Severe sour environments, with low pH and high H2S content, can pose challenges for material selection and design of carbon steel pipelines. Steel can be susceptible to both corrosion and embrittlement if not properly designed and maintained. The standard remedy for preventing embrittlement and cracking susceptibility is to control the hardness below a maximum threshold value. Material qualification testing for sour service applications involves hardness testing on the throughthickness surface, at a prescribed distance from the internal diameter (ID) surface of the pipe. However, recent pipeline failures have demonstrated this testing approach to be inadequate for detecting thin zones of high hardness. These "Local Hard Zones", or LHZ, have been traced to Thermo-Mechanically Control Processed (TMCP) steel plate manufacturing, and are susceptible to sulfide stress cracking.
This article details a series of hardness testing campaigns carried out on various heats of TMCP steel. The results will demonstrate the benefits (and risks) of using lower indent loads to sample near-surface microstructures and properly characterize cracking susceptibility. Practical guidance is provided for using improved hardness testing practices during sour service material qualification.
Recent high-profile failures of sour service pipelines have revealed potential risks from the use of C-Mn steel pipe manufactured via Thermo-Mechanically Controlled Processing (TMCP). In this method, steel plates are formed using controlled rolling and an accelerated cooling stage prior to pipe forming and seam welding. It has been concluded that the large volumes of water used during accelerated cooling, may cause local overcooling in very thin layers on the plate surface. While the bulk of the plate achieves the intended microstructures and mechanical properties desired in sour-grade pipe, the overcooled regions can present excessive hardness on the surface that may be susceptible to sour cracking mechanisms. These "Local Hard Zones", or LHZ, have been thoroughly investigated (Fairchild, Newbury, Anderson, and Thirumalai, 2019) and were concluded to be the root cause for the Kashagan pipeline failures. Unfortunately, mitigation of LHZs via conventional pipe qualification testing is challenging, owing to their rarity. LHZs may only occupy a small percentage of the plate surface, and even within only a small percentage of plates. Although it has been theorized that the rarity of LHZ formation is connected to plate descaling processes prior to accelerated cooling, it is still difficult to predict where and when they will form.