Microstructural Evolution during Welding and its Influence on the Impact Toughness of Heat-Affected Sub-Zones of a W-Co Containing Martensitic Steel

Abstract

This study investigates welding thermal effects on microstructure and impact toughness in the heat-affected zone (HAZ) of novel 9-12% Cr martensitic steel FW2. The HAZ is divided into sub-zones including the near-weld zone (NWZ), the coarse-grained HAZ (CGHAZ), the fine-grained HAZ (FGHAZ), the inter-critical HAZ (ICHAZ), and the over-tempered HAZ (OTHAZ), according to the peak welding temperature. Reduction in grain size and residual \(\delta\) ferrite was observed in the NWZ, suggesting that \(\gamma \to \delta\) and incomplete \(\delta \to \gamma\) transformations occurred during welding. Using welding thermal simulation, homogeneous microstructures representing three sub-zones (CGHAZ, FGHAZ, and OTHAZ) were obtained, and Charpy impact tests of these sub-zones were performed. The FGHAZ demonstrated enhanced impact toughness due to refined prior austenite grains (PAGs). In the OTHAZ, the martensite underwent recovery under high-temperature tempering, where dislocation motion formed new high-angle grain boundaries (HAGBs), increasing HAGB density and consequently improving impact toughness. In actual welded joints, the impact energy of the FW2 HAZ was intermediate between that of the CGHAZ and OTHAZ, indicating that directly evaluating the impact toughness of the FW2 HAZ based solely on welded joint would be non-conservative.