Evaluation of microstructure and mechanical properties of 0.5Cr-0.5Mo-0.25V steam piping after long-term service in creep conditions

Abstract

This work is part of the ongoing research by the South African power utility aimed at addressing the issues related to creep damage and safe extension of service life to meet the country’s increasing electricity demand. Creep damage in low-alloy steels remains a major concern for the safe and reliable operation of high-temperature components in power plants. This study evaluates the microstructural evolution and mechanical property degradation of ½Cr–½Mo–¼V (14MoV6-3) steam piping that was in service for more than 270,000 hours. Optical microscopy, scanning electron microscopy, and X-ray diffraction were used to characterize microstructural evolution of the new and service exposed materials, while Thermo-Calc software was employed to predict equilibrium carbides. Mechanical performance was assessed through hardness, tensile tests at room temperature and high temperatures (500°C –560°C), impact testing at-30℃ to 200℃, and creep testing using standard and Gleeble® machine. The results revealed significant microstructural changes in the service-exposed material, with the disappearance of ferrite–pearlite/bainite structures and the presence of ferrite and carbides, predominantly M23C6 and M6C along grain boundaries. Mechanical properties largely remained within the lower acceptance bounds but showed pronounced deterioration at elevated temperatures. Replica analysis confirmed creep damage to be most severe in the heat affected zone, consistent with Type-IV failure. Cross-weld creep testing aligned with the lower limit of the European Creep Collaborative Committee (ECCC) master curve. The deviations from ECCC curve were attributed to prior long-term exposure. Overall, the findings highlight that prolonged creep service significantly reduces creep strength, underscoring the need for additional testing to establish reliable strength reduction factors for life extension assessments.