Precipitation hardened nickel alloys are commonly used in oil and gas production equipment where materials must provide both outstanding mechanical strength and corrosion resistance. Additive manufacturing (AM) provides a new approach to the design and manufacture of components from metal powder and provides some unique advantages over traditional manufacturing. However, components produced by AM are not currently acceptable per NACE(1) MR0175/ISO(2)15156 since no AM alloys have been balloted and approved for inclusion. There are uncertainties and knowledge gaps around the performance of AM material in sour service. AM Alloy 718 (UNS N07718) test material was manufactured by laser powder bed fusion (LPBF) with final heat treatment per API(3) 6ACRA 150K designation by three suppliers. After benchmarking the basic mechanical properties in air, the AM Alloy 718 test material was evaluated by slow strain rate (SSR) testing and proof ring testing in sour environments similar to those used for the wrought Alloy 718 150K ballot for NACE MR0175/ISO 15156. The resistance to stress corrosion cracking (SCC) and galvanically induced hydrogen stress cracking (GHSC) were investigated and compared to the wrought Alloy 718 150K results.
Alloy UNS N07718 (hereafter abbreviated as 718) is one of the most versatile precipitation-hardened nickel-based corrosion-resistant alloys (CRAs) used for both surface and sub-sea components in oil and gas production service. API 6ACRA1 provides heat treatment windows and acceptance criteria for 718 in these oil and gas production environments, in which the heat treatment is intended to obtain high strength and to minimize the formation of δ-phase at grain boundaries. As pointed out in NACE MR0175 Part 32 (Table 1), field failures of 718 components in sour service are primarily related to stress corrosion cracking (SCC) at elevated temperatures and hydrogen embrittlement in the lower temperature range. The latter is specifically called galvanically induced hydrogen stress cracking (GHSC or GIHSC), which is typically caused by atomic hydrogen uptake from galvanic corrosion or cathodic protection (CP) when 718 is used with steel components in a seawater environment. CP is normally used to protect steel component from corrosion in subsea environments. Therefore, the qualification of precipitation-hardened nickel-based alloys in sour service is aimed to address these two primary cracking mechanisms, as documented in the recent literature3,4 for balloting the 150K designation of wrought 718 in NACE MR0175/ISO 15156 Part 3,2 Technical Circular 2 (Table A.32, reproduced in Table 2), with link to the heat treatment in API 6ACRA1 for the 150K grade through footnote c.