Microstructural and performance evolution of HR3C steel and HR3C/T92 dissimilar steel joints in ultra-supercritical boilers after service

To investigate the service reliability of key heating surface materials in ultra-supercritical boilers, this study focuses on long-term serviced HR3C steel base metal and HR3C/T92 dissimilar steel welded joints. Systematic research on their microstructural evolution laws and mechanical property degradation characteristics was conducted using metallographic analysis, EBSD (Electron Backscatter Diffraction) testing, room/high-temperature tensile tests, low-cycle fatigue tests, and fracture observation. Results indicate that the HR3C base metal undergoes regionally differentiated microstructural evolution under the synergistic action of long-term high temperature, service stress and working medium, where grain boundary carbide aggregation induces distinct embrittlement. The HR3C/T92 welded joint achieves stable metallurgical bonding, with the degree of microstructural degradation on its HR3C side being weaker than that of the base metal. By contrast, the joint’s high-temperature strength is restricted by the decarburized softening zone caused by carbon migration. The low-cycle fatigue lives of both materials decrease significantly with increasing strain amplitude, complying strictly with the Manson-Coffin relationship, and the joint presents a markedly shorter fatigue life than the base metal. This study provides a solid experimental basis for the service evaluation, residual life prediction and failure control of ultra-supercritical boiler components.