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Domain Diagrams for the Sulfide Stress Cracking Resistance of High Strength Low Alloy steel 41xx Bar Stocks

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. 

Product Number: 51320-14590-SG
Author: Vipul Shinde, Manuel Marya
Publication Date: 2020