Fracture Toughness Analysis of Ni–Cr–Mo Low-Alloy Steel for Advanced Nuclear Power

The fracture toughness of nuclear reactor pressure vessel (RPV) steel is an important basis for the structural integrity evaluation of equipment. SA508 Gr.4N (Cr-Ni-Mo) low-alloy steel has attracted people's attention because of its excellent strength and toughness, and it is considered as a candidate material for the next generation of RPV. The fracture toughness of SA508 Gr.4N alloy steel was analyzed from the perspective of macroscopic mechanical properties and microstructure, and compared with that of the SA508 Gr.3 (Mn-Ni-Mo) steel used in commercial PWR nuclear power plants. SA508 Gr.4N steel showed better toughness reserve than SA508 Gr.3 steel in terms of fracture toughness parameters such as the reference nil-ductility transition temperature RT_NDT, brittleness transition characteristic temperature T_41J, upper shelf energy and master curve reference temperature T₀. The reasons for the excellent fracture toughness of SA508 Gr.4N steel were analyzed from the aspects of microstructure, precipitation and grain boundary structure.