Effect of hydrogen on the mechanical properties of alloy 945X (UNS N09945) and influence of microstructural features

This study investigated the hydrogen embrittlement sensitivity of the precipitation hardened (PH) alloy 945X in three different metallurgical states. The three variants examined (obtained by different heat treatments) were the standard oil and gas industry condition, and two alternative microstructures with variations in fraction and morphology of γ′, γ″ and δ phases. For each metallurgical state, mechanical tests were carried out on both uncharged and hydrogen pre-charged specimens in order to evaluate the deleterious influence of hydrogen on mechanical properties. Material characterisation and post-test fractography was performed to understand the link between microstructural features, fracture behaviour, and susceptibility to hydrogen embrittlement. Fractographic analysis showed that, in the presence of hydrogen, intergranular fracture occurred for all the heat treatments, regardless the presence of δ-phase at grain boundaries. There was no simple correlation between the volume fraction of δ-phase and susceptibility to hydrogen assisted embrittlement. Rather, it was demonstrated that the morphology and distribution of δ-phase along grain boundaries plays a key role and the other precipitate phases also have an influence through their influence on the ease of strain localization.