Microstructure characterisation of long-term aged advanced austenitic stainless steels for power plant applications

Power plants that are fuelled by fossil fuels form a large part of the global electricity generation sector and will continue to do so in the future as the global demand for electricity increases. However, there is also a growing concern for the environment and stricter legislation has been introduced to reduce the release of greenhouse gases into the atmosphere, including CO2. One way to reduce CO2 released by power plants is to increase the efficiency of the steam cycle by increasing the steam temperature and pressure within the boiler. The increase of the steam temperature and pressure beyond supercritical temperatures and pressures (600°C and 30 MPa) relies on materials being able to withstand these conditions. One material class that is used in the boilers of power plants that operate at or above supercritical conditions is advanced austenitic stainless steels. In this research, two advanced austenitic stainless steels have been studied. The two materials are Super 304H and HR3C, which are used as reheater and superheater tubes in the boilers of power plants. Reheater and superheater tubes see some of the most severe conditions in a power plant boiler due to the high temperature and high pressure steam that travels through them. Furthermore, they are also operating in the creep regime. Therefore, it is important to understand the microstructural evolution of Super 304H and HR3C tubes that have been exposed to high temperatures for long periods of time. The presence of second phases that evolve during service exposure can have an impact on the creep rupture properties and lifetime of components manufactured from Super 304H and HR3C.  [Continues ...]