Abstract
Low-alloy steel of API 5L grade X65 is commonly used as line pipe material for sour service applications. 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. Within this study a systematic investigation of material properties related to corrosion resistance was performed for low sulfur X65 TMCP-based steel plates with a thickness of 30 mm and 35 mm. Plate-to-pipe forming simulation to different wall thickness to diameter ratios (t/D) was undertaken and HIC testing according to NACE TM0284 was performed to investigate the HIC resistance after cold forming of heavy plates to SAWL pipes. Wall thickness to diameter ratios of 5 % and above were found to be accomplishable. In addition, hydrogen permeation measurements were used to further investigate the influence of the microstructure on the hydrogen diffusion. SSC resistance tests according to NACE TM0316 were carried out under standard test conditions (1 bar H2S) and elevated pressure for fully machined specimens and for specimens containing the as-received plate surface. The SOHIC resistance was assessed using the new “Twist-and-Bend”-SOHIC test, which is currently under standardization within AMPP, based on applied twist and load. The results of electron backscatter diffraction (EBSD) measurements on plate material were correlated with the hydrogen permeation measurements. The results support the understanding of significant material-related factors, which is essential for the development of high-quality line pipe steel for sour service applications.
