Hydrogen embrittlement testing of alloy 718 for oil and gas applications

Hydrogen embrittlement of Ni-base corrosion resistant alloys (CRAs) can occur in deep sea oil wells when components are exposed to hydrogen while under stress. The effects of microstructure on hydrogen embrittlement of precipitation hardened Ni-base CRAs, such as alloy 718, are not fully understood. There are several test methods used in the oil and gas industry for evaluating the susceptibility of CRAs to hydrogen embrittlement. Slow strain rate (SSR) tensile testing of smooth specimens is useful for comparing the susceptibilities of different alloys and microstructures to hydrogen embrittlement in the testing environment and is the most commonly performed test method, mostly due to the short duration of the test. There are several accelerated fracture mechanics test methods that have been developed to produce useful design data in the form of threshold stress intensity factors (Kth) for crack growth in a hydrogen-rich environment and have a much shorter test duration compared to constant displacement/constant load fracture mechanics testing. One type of accelerated fracture mechanics test method is the rising step load (RSL) test method, which attempts to simulate the loading conditions experienced by components in application by holding the specimen at constant displacements for a majority of the test. There is a lack of well-known correlations between the effects on hydrogen embrittlement susceptibility and fracture mode of dynamic straining of smooth specimens in the SSR test and accelerated fracture mechanics testing of notched or pre-cracked specimens. In this investigation, alloy 718 was annealed and aged to produce microstructures with variations in grain size, grain boundary precipitation, and strength level. The peak-aged and over-aged conditions exhibited a high strength and a low strength, respectively, and were produced through heat treatments that conformed to the API standard. An under-aged condition had no grain boundary δ phase and a similar yield strength to the over-aged condition. A high δ condition was produced through a double aging heat treatment that precipitated large δ phase particles on a significantly greater fraction of grain boundaries compared to the other conditions and produced a comparable strength level as the peak-aged condition. A high temperature and time annealing heat treatment was performed on these four conditions which resulted in a relatively large grain size. A small grain size condition was also produced by performing a lower temperature and time annealing heat treatment and was peak-aged. Grain boundary precipitation was evaluated through scanning electron microscopy (SEM), and the mechanical properties of the different microstructures were determined. The effect of microstructure on hydrogen embrittlement susceptibility of alloy 718 was primarily assessed through SSR experiments performed with cathodic hydrogen charging and in air on all microstructural conditions. RSL tests were performed on circular notch tensile (CNT) specimens of the peak-aged, over-aged, and…