Mechanical Properties of Newly Developed API X80 Grade HFW Linepipe for Long-Term Exposure at Elevated Temperature

API X80 grade UOE double submerged arc-welded pipe has been applied to steam injection oil sand recovery systems to increase the volume of steam to be injected and decrease the installation cost. The pipes for the systems are subjected to high temperature for a long period, such as 350 °C for 20 years. Therefore, it is important to ensure the reliability of the pipes during and after long-term operation. In this study, based on the recent development of high-frequency electric-resistance-welded (HFW) linepipe with a high-quality weld seam, the durability of newly developed API X80 grade HFW linepipe for long-term high-temperature operation was investigated. The change in the microstructure of the pipe body and weld seam was small after exposure to 400 °C and lower temperatures. The tensile strength of the base metal and weld seam after heat treatment with temperatures as high as 400 °C can be determined using the Larson-Miller parameter, which depends on the temperature and holding time of the heat treatment. The newly developed API X80 grade HFW linepipe was considered to have sufficient tensile strength during and after long-term operation at 350 °C for 20 years, similar to API X80 grade UOE pipe. No significant change in the Charpy absorbed energy during long-term heating was observed. Creep tests indicated that the time to rupture at 400 °C or lower exceeded 10⁶ hours, and the creep effect was considered almost negligible at temperatures less than 400 °C. The rupture stress at approximately 350 °C was estimated to be far higher than the typical hoop stress of approximately 200 MPa on the steam distribution system. High-temperature fatigue properties were also measured to ensure reliability under varying stress conditions.