Heat Treatment Optimisation of Electron Beam Welded Reactor Pressure Vessel Steel

Electron beam welding (EBW) is a promising technology for the joining of large steel components such as pressure vessels. Notably, EBW does not use a filler metal, meaning that the weld and parent material (PM) should have the same composition. Accordingly, it may be possible to homogenise microstructure and properties across the weld given a suitable post-weld heat treatment (PWHT), essentially making the weld ‘disappear’. To investigate this possibility, three different PWHTs were applied to an SA508 Grade 4N weld. A stress-relief PWHT only had a relatively minor impact on the variations in microstructure and hardness across the fusion zone (FZ), heat-affected zone (HAZ) and PM versus the as-welded state, and compositional microsegregation in the FZ was unchanged. Hardness was successfully homogenised across the parent and weld in heat treatments involving a traditional 860 °C austenitisation, quench and temper, but compositional microsegregation persisted in the FZ. The addition of a homogenisation step at 1200 °C eliminated this microsegregation, with the weld area only remaining visible owing to the presence of compositional banding in the PM. However, homogenisation also led to the formation of undesirable prior austenite grain structures following subsequent re-austenitisation at 860 °C. Nevertheless, the application of PWHTs involving a full re-austenitisation seems promising for future applications.