Recent Development in the Characteristics of Alloy 625 for A-USC Steam Turbine Casting

Raising the operating temperature and pressure of the steam turbine can improve the thermal efficiency in order to reduce the fossil fuel consuming and CO2 emissions. Alloy 625 is a solution strengthened Ni-base superalloy and has been considered as one possible candidate for A-USC steam turbine castings. In this research, we investigate the chemical composition, microstructure, tensile and creep behavior, and long-term thermal stability of an Alloy 625 step block, which was produced by sand casting. Nb segregation has been observed in the thick section. Segregation can influence the mechanical properties and cause defects in large casings and valves. The alloy exhibits excellent creep properties and thermal stability, however, the ductility of Alloy 625 casting decreased significantly after 1000 h at 700 °C. Alloy 625 can meet the design requirements of A-USC steam turbine, but also need further research on the production process.