Thermal cycle-dependent metallurgical variations and their effects on the through-thickness mechanical properties in thick section narrow-gap welds

The primary components in pressurised water reactors are manufactured by welding thick sections of either SA508 or SA533 pressure vessel steel using processes such as submerged arc welding (SAW) or gas-tungsten arc welding (GTAW). Narrow-groove (NG) variants of these processes have reduced welding times, but thick-section welds still require a large number of passes. In this work, the effects of a large number of welding thermal cycles on the through-thickness variability in microstructure and mechanical properties have been analysed for NG-GTAW and NG-SAW joints made in 78 mm thick SA533 steel. Microstructures were characterised using optical and scanning electron microscopy, while mechanical properties were captured in cross-weld tensile tests using digital image correlation and through tests on coupons extracted exclusively from the weld metal and from the heat-affected zone. Charpy impact testing was used to assess toughness. While the toughness was relatively consistent throughout the SAW joint, significant through-thickness variations in toughness were observed in the NG-GTAW joint, which can be attributed to the varying degree to which the weldment was tempered by subsequent welding thermal cycles.