Characterisation and Performance Testing of Additively Manufactured Alloy 625 in High Temperature Water

Additive manufacturing (AM) of nickel-based alloys offers design flexibility and reduced lead times for critical components in pressurised water reactors (PWRs). However, its influence on stress corrosion cracking (SCC) behaviour remains poorly understood. This work investigates the microstructural evolution and SCC initiation susceptibility of Laser Powder Bed Fusion (LPBF) Alloy 625, with emphasis on the role of hot isostatic pressing (HIP). The as-built LPBF microstructure revealed a columnar dendritic structure with pronounced interdendritic segregation of Nb, Mo, and Ti. HIP treatment effectively homogenised the microstructure, resulting in equiaxed grains containing Nb-enriched grain boundary carbides. Cold work introduced fractured Nb-rich grain boundary carbides and nanoscale cavities, providing a mechanical baseline for distinguishing purely deformation-induced damage from environmentally assisted cracking. Accelerated oxidation tests of HIP-AM samples in hydrogenated steam at 480°C revealed selective degradation of Nb-rich carbides, forming voids along grain boundaries. Slow strain rate tensile (SSRT) tests were conducted on 20% cold-rolled HIP-AM specimens in simulated PWR water (342°C, 30 cc/kg H₂). Minimal intergranular SCC (IGSCC) was observed at 4.8% strain, whereas clear IGSCC initiation occurred at 26% strain, indicating high but finite SCC resistance under combined mechanical and environmental loading. These findings highlight that while HIP improves microstructural homogeneity, it introduces grain boundary features susceptible to oxidation-assisted degradation. The results provide critical insights for optimising post-processing strategies and assessing the long-term integrity of AM Alloy 625 in high temperature reactor water.