The Effects of Thermal History on Toughness of Ni-based Corrosion Resistant Alloys during in-situ Hydrogen Charging

Hydrogen embrittlement (HE) susceptibility was investigated for Alloy 718 and Alloy 945X specimens heat treated to a set of conditions within the specifications of API Standard 6ACRA. Heat treatments were selected to simulate the potential variation in thermal history in thick sections of bar or forged products and produce various amounts of discontinuous grain boundary δ phase in Alloy 718 and M₂₃C₆ carbides in Alloy 945X, while maintaining a constant hardness in the range of 35-45 HRC for Alloy 718 and 34-42 HRC for Alloy 945X. Time-temperature-transformation (TTT) diagrams and experimentation were used to select a set of heat treatments containing no δ phase, a small quantity of δ, and a larger quantity of δ, in Alloy 718. A similar approach was taken regarding M₂₃C₆ carbides in Alloy 945X. Incremental step loading (ISL) tests were conducted under in situ cathodic charging on circular notch tensile (CNT) specimens in a 0.5 M H₂SO₄ solution. During the test, the direct current potential drop (DCPD) was measured across the notch to determine the stress intensity associated with crack initiation and unstable crack growth. Results indicate that even very small quantities of δ phase in Alloy 718 are detrimental to HE resistance.