THE INFLUENCE OF PALLADIUM ON THE HYDROGEN-ASSISTED CRACKING RESISTANCE OF PH-13-8 MO STAINLESS-STEEL

We compare the hydrogen-assisted cracking resistance of wrought PH 13-8 Mo stainless steel alloyed with 0.4 to 1.0 wt pct palladium to the conventional alloy when aged to yield strengths of 1170 to 1250 MPa. Intergranular hydrogen cracking is suppressed with Pd in both static load and constant extension rate tests conducted with electrochemical hydrogen charging. These results are analyzed to elucidate the role of Pd in suppressing intergranular cracking. Palladium is found both in substitutional solid solution in the martensitic phase and also in the form of randomly distributed PdAl precipitates in all Pd-modified alloys. Interfacial segregation of Pd to grain boundaries and lath boundaries is not observed at any levels above a detection limit of approximately 0.5 monolayers. Hydrogen permeation analyses indicate that hydrogen ingress is not inhibited by Pd but that apparent diffusion coefficients are lowered relative to the conventional alloy. Lower diffusion coefficients are consistent with the creation of a strong but reversible hydrogen trap, identified as the uniformly distributed PdAl phase. We hypothesize that PdAl trap sites force a redistribution of trapped hydrogen, which lowers the amount of interfacially segregated hydrogen at prior austenite grain boundaries for the electrochemical conditions applied. These assertions are supported by a simplistic trapping model for PH 13-8 Mo which shows that both the hydrogen trap binding energy and the trap density for the PdAl trapping site are greater than the hydrogen trap binding energy and density for prior austenite grain boundaries.