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.
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. As the complete production cycle from blast furnace to finished pipe is expensive and time consuming, understanding the influence of the relevant production steps is vital for plate and pipe development and manufacture. Within this work the impact of material properties of thick-wall X65 TMCP (Thermo-Mechanically Controlled Processed) plate on sour service performance is shown and discussed for the various steps of the production cycle.
