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.
Precipitation hardened (PH) nickel-base alloys are frequently used as engineering materials in the Oil & Gas industry.1,2 They excel because of their outstanding combination of strength, toughness, and corrosion resistance. In that regard, alloy N07725 is of high interest as it offers better corrosion resistance than the widely used N07718, while also offering better high temperature strength than solid-solution nickel-base alloys.3 However, recent failures of installed components made of age-hardened nickel-base alloys have highlighted their susceptibility to hydrogen induced stress cracking (HISC).4,5,6,7 Compared to N07718, standard available N07725 material shows higher HISC susceptibility although the mechanical and microstructural requirements of the well-known API 6ACRA standard are fulfilled.8
Several research groups investigated the root cause of this unexpected embrittlement in N07725 and reported previously unknown grain boundary (GB) precipitates in HISC-susceptible microstructures.9,10 The occurrence of these precipitates’ correlates to the poor HISC resistance. Hence, avoiding these phases would yield a significant improvement. The presented work intends to deepen these recent scientific discoveries and transfer them to industrial-scale alloy production.
