Characterization of Hydrogen Ingress in High-Strength Alloys

Abstract

Abstract : The effect of heat treatment on irreversible hydrogen trapping was investigated for high strength steels (4340, 18Ni (250), and AerMet 100), alloy K-500, and precipitation-hardened alloys (X-750 and 18Ni (250) steel), with the goal of providing insight into the factors governing the intrinsic susceptibility to hydrogen embrittlement (HE). A potentiostatic pulse technique was used to determine irreversible trapping constants (k), which were compared with changes in strength and microstructure. Irreversible trapping in AerMet 100 is associated with two types of carbide, depending on the aging temperature. 4340 steel also undergoes a change in its principal type of irreversible trap with decreasing yield strength. The type of heat treatment for alloy K-500 can produce differences in trapping. Annealing increases k considerably, whereas aging has a negligible effect for the annealed alloy but can result in an increase for the unannealed alloy. 18Ni steel and alloy X-750 both show an increase in k with aging. Carbonitride particles provide the principal irreversible traps in the unaged alloys and appear to be one of the principal traps in aged X-750 but not in the aged steel. The order of the k values for AerMet 100, 4340, 18Ni (250), and two previously studied steels --H11 and 18Ni (300)--inversely parallels their threshold stress intensities for stress corrosion cracking. A correlation was found between k and the observed resistance to HE also for annealed/aged and direct-aged alloy K-500, alloys X-750 and 718, and 18Ni (250) steel and alloy 718.