Synergistic effects of TiN precipitation and Mg-treated oxide metallurgy on CGHAZ toughness in high-heat-input welding

High-heat-input welding greatly improves construction efficiency but inevitably deteriorates the toughness of the coarse-grained heat-affected zone (CGHAZ) due to austenite grain coarsening and the formation of brittle microstructures. In this study, the synergistic effects of the precipitation of TiN and Mg-treated oxide metallurgy were systematically investigated. Four experimental steels with varying Ti–N levels and with or without Mg treatment were designed based on the optimized Ti–N level established in our previous work. Electrogas welding with a heat input of 550 kJ/cm was applied, and CGHAZ was examined through Charpy impact testing, microstructural characterization, quantitative inclusion analysis, and in situ observation using laser scanning confocal microscopy. The results show that under the optimized Ti–N level, Mg treatment cannot further refine austenite grains and provides only a slight toughness increase. In contrast, excessive high Ti–N level produce coarse liquid-precipitated TiN, severely reducing toughness, but Mg treatment effectively refines these coarse TiN particles and improves their morphology, leading to a remarkable toughness enhancement to 160 ± 21 J. These findings reveal a previously underappreciated synergy between the precipitation of TiN and Mg-treated oxide metallurgy and establish a new alloy-design framework for developing steels suitable for high heat-input welded structures.