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.  

The current paper deals with a thorough analysis of these newly recognized LHZ with Scanning Electron Microscope (SEM) and Electron Back Scattering Diffraction (EBSD) investigations through the wall thickness of pipes. Internal diameter (ID) surface, intermediate zone and bulk metal microstructures showed an increase of a strong misorientation while approaching the ID. Thus, LHZ is characterized by the presence of lath and especially lower bainite type microstructures associated to high local hardness above the NACE MR 0175 / ISO 15156 limits for sour service applications.

Low-alloy steel of API⁽¹⁾ 5L grade X65 is commonly used as line pipe material for sour service applications, where, due to stringent operation limits, resistivity to Hydrogen Induced Cracking (HIC), Sulfide Stress Cracking (SSC) and Stress-Oriented Hydrogen Induced Cracking (SOHIC) is required and tested before and after plate into pipe forming. Within recent years more challenging requirements have been introduced, including more severe corrosion test conditions, increased mechanical properties and extended limitations regarding hardness, to enable application to increasingly severe sour service conditions. For steel plants and plate manufacturers this entails the necessity to further improve the quality of their products together with separate improvements and modifications on the production plants and processes that have been implemented anyway.