Recently, there has been discussion in the literature about the potential effect of local hard zones (LHZ) in thermo-mechanically controlled processing (TMCP) linepipe steels on the integrity of pipelines in severe sour service. This has generated interest to understand the sulfide stress cracking (SSC) propagation resistance of base linepipe steels. In this work we studied SSC propagation resistance using double cantilever beam (DCB) and single edge notched bend (SENB) testing. To develop a general understanding of SSC behavior, TMCP steels were compared with both seamless linepipe steels and oil country tubular goods (OCTG) steels. It was observed that linepipe steels showed generally higher materials resistance in both DCB and SENB results compared to OCTG steels in severe sour conditions, but the differences were less in the milder sour conditions. However, there were significant experimental challenges in measuring SSC propagation resistance in base linepipe steels. Therefore additional work is needed for developing standardized testing to measure SSC propagation resistance in base linepipe steels.
Sulfide stress cracking (SSC) is the cracking of metal involving corrosion and tensile stress (residual and/or applied) in the presence of water and H2S in production fluids. SSC is a form of hydrogen embrittlement (HE) induced by atomic hydrogen that is produced by sour corrosion on the metal surface (Berkowitz and Horowitz, 1982, Berkowitz and Heubaum, 1984). Hydrogen uptake is promoted since the presence of H2S prevents atomic hydrogen from recombining into hydrogen molecule, as hydrogen recombination poison (Berkowitz et al., 1976). The atomic hydrogen can diffuse into the metal, reduce ductility, and increase susceptibility to cracking (Fig. 1). High-strength metallic materials and hard weld zones are prone to SSC (NACE_MR-0175, 2015).
For decades, the oil and gas industry has been using carbon steel pipe for sour service applications (both for down-hole casings and pipelines). For large diameter pipelines, the material of choice has been thermomechanically control processed (TMCP) pipe. TMCP technology has been evolving since the early 1980s to develop a plate manufacturing method that combines microalloying elements (Ti, Nb, V), controlled hot rolling, and accelerated cooling control (ACC) (Tamura et al., 1988). TMCP plates can be produced in thicknesses up to ∼120 mm for structural purposes; however, for large diameter pipe, thicknesses are typically 15 – 40 mm (Fairchild et al., 2019).
