Study of the effect of hydrogen charging on the tensile properties and microstructure of four variant heat treatments of nickel alloy 718

Abstract

This study examined the hydrogen embrittlement sensitivity of nickel alloy 718 given four different heat treatments to obtain various microstructural states. The four heat treatments examined are the oil and gas 718 heat treatment, the aerospace 718 heat treatment, and two variant two-step heat treatments, with a difference in aging heat treatment named 718 low band and high band. Each heat treatment leads to differences in the precipitate morphologies of γ′, γ′′ and δ phases.Material characterisation and fractography of the examined heat treatments were performed using a high resolution FEG SEM. Three specimens of each condition were pre-charged with hydrogen and tensile properties were compared with those of non-charged specimens. It was observed that hydrogen embrittlement was associated with intergranular and transgranular microcrack formation, leading to an intergranular brittle fracture. δ phase may assist the intergranular crack propagation, and this was shown to be particularly true when this phase is coarse enough to produce crack initiation, but this is not the only factor determining embrittlement. Other microstructural features play a role, as does the strength of the material. The aerospace heat treatment, which gives the highest strength and ductility in the uncharged state, shows the greatest reduction in properties with hydrogen charging.