Hydrogen embrittlement assessment of alloy 718 with precipitate variations

Abstract

Hydrogen embrittlement of Ni-base corrosion resistant alloys (CRAs) can occur when components under stress are exposed to hydrogen in deep sea oil wells. Ni-base CRAs, such as alloy 718, are primarily strengthened by precipitation of γ” and γ’, and the effect of these precipitates on hydrogen embrittlement is not well understood. To better understand hydrogen embrittlement of γ” and γ’ strengthened Ni base CRAs, the hydrogen embrittlement susceptibilities and fracture modes of alloy 718 microstructures with variations in the γ” and γ’ precipitates and without grain boundary δ phase were evaluated through incremental step load (ISL) and rising displacement (RD) testing of circular notch tensile (CNT) specimens coupled with the direct current potential drop (DCPD) technique and with in situ cathodic hydrogen charging. Ki, the stress intensity factor for crack initiation, was generally independent of microstructure. Crack growth resistance and Ku, the stress intensity factor for the onset of unstable crack growth, were highly dependent on variations in the γ” and γ’ precipitates. At a constant strength level, the transition from under aged to over aged γ” decreased crack growth resistance and Ku. Increasing size of under aged γ” decreased hydrogen embrittlement susceptibility, in terms of increasing crack growth resistance and Ku, despite increasing the strength level. These results demonstrate that high strength conditions with high hydrogen embrittlement resistance can be produced. Crack initiation appeared as fine faceted transgranular cracks on {111} planes and occurred at the transition from Stage I to Stage II of strain hardening when the slip band spacing is surmised to reach a minimum value. The requirements for crack initiation are localization of deformation to the nucleated slip bands and a high concentration of hydrogen. Crack growth was dominated by transgranular cleavage on {100} planes with visible river marks, cracking on Σ3 twin boundaries, and cracking on general grain boundaries and was caused by dislocations piled up at Lomer-Cottrell locks and grain boundaries in combination with hydrogen absorbed directly at the crack tip. The γ” and γ’ precipitates alter the strain hardening behavior which changes the crack tip stresses and strains required to propagate the crack and thereby influences crack growth resistance. There was no evidence of trapping at the γ” and γ’ precipitates from thermal desorption spectroscopy (TDS) experiments of specimens pre charged with gaseous deuterium. The hydrogen embrittlement response of a microstructure with an intermediate strength level that contained δ phase was also evaluated. Transgranular crack initiation on {111} planes was observed for the δ containing microstructure at a similar Ki to microstructures without δ phase indicating that δ phase does not affect this fracture mode. Grain boundary δ phase caused unstable intergranular crack propagation at a low applied stress. The ISL test coupled with the DCPD technique allowed for identification of crack initiation and the onset of unstable crack growth and produced crack growth resistance curves. Slow strain rate (SSR) tensile testing of smooth specimens was also performed with in situ cathodic hydrogen charging, and the total elongation ratio from the SSR test exhibited an inverse relationship with Ku and crack growth resistance. This discrepancy highlighted that there are several uncertainties with regards to the mechanical property ratios produced in the SSR test.