Characterisation of the microstructural evolution of aged super 304H (UNS30432) advanced austenitic stainless steel

Advanced austenitic stainless steels, such as Super 304H, have been used in reheater and superheater tubes in supercritical and ultra-supercritical power plants for many years now. It is important to characterise the microstructure of ex-service reheater and superheater tubes as this will help to understand long-term microstructural evolution and degradation of the material which can impact the performance and lifetime of the components that are in service. In this research, the microstructure of an ex-service Super 304H reheater tube that has been in service for 99,000 hours at an approximate metal temperature of 873K (600°C), has been characterised. The characterisation techniques used were electron microscopy based and include imaging and chemical analysis techniques. Seven phases were observed as a result of the characterisation work. The phases observed were, MX carbo-nitrides rich in niobium, copper rich particles, M23C6, sigma, Z phase, a cored phase and a BCC phase.