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51318-11478-High Strength N07718 with Improved Resistance to Cracking in H2S-Containing Environments

This paper describes the development of an alternative heat treatment designed to increase the strength of 718 (UNS N07718) above the current level, which is bound by the 40 HRC limit of NACE MR01751 and the heat treatment requirements of API 6ACRA.

Product Number: 51318-11478-SG
Author: Sabine Kremel / Martin Wöls / Greg Chitwood / Luciana Intiso
Publication Date: 2018
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This paper describes the development of an alternative heat treatment designed to increase the strength of 718 (UNS N07718) above the current level, which is bound by the 40 HRC limit of NACE MR01751 and the heat treatment requirements of API 6ACRA.2 The investigation was triggered by hydrogen embrittlement failures reported for PH Ni-base alloys, where unfavorable microstructure was reported to be a contributing factor. Increased strength was also suspected, but this comes at a time when the industry needs to push strength higher to exploit future high-pressure/high-temperature (HP/HT) developments. Initial slow strain rate testing (SSRT) of a two-step heat treatment in a sour environment containing 28 bar H2S and 25% NaCl at 150°C revealed improvement in critical SSRT ratios (ep and RoA) over that achievable with the API heat treatment; strength increased by approximately 30 ksi and ductility and toughness were maintained. This testing was followed by more extensive sulfide stress cracking (SSC), galvanically-induced hydrogen stress cracking (GHSC), and stress corrosion cracking (SCC) to qualify the material for MR0175 at 205°C with up to 35 bar H2S. This paper documents the corrosion testing conducted to compare the alternative heat treatment with that required by API 6ACRA, and provides an explanation for the improved environmental cracking resistance through investigation of cracking morphology, microstructure, and metallurgical properties. 

Key words: 718-API, 718 high strength, SSC, GHSC, SCC, SSRT, HP/HT sour gas testing

This paper describes the development of an alternative heat treatment designed to increase the strength of 718 (UNS N07718) above the current level, which is bound by the 40 HRC limit of NACE MR01751 and the heat treatment requirements of API 6ACRA.2 The investigation was triggered by hydrogen embrittlement failures reported for PH Ni-base alloys, where unfavorable microstructure was reported to be a contributing factor. Increased strength was also suspected, but this comes at a time when the industry needs to push strength higher to exploit future high-pressure/high-temperature (HP/HT) developments. Initial slow strain rate testing (SSRT) of a two-step heat treatment in a sour environment containing 28 bar H2S and 25% NaCl at 150°C revealed improvement in critical SSRT ratios (ep and RoA) over that achievable with the API heat treatment; strength increased by approximately 30 ksi and ductility and toughness were maintained. This testing was followed by more extensive sulfide stress cracking (SSC), galvanically-induced hydrogen stress cracking (GHSC), and stress corrosion cracking (SCC) to qualify the material for MR0175 at 205°C with up to 35 bar H2S. This paper documents the corrosion testing conducted to compare the alternative heat treatment with that required by API 6ACRA, and provides an explanation for the improved environmental cracking resistance through investigation of cracking morphology, microstructure, and metallurgical properties. 

Key words: 718-API, 718 high strength, SSC, GHSC, SCC, SSRT, HP/HT sour gas testing

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