Effects of initial microstructures on the microstructural evolution and corresponding mechanical property of K424 superalloy after overheating exposure

Service safety and maintenance costs of turbine components after overheating exposure are closely related to the initial microstructures and subsequent microstructural evolutions after overheating. However, comparative investigation about microstructural evolutions and corresponding mechanical properties of cast polycrystalline superalloys with different initial microstructures after overheating exposure has been rarely reported. In this paper, stress rupture tests at 975 °C/196 MPa, tensile tests at room temperature and microhardness measurements were conducted in both as-cast and standard heat-treated K424 alloy after overheating at the temperature range from 1000 °C to 1200 °C for 2 h. The microstructural evolutions and fracture features were then investigated. The results showed that the mechanical properties didn’t decrease for standard heat treated specimens and even increased gradually for as-cast specimens with increasing the overheating temperature. Microstructural characterization indicated that the grain structures, the morphologies of MC carbides and γ/γ′ eutectics showed no significant change after overheating at different temperatures. The mechanical properties were improved by the more cuboidal γ′ precipitates in the dendritic and interdendritic region, the sufficient re-precipitation of ultra-fine γ′ particles as well as the dissolution of γ′ particles along grain boundary. The initial microstructures had a significant influence on the microstructural evolution and corresponding mechanical properties after overheating exposure. In addition, the high temperature and room temperature fracture behaviors of overheated K424 alloy were also discussed.