Phase Characterization of CRA Fastener INCONEL718 in Relation of Hydrogen Assisted Cracking

Nickel base alloy INCONEL718 has been provided as two bolts, one was solution treated (ST) at 1035C°-2hr and other solution treated and single aged (STA) at 790C°-7hr. Presence of second phase particles in γ grains found to be responsible for trapping hydrogen between 'matrix-particle' interfaces and hence need to be identified. For that microstructure evolution from centre to edge for each bolt was investigated by Optical microscope (OM), Scanning electron microscope (SEM) and Transmission electron microscope (TEM). Specifically, the evolution of metal carbides (MC) and grain boundary delta phase (δ) (which are potential hydrogen traps) along with γ* and γ** was the primary focus for the current study. In fact, this characterization helps in predicting the actual hydrogen embrittlement by slow strain rate (SSR) and tensile tests and correlate with fractography data later on. Vicker's hardness (HV) measurements, critical to hydrogen embrittlement (HE), were carried out at each bolt to get an insight about morphology of γ* and γ** precipitates.