Effect of heat treatment on the microstructure and properties of nickel-based superalloy thin-wall pipe for the fourth-generation nuclear reactor

In recent years, the development of next-generation nuclear reactors with enhanced requirements for the safe and reliable production of nuclear energy has been attracting increasing attention. The thorium molten salt reactor (TMSR) has been regarded as the most promising prospective next-generation nuclear reactor because of its high security, desirable online refueling properties, minimization of nuclear waste, nuclear non-proliferation, etc. The structural materials for molten salt reactors should exhibit high temperature resistance and good corrosion and neutron irradiation resistance. The Ni-Cr-Mo-based superalloy GH3535 is the preferred material for TMSR applications because of its superior corrosion resistance and good mechanical properties. The Mo content of GH3535 is 15%~18% (mass fraction), which leads to the precipitation of a large amount of Mo-enriched M₆C carbides in the matrix. Numerous studies have shown that the precipitation of these carbides directly affects the grain size and mechanical properties of GH3535 alloy. In this study, the effects of heat treatment on the grain size, carbide distribution, and mechanical properties of GH3535 alloy were investigated by cold-rolled-pipe tests. To provide the experimental and theoretical basis for applying heat treatment to control the properties of GH3535 alloy, the thermodynamic and kinetics characteristics of GH3535 were calculated using the JMatPro simulation software. The influence of heat treatments on the size and homogeneity of grains, the precipitation character of carbides, and the mechanical properties of the alloy were investigated. The results show that the equilibrium precipitate of the GH3535 at temperatures between 900℃ and 1500℃ is a Mo-rich carbide of M₆C type and that the initial precipitation temperature of this M₆C-type carbide is in the liquid-solid phase range. The grains grow slowly when the solution temperature is less than 1200℃. When the solution temperature is increased to 1230℃, the grains grow quickly to an average size of 160 μm; the grains are homogeneous when the temperature is maintained at 1180℃ for 10 min. Tensile tests show that a higher solution temperature decreases the strength and increases the elongation. The tensile fracture mechanism of GH3535 alloy is microporous aggregation.