The material trend in the Oil & Gas sector is for high strength materials with high levels of corrosion resistance to resist increasingly harsh sour downhole environments. Compared to sweet wells the presence of hydrogen sulphide , elemental sulphur and hydrogen generally requires material selection of tubular and bar products in high performance stainless steels and nickel base alloys to withstand the pressures and temperatures. The materials of choice must be corrosion resistant, cost effective, reliable and have the strength required for the well conditions. Age hardened nickel base alloys and cold worked solid solution nickel-base alloys offer many advantages such as high strength, general and localised corrosion resistance, and stress corrosion cracking resistance.

The material selection for downhole and well head equipment such as hangers, sub-surface safety valves, pumps and packers require age-hardenable materials to obtain the strength in heavier cross sections which cannot be strengthened by cold work. The commonly used nickel alloys for the sour service applications are alloy 925, alloy 718 and alloy 725 with the more recently developed alloy 945 and alloy 945X designed for HP/HT and sour wells. The metallurgical stability of these materials being increasingly important in order to optimise the mechanical and corrosion resistant properties, particularly as larger section thicknesses of higher strength materials.

The effect of the microstructure of these materials is shown to have a significant effect on the resistance to hydrogen attack and corrosion in sour environments. Optimising the compositional control, thermomechanical processing and microstructure is shown to give significant improvements in resistance to sour corrosion and hydrogen stress cracking resistance of materials used for critical downhole components. Over recent years there has been increasing industry demand to improve quality control and categorise the various PH Nickel alloy grades resistance to Hydrogen Stress Cracking (HSC) for critical High Pressure-High Temperature environments. The HSC mechanism has been less well understood in high strength precipitation hardenable subsea and downhole components than a Corrosion Resistant Alloys high temperature sour corrosion material limits. HSC is a complex corrosion mechanism with many factors including composition, strength, microstructure and grain boundary cleanliness. Evaluation efforts have used multiple techniques to measure the effects of HSC resistance, with this paper concentrating on the Slow Strain Rate Test (SSRT) method according to TM0198 Appendix C and using the quality control standard API6ACRA.

The purpose of the paper is to present results using the TM0198 slow strain rate test method in a hydrogen charging environment and show the HSC resistance of the grades 925, 718, 945, 945X, and 725. This paper shows how the composition can be controlled within the defined limits of the alloy grade to optimise the HSC resistance by reducing precipitation of deleterious phases and reduce mill heat batch variation. The SSRT results are compared with mechanical properties determined according to API6ACRA and detailed microstructural analysis.