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Effects of aging condition on the hydrogen embrittlement susceptibility of precipitation hardenable nickel-base alloys
Kent, Michelle N.
Kent, Michelle N.
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2022
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During heat treatment, the thermal history can vary across a thick section of bar or forged product, resulting in a variation in mechanical properties, including hydrogen embrittlement (HE) susceptibility in Ni-base alloys. While it is well-established that δ phase precipitation occurs along grain boundaries in Alloy 718, it is unclear how much δ phase is necessary to diminish the HE resistance. Alloy 945X was designed to improve the HE resistance relative to Alloy 718 by avoiding δ phase precipitation; however, M23C6 carbides can form along grain boundaries in Alloy 945X under typical aging conditions. This work investigates the HE susceptibilities of Alloys 718 and 945X conditions aged at industrially-relevant temperatures that span the δ and M23C6 precipitation thresholds, respectively. The first objective of the study was to determine the sensitivity of HE susceptibility to small amounts of δ phase in Alloy 718. The second objective was to determine whether M23C6 phase increased the HE susceptibility, as the effects of M23C6 on HE susceptibility are not well established. Thirdly, the effects of δ and M23C6 were compared to determine whether M23C6 was as deleterious to HE resistance as δ phase. Finally, the HE susceptibility data were compared to the ambient mechanical properties to determine whether there was a relationship between HE susceptibility and Charpy impact toughness or tensile properties. Each heat treated condition was subjected to ambient mechanical property testing including hardness, tensile, and Charpy impact toughness testing. For hydrogen embrittlement testing, incremental step load tests were performed with circular notched tensile specimens subjected to in situ cathodic charging while crack initiation and growth were monitored using the direct current potential drop technique. For conditions with similar hardness values that spanned the δ precipitation threshold, fine, infrequent, and discontinuous δ phase precipitation resulted in an increase in HE susceptibility and increasingly intergranular fracture morphology, indicating that even a small amount of δ can reduce the HE resistance. The HE susceptibility also increased with M23C6 precipitation and growth in Alloy 945X conditions with similar hardness. The Alloy 945X conditions with M23C6 contained more frequent and more continuous grain boundary precipitates than the Alloy 718 conditions that contained δ phase; however, the HE susceptibility of M23C6-containing Alloy 945X was similar to the HE susceptibility of δ containing Alloy 718, suggesting that M23C6 is not as deleterious to HE susceptibility as δ phase, though both phases significantly reduce the HE resistance. Increasing HE susceptibility correlates with decreasing Charpy impact toughness, likely because grain boundary precipitates diminish both of these properties.
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