Effect of heat treatment on hydrogen trapping in alloy K-500

The effect of heat treatment on irreversible hydrogen trapping was investigated for alloy K-500 (UNS N05500) with a goal of providing more detailed insight into factors governing the intrinsic susceptibility to hydrogen embrittlement (HE). A potentiostatic pulse technique was used to determine the irreversible trapping constants (k) and hydrogen entry fluxes for annealed, annealed and aged (AA), and 12-h and 16-h direct-aged (DA) specimens of cold-drawn alloy K-500 that had been used earlier to provide as-received (unannealed, cold-drawn) and 8-h DA specimens. The type of heat treatment produced marked differences in irreversible trapping. Intrinsic susceptibility to HE, as defined by k, was increased considerably by annealing. Aging had a negligible effect on the intrinsic susceptibility for the annealed alloy, but resulted in a sizable increase for the unannealed alloy if performed for extended times, Intrinsic susceptibilities for the AA and DA alloys were correlated with the observed resistances to HE, implying that the previously reported decrease in resistance to HE (based upon tensile strength) produced by annealing was caused to a large extent, if not entirely, by a change in the irreversible traps.