Depth-resolved irradiation hardening across electron beam welded XM-19

XM-19 austenitic stainless steel combines high strength and high-temperature corrosion resistance with good post-irradiation ductility and radiation resistance, making it attractive for small modular reactor (SMR) structures. Electron beam welding (EBW), widely used for thick reactor internals, imposes a steep thermal history that produces microstructural heterogeneity and may alter the local irradiation response. Here, an EBW XM-19 joint was irradiated with 2.0 MeV protons at 360 °C, and the zone-dependent irradiation hardening of the fusion zone (FZ), coarse-grained HAZ (CG-HAZ), fine-grained HAZ (FG-HAZ), and base metal (BM) was quantified by depth-resolved TD nanoindentation with Nix–Gao indentation size effect (ISE) correction. EBSD confirmed a heterogeneous starting state with zone-dependent grain size and local misorientation. Stopping and Range of Ions in Matter (SRIM), cross-sectional electron channeling contrast imaging (ECCI), and nanoindentation consistently identified a damaged layer extending ~19.8 μm from the surface, and the stable nominal 1 dpa ± 0.2 (0.8–1.2 dpa) window was adopted as the primary comparison basis. At nominal 1 dpa, the self-referenced hardening was 1.14, 1.01, 1.00, and 0.72 GPa for BM, FZ, FG-HAZ, and CG-HAZ; after normalization by the unirradiated hardness, CG-HAZ showed the lowest and FZ and FG-HAZ the highest irradiation-hardening sensitivity. When referenced to the unirradiated BM, the weld zones were initially softer than the base metal and hardened non-uniformly after irradiation. Consequently, the cross-weld hardness heterogeneity (Hmax - Hmin) widened from 0.69 to 0.98 GPa, indicating that irradiation may amplify rather than relax the weld-induced heterogeneity.