High-strength low-alloy steel bar stocks with 110ksi (758MPa) and 125ksi (862MPa) specified minimum yield strength (SMYS) are in demand for temporary and permanent downhole tools for sour service. NACE MR0175/ISO15156 currently allows the use of low-alloy steel bar stocks without any environmental restrictions up to 22HRC, which, by most specifications, corresponds to 80ksi (552MPa) SMYS. At higher SMYS, and with exclusions of API and proprietary sour tubular grades, NACE MR0175/ISO15156 does not address solid bar stocks, a gap and opportunity addressed by this investigation. Specifically, in this paper, the sulfide-stress cracking (SSC) of commercial UNS G41xxx (41xx) alloys, including 41xxMod (i.e., carefully selected or mill modified) is investigated following a series of NACE TM0177 Method A tests in either Solution B or A (NACE Region 3). Domain diagrams for 41xx alloys are disclosed, all demonstrating that 41xx solid bar stocks are SSC resistant above 150°F (66°C) when under a new and improved specification. When SMYS is raised to 125ksi (862MPa) with 34HRC max, a safe minimum temperature of 175°F (79.5°C) is confirmed for hollow bars, well in line with current NACE MR0175/ISO15156. The metallurgical and hardness requirements of 41xx alloys are also briefly discussed, along with opportunities for further modifications of 41xx bar stocks.
Stainless steel UNS S17400 (17-4PH) has been successfully used in oilfield services outside the traditional NACE MR0175/ISO 15156 limits for permanent equipment. The exact operational envelops of 17-4PH (HH1150), including the tensile threshold stress, sour gas partial pressure, temperature, and exposure time that enable the crack-free usage of 17-4PH (HH1150) are not well established. For service equipment, NACE MR0175/ISO15156 currently provides exemptions from the tight environmental restrictions of permanent equipment, but instead limits the maximum applied stress to a debatable 60% of the specified minimum yield strength (SMYS). In this investigation, the sulfide stress-corrosion cracking of 17-4PH is revisited through 51 new NACE TM0177 Methods A tests conducted over 240 hours minimum (480hrs in certain cases). Under unrestricted sour gas partial pressures, the threshold tensile stress below which cracking does not occur is between 45% and 60% of the SMYS at ambient temperature. Alloy 17-4PH is also less susceptible to sulfide stress cracking as temperature increases from 70°F (21°C) to 350°F (177°C). Risk of sulfide stress cracking is also greatly mitigated when delta ferrite is controlled. With reduced delta ferrite, as provided by two out of three tested heats, and reverted austenite promoted by both chemical composition and longer aging treatments, no cracking is seen at 60% stress level up to 45psi H2S (0.31MPa); at 45% stress level, this value is increased to 120psi (0.83MPa) based on newly-collected test data.