJ/d a, were measured using precracked compact specimens. Specimens were el ectrochemically charged with hydrogen prior to fracture testing in air . After charging, a monotonically increasing load-line displacement wa s applied to produce the J-integral curve for stable crack growth. Cra ck length was monitored by the direct-current electric potential metho d; current switching eliminated thermal voltage contributions. The fra cture properties of PH 13-8 Mo stainless steel were severely degraded by hydrogen. J(i) for material in the hydrogen-charged condition was d egraded by as much as 98% compared to the uncharged condition. The fra cture mode exhibited dramatic transitions from microvoid coalescence i n uncharged specimens to intergranular cracking in charged specimens t ested at slow displacement rate. Toughness tests performed at higher d isplacement rates exhibited less susceptibility and failed by a mixed mode of transgranular facets and microvoid coalescence. The rate depen dence is an indication that the higher displacement rates limit hydrog en transport to the crack-tip process zone. The contributions of hydro gen transport by diffusion and by dislocation transport to the crack-t ip process zone are discussed. The displacement rate is related to a l ocal controlling crack-tip parameter, or crack-tip strain rate.