Mechanism of stabilization treatment on regulating grain boundary carbides and thermal exposure plasticity-toughness in high-carbon 800H alloy

Long-term service at 650 °C imposes stringent requirements on the plasticity and toughness of the high-carbon 800H alloy used for control rods in high-temperature gas-cooled reactors, with grain boundary carbide coarsening being the key bottleneck restricting its mechanical performance. This study systematically investigates the effect of stabilization treatment on the microstructure evolution and mechanical properties of the high-carbon 800H alloy during long-term thermal exposure at 650 °C. The results show that performing a stabilization treatment at 1020 °C for 2 h prior to thermal exposure significantly enhances the impact toughness and tensile ductility of the alloy. After thermal exposure for 300 h, 1000 h and 3000 h, the impact energy increases by 29.03 %, 75.76 % and 30.07 %, respectively, accompanied by a simultaneous improvement in ductility. The stabilization treatment promotes the preferential precipitation of thermally stable Ti(C,N) at grain boundaries, effectively suppressing the coarsening and continuous film-like formation of M₂₃C₆ carbides during subsequent thermal exposure. After 1000 h of thermal exposure, the average aspect ratio of grain boundary carbides decreases by 80.9 %, and the average interparticle spacing increases by a factor of 4.4. Meanwhile, Ti(C,N) forms incoherent interfaces with the γ matrix on both sides, promoting random nucleation of voids and retarding crack propagation along grain boundaries. By suppressing grain boundary carbide coarsening, the stabilization treatment endows the 800H alloy with an excellent combination of strength and toughness, providing an important theoretical basis for its long-term service in advanced nuclear energy systems.