The hydrogen embrittlement susceptibility of several UNS N07718 microstructures was determined with different in situ testing techniques. Hydrogen-affected stress intensity factors for crack initiation and for unstable crack growth were determined through incremental step load (ISL) and rising displacement (RD) testing coupled with the direct current potential difference (DCPD) method. Slow strain rate (SSR) tensile tests were also performed to evaluate hydrogen embrittlement susceptibility. Microstructures with no δ phase and three different strength levels and a microstructure with an intermediate strength level that contained δ phase were evaluated. For UNS N07718 microstructures without δ phase, the stress intensity factor for unstable crack growth increased with increasing yield strength and exhibited an inverse relationship with the total elongation ratio from the SSR tests. The presence of δ phase decreased the stress intensity factor for unstable crack growth but did not affect the stress intensity for crack initiation. Fracture modes were evaluated with scanning electron microscopy and were independent of testing method.
UNS(1) N07718, commonly referred to as alloy 718, is one of several precipitation-hardened nickel-base corrosion resistant alloys (CRAs) used for components in deep sea oil wells. Ni-base CRAs are used for packers, tubing hangers, fasteners, valves, and bolting components in deep-sea oil wells because of their high-strength, high toughness, and corrosion resistance; however, field failures of these components have occurred due to hydrogen embrittlement.1-3
Precipitation-hardened Ni-base CRAs are primarily strengthened by coherent and/or semi-coherent Y" and Y' precipitates formed during aging. Thermomechanical processing and aging may also result in the formation of additional precipitates at grain boundaries, such as δ phase, and carbides within grains. It is well known that δ phase can increase the hydrogen embrittlement susceptibility of Ni-base CRAs by inducing hydrogen-affected intergranular and/or interphase cracking.1, 2, 4-6 API(2) standard 6ACRA provides guidelines for annealing and aging heat treatments and acceptable mechanical properties of UNS N07718 and other Ni-base CRAs for use in downhole environments.7 In certain service conditions, UNS N07718 can experience hydrogen embrittlement despite being heat treated according to the API 6ACRA standard due at least in part to limited δ-phase precipitation along grain boundaries,2 3 these precipitates are formed during the aging step of the heat treatment. The effect of the Y" and Y' precipitates on hydrogen embrittlement in Ni-base CRAs when δ phase is not present is unclear, in part due to the difficulty of producing high strength microstructures with no δ phase.
