Grain-scale deformation and fracture mechanisms in tempered nuclear weld metals revealed by in-situ SEM and crystal plasticity modeling

The structural integrity of nuclear reactor pressure vessel (RPV) weld metal is paramount for nuclear power plant safety. In this study, a nuclear-grade low-alloy weld-deposited metal was tempered at 615 °C for different durations, and its microstructural evolution, local deformation behavior, and fracture mechanism were investigated using SEM, EBSD, TEM, in-situ SEM tensile testing, DIC, and EBSD-based CPFEM. The as-welded (AW) condition contained martensite–austenite (M–A) islands, with a retained austenite fraction of approximately 4.4%. After tempering for 1 h, the retained austenite fraction decreased to 0.2%, accompanied by rapid M–A decomposition, nanoscale M3C precipitation, increased dislocation density, and enhanced bainitic ferrite (BF) constraint. Prolonged tempering for 20 h eliminated M–A islands but promoted ferritic matrix rebalancing, carbide coarsening, and grain-boundary localization. Tensile testing showed that the 1H condition achieved the best strength–uniform elongation balance, whereas the 20H condition exhibited significant property degradation. In-situ SEM/DIC observations and Schmid factor analysis revealed that polygonal ferrite (PF) was the primary plastic deformation carrier, while M–A islands and BF acted as less-deformable constituents that altered slip localization. CPFEM further demonstrated that cracked M–A/PF interfaces exhibited higher local strain than uncracked interfaces, indicating that interfacial strain incompatibility dominated M–A-induced crack initiation. In contrast, BF constrained plastic slip-band propagation and delayed local fracture localization in the 1H condition, whereas carbide coarsening and continuous PF grain-boundary localization accelerated microvoid coalescence in the 20H condition. These results establish a grain-scale process–structure–deformation–fracture relationship for tempered nuclear weld metals and provide guidance for optimizing post-weld tempering treatments.