On the microstructure and yield strength of a Cu-free high-phosphorus high-nickel RPV steel before and after neutron irradiation

The extended operation of light water reactors (LWRs) requires maintaining the structural integrity of their pressure vessels (RPVs), which are susceptible to neutron-irradiation-induced embrittlement. This embrittlement leads to upward shifts in ductile-to-brittle transition temperature (DBTT) of low-alloy RPV steels. While previous studies have extensively explored embrittlement mechanisms, comprehensive, multiscale characterization of both pre-irradiation and post-irradiation microstructures remains limited. Here, we investigate the microstructure of a split-melt RPV steel both before and after high-fluence neutron irradiation. We observed Mn-Ni-Si-P rich precipitates following irradiation in the matrix and at dislocations, as well as significant pre- and post-irradiation segregation of various solutes to dislocations and grain boundaries. In contrast, the structures of dislocations, grain boundaries, and Mn- and Mo-carbides, which mediate both fracture and strength properties, were not affected by irradiation. The multiscale and multi-feature microstructural data were utilized to model the yield stress of the steel under both unirradiated and irradiated conditions. The model demonstrates good consistency with experimental observations, helping interpret how microstructural changes influence mechanical properties relevant to LWR RPV performance.