Effect of cooling method during heat treatment on microstructure and mechanical properties of FGH96 powder metallurgy superalloy

Heat treatment is the most critical thermal process determining the properties of powder metallurgy（PM）superalloy components, with FGH96 currently being one of the most prevalent PM Ni-based superalloys. This work investigates the effects of two distinct solution heat treatment cooling methods—full air cooling quenching and combined air-oil cooling quenching—on the microstructure and properties of FGH96 alloy ring parts. The results indicate that both cooling methods yield equivalent grain sizes, ranging from grade 6.5 to 7. Notably, full air-cooled rings exhibit more homogeneous distribution of secondary γʹ phase. In contrast, rings subjected to combined air-oil cooling exhibit coarser secondary γʹ phases, with reduced quantity on the inner side compared to the outer side, attributable to internally diminished cooling rate. During the later stage of quenching, ring parts undergoing full air cooling experience a slower cooling rate than those using combined air-oil cooling. Furthermore, fine γʹ phases, possessing sizes between the secondary and tertiary γʹ phases, precipitate along the grain boundaries using full air cooling method, leading to grain boundary strengthening. This enhances tensile strength but reduces elongation and plastic elongation at 68 h in high-temperature creep tests. Additionally, due to the more uniform cooling rate throughout the ring during full air cooling quenching, the surface residual stress reduces with more uniform distribution, thereby augmenting dimensional stability during subsequent machining processes.