This paper studies the effect of deleterious grain boundary (GB) phase and its effect on the Hydrogen Induced Stress Cracking (HISC) resistance of N07725. Two related tasks are addressed and discussed. The first task deals with N07725 produced on a lab-scale and focuses on the occurrence of GB precipitates and their detrimental effect on HISC. It will be demonstrated that a modified N07725 alloy can be nearly free of GB precipitates, even with an API conforming heat treatment. High-resolution SEM analysis is used to identify the amount of GB precipitation in optimized and “according to standard” N07725 alloys. Subsequently, micro-mechanical testing data confirms the increased hydrogen resistance in absence of GB precipitates. The second task of this paper is to analyze an N07725 alloy, which was further optimized on large-scale production. The optimized material complies with the API 6ACRA standard and demonstrates improved resistance to HISC. The results presented include characterization of the GBs, HISC data according to NACE TM0198 – Appendix C, a characterization of the crack appearance as well as mechanical testing results.

Subject
Scanning electron microscopy, Test methods, Energy dispersive X-ray spectroscopy, Materials, Materials performance, Microstructure, Precipitates, Nickel based alloys, Elongation, Risk reduction, Alloys, Grain, Hydrogen
Keywords:
corrosion resistant alloys, nickel-base alloys, materials performance, hydrogen embrittlement, HISC, HR-REM, tensile testing, Charpy impact testing, SSR tests
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2023
Association for Materials Protection and Performance (AMPP)
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