The precipitation hardened nickel alloys are designed for Oil & Gas applications requiring high mechanical strength and toughness combined with high corrosion resistance in sour environments. 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. This is a complex corrosion mechanism with many factors including composition, strength, microstructure and grain boundary cleanliness1,2 . Evaluation efforts have used multiple techniques to measure the effects of HSC resistance with this paper concentrating on the Slow Strain Rate Test (SSRT) according to TM0198 Appendix C and the quality control of 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 Hydrogen Stress Cracking resistance of the UNS grades N09945, N09946, N07718 and N07725 in relation to mechanical properties and microstructural characterization of mill heats. Process control of the composition and processing has shown the HSC resistance of N09946 can be optimized with the plastic elongation ratio values in the range of 0.7 to 0.4 dependent on the grade and heat treatment. The processing of N07725 manufactured according to API6ACRA has shown the materials elongation ratio as a measure of resistance to HSC can be significantly improved from values of approximately 0.16 to ratio’s above 0.45. The SSRT results are compared with mechanical properties determined according to API6ACRA and detailed microstructural analysis.

Subject
Acids, Materials, Precipitation (solids), Materials performance, Precipitates, Nickel based alloys, Plastics, Elongation, Heat, Corrosion resistance, Grain boundary, Slow strain rate technique, Hydrogen stress cracking
Keywords:
Precipitation hardened nickel alloys, Hydrogen stress cracking, Slow strain rate testing
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2022
Association for Materials Protection and Performance (AMPP)
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