Microstructure-Property Relationship and Embrittlement Mechanism of HR3C Steel with High-Temperature Service of 65000 h

Abstract

HR3C steel, an austenitic helat-resistant steels, due to its good strength, high temperature behaviour and cost effectiveness, which lead to the extensive use in ultra-supercritical boilers. In order to study the microstructure characteristics and embrittlement mechanism of superheater tube of 660 MW thermal power boiler, the microstructure and chemical composition as well as the mechanical performance of HR3C steel pipe after service were tested. The results showed that HR3C steel, after approximately 65,000 hours of service, exhibited a continuous sheet-like distribution of M23C6 phasealong the grain boundaries, accompanied by needle-like or strip-like M23C6 phase growing into the grain boundaries, as well as the presence of Z phase (NbCrN) and σ-equivalent precipitation phase along the periphery of the grain boundaries. Following service, the hardness of HR3C steel experienced a slight increase, the tensile strength remained relatively unchanged, and the yield strength exhibited an increase of approximately 15%. However, the elongation after fracture significantly decreased, resulting in a decrease in plasticity decreased by 64% to 73% compared to its original state. HR3C steel displayed notable embrittlement after 65,000 hours of service at 650 °C, with a 96% reduction in impact toughness. The precipitation of M23C6 and σ phase were identified as the primary causes of embrittlement in HR3C steel.