Microstructural evolution in cast Haynes 282 for applications in advanced power plants

There is a worldwide drive to increase the efficiency of power plants in order to reduce the amount of fossil fuel consumed and associated CO2 emissions. Raising the operating temperature and pressure can improve the thermal efficiency, however, this necessitate the use of materials which have high temperature performance. Steels are currently used at temperature up to 600°C with the efficiency of 38-40 %. Advanced Ultra Supercritical (A-USC) design plans power plants to operate at steam temperatures of 700°C and pressure up to 35 MPa with a lifetime of at least 100 000 hours. Ni-base superalloys are leading materials due to their significant strength and creep resistance. Haynes 282 is one possible candidate to meet the A-USC conditions for turbine engines. This alloy is a γ′ precipitation strengthened material and exhibits very good creep properties and thermal stability. The alloy examined in this research was produced by sand casting, and therefore the aim of this research is to investigate the microstructural evolution in large scale cast components. The alloy has been examined in both the as-cast condition and as a function of a range of different pre-service heat treatments. The microstructural changes during different heat treatments have been fully identified and quantified. The results have also been compared with predictions from thermodynamic equilibrium calculations using a Ni alloy database. It has been found that variations in the heat treatment conditions can have a significant effect on microstructural development and hence, potentially, the mechanical properties of Haynes 282 alloy.