Steel pipelines are sometimes subjected to demanding sour environments resulting from the presence of high H2S contents. Pipeline materials, therefore, must be resilient against sulfide stress cracking (SSC) which is caused by H2S. Beginning in the 1980s, thermo-mechanically controlled processed (TMCP) steels have been widely used for the manufacturing of large-diameter sour service pipelines. The failure of the Kashagan pipelines in 2013 raised concern regarding the use of TMCP steels in sour environments. These concerns arise from the potential for local hard zones (LHZs) to be produced on the surface of the line pipe during TMCP processes, ultimately leading to through-wall SSC failures. In the present study, several X60 - X65 TMCP steels (both with and without LHZs) have been tested under different Region 3 (R3) conditions in the NACE MR0175/ISO15156-2 pH-H2S partial pressure diagram. It can be concluded that the presence of LHZs increases TMCP steels’ sour cracking susceptibility; however, TMCP steels without LHZs pass the SSC tests at even the most severe R3 environments. Traditional HRC or HV10 testing are not able to detect LHZs, and so lower load HV 0.5 or HV 0.1 tests are necessary. For TMCP steels, the current R3 may be further divided into R3-a and R3-b sub-regions. The sour cracking severity of R3-a is less than that of R3-b. Additional actions, like enhanced mill qualification of the TMCP plate, should be considered to ensure that no LHZs exist in steels to be utilized in R3-b environments.
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.