Microstructure-mechanical behaviour correlation for electron-beam welded SA508 grade 3 steel after post-weld normalising treatment

Electron beam welding (EBW) has emerged as a viable process for joining of thick section structural components in pressurised water reactors, with the potential for post-weld treatments to optimise local microstructures and mechanical performance. In this work, we evaluated the uniformity in local microstructure and mechanical properties of an 80 mm-thick electron beam weld in forged SA508 Grade 3 steel, after a Post-Weld Normalising Heat Treatment (PWNT) involving two steps: re-austenitisation at 875 °C for 4 hours, water quenching, followed by tempering at 642 ºC for 4 hours. The variability in the prior austenite grain size was evaluated by large area electron backscatter diffraction (EBSD), and room-temperature tensile testing and hardness mapping were also carried out. The results were compared with those for two reference welds of the same steel grade: one in the as-welded condition, and another that was subject to a conventional single-step (stress-relieving) post weld heat treatment (PWHT) at 607 ºC for 2.5 hours. The two-step PWNT reduced the axial size of the largest prior-austenite grains in the fusion zone from ≥ 400 μm to ≤ 150 μm, whereas the cross-weld variation in hardness reduced from ~ 25% for a conventional PWHT to ~ 10% for a PWNT, improving reliability and overall mechanical property consistency.