Microstructural Degradation of Super304H after Long-Term Service Exposure

Super304H is austenitic steel used predominantly for boiler and turbine components in thermal power plants due to the high strength, excellent creep and oxidation resistance in high-temperature steam environments. Microstructural degradation is inevitable upon long-term high-temperature exposure. Therefore, understanding the processes of degradation is critical for the determination of residual lifetime of the parts. In this study, the hardness, tensile strength and creep strength of a Super304H steel that was in service as a reheater for ~8 years at about 600°C was investigated with corresponding microstructural analysis. Grain growth is evident, but microhardness and tensile strength did not decrease, due to the precipitation of nanosized NbX and Cu-rich particles in grain interior. However, the creep rupture life of the aged S304H steel is ~90% lower at 650°C than that of the virgin S304H, due to the coarsening of σ phase and formation of M 23 C 6 on the grain boundary. Our measurements and analysis of the specimens in this study indicates the remaining service life longer than those in the NIMS database.