Microstructure and Property Evolution of T91 Steel under Deep Peak-Shaving Service Conditions

To address the safety assessment challenges of T91 steel heating surface components in the context of coal-fired power plant transformation toward deep peak-shaving, this study systematically investigates the evolution laws of microstructure and mechanical properties of in-service T91 pipes from different positions in a power plant after peak-shaving operation. Material properties were evaluated in accordance with standards such as GB/T 5310-2017 through metallographic analysis, tensile testing, impact testing, and hardness measurement, while the mechanisms linking microstructure to property degradation were explored. Results show that oxidation and decarburization occur on the outer surface of T91 pipes at all positions, with differences in the thickness of oxide/decarburized layers between the fire-facing and back-fire surfaces; the oxide layer exhibits fracture characteristics. After service, the tensile strength, yield strength, elongation, and hardness of the material still meet national standards. The performance at the platen and final superheater inlet is superior to that at the final reheater inlet, consistent with differences in operating temperature and pressure. This study indicates that the T91 steel provided in this study is in the early stage of its service life, providing a theoretical basis for the safe operation of the unit.