Environmental effects of microstructure stability on nickel-base superalloys

Abstract

Next generation nuclear reactor VHTR (Very High Temperature gas-cooled Reactor) wants to achieve higher thermal efficiency and hydrogen production. IHX (Intermediate Heat eXchanger) will be exposed to the highest temperature condition among lots of structural components. Solid-solution hardening nickel-base superalloys Alloy 617 and Haynes 230 are expected to use for this applications. Studies on oxidation test and time dependent deformation at 900℃ were conducted before. This study is focused on the microstructure evolution and mechanical properties at other temperature ranges. Furthermore, considering heat treatment history especially cooling rate effects on microstructure evolution, those of two superalloys are cooled down to room temperature by air and furnace. Materials behavior at intermediation temperature ranges from 600-900℃ and diffusion bonding condition (1150℃) were evaluated. Vicker`s hardness test and small-size tensile test were carried out for each specimen at room temperature. Hardness number and tensile strength were higher than any other temperature condition at 700℃ due to gamma prime phases for Alloy 617. As the aluminum contents of Haynes 230 is far less than Alloy 617, there is no big difference for Haynes 230 at intermediate temperature ranges. The value of mechanical property of alloys at 1150℃ air cooling condition was severely decreased and fully ductile fracture was detected for both alloys. On the other hand, the values showed the tendency of return to the intermediate temperature ranges when the specimen was slowly cooled down. Characteristic precipitates along the grain boundaries were detected. There was no other singularity up to 700℃ for Alloy 617. However, lots of tiny $M_{23}C_6$ type carbide were formed after 800℃ heat treatment, and those of carbide got bigger and bigger as the heat treatment temperature increased up to 900℃. For diffusion bonding temperature, grain boundary carbide again diminished when the specimen was e...