Abstract
High strength Nickel alloys are widely used in subsea and downhole O&G applications for their excellent combination of mechanical properties, toughness and corrosion resistance in sour environments. The trend in the O&G industry is for using higher strength materials for high pressure – high temperature service, however as strength increases materials may also become more susceptible to ambient temperature failure mechanisms associated with hydrogen absorption. In recent years resistance to Hydrogen Stress Cracking (HSC) and Hydrogen Embrittlement as well as sour corrosion resistance have become of increasing interest to the industry due to a number of reported failures of high strength precipitation hardened Nickel alloy grades used in completion tools. The failures of the materials have been attributed to unfavourable microstructures increasing their susceptibility to HSC. A number of factors are known to influence HSC and this paper reviews previous work and shows laboratory results using different test techniques to demonstrate the influence of yield strength and microstructure on the resistance to HSC of precipitation hardenable Nickel alloys N07718, N09925, N07725, N09945 and N09946. Tests have been conducted in a Hydrogen charging environment while conducting a slow strain rate test with plain and notched specimens and assessing the effect of hydrogen absorption on the materials’ properties. The resistance of the Nickel alloys to HSC is correlated with material composition, microstructure and mechanical properties. The effect of a stress concentration is shown influence the notch tensile strength of the materials in a hydrogen charged environment.
