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.
Seamless X60QOS and X65QOS line pipes are widely used for offshore and onshore Sour Service applications. Sour Service refers to the risk of hydrogen related cracking as Sulfide Stress Cracking (SSC). The International standard (NACE MR0175 / ISO 15156) provides requirements for assessing the resistance to SSC, specifically on how to qualify for use in region 3 of the environmental severity diagram (Figure 1 in paragraph 126.96.36.199 of part 2). It is requested to expose materials in an acid solution saturated by 1 bar of H2S (NACE TM0177 Solution A) and to apply a tensile stress above 80% AYS by means of different methods: uniaxial tensile, C-ring or Four-Points Bend. However, for very sour fields presenting H2S partial pressures much higher than 1 bar, the preservation of the SSC resistance might be questioned and is presently a major concern for the O&G industry.
The present paper is dedicated to the evaluation of the SSC resistance of seamless quenched and tempered X65 grades, including the girth weld in the standard NACE TM0177 Solution A up to 15 bar of H2S partial pressure. Corrosion tests consisted of four-point bend tests performed in autoclave vessels. Different test configurations were investigated as specimen sampling locations through the wall thickness and surface state preparation.