Influence of heat input on droplet transfer process, microstructure, and mechanical properties during underwater submerged arc welding

Underwater welding has become increasingly important with the rapid development of marine resource exploitation and offshore engineering. In this study, underwater submerged arc welding was performed on X65 pipeline steel. Different heat inputs were obtained by varying the welding current under constant voltage and welding speed conditions, and the effects of heat input on droplet transfer behavior, arc stability, weld microstructure, and mechanical properties were systematically investigated. The results show that increasing heat input significantly altered the droplet transfer behavior during underwater submerged arc welding. The fraction of globular repelled transfer decreased progressively, while the proportion of surface tension transfer increased from 5.3% to 11.5%. In addition, the droplet transfer period was shortened, and the droplet diameter increased. Sound welds were obtained under all tested heat input conditions. However, significant microstructural evolutions were observed with the increase of heat input. The fraction of acicular ferrite gradually decreased, while the contents of bulk ferrites and side-plate ferrite increased, accompanied by continuous grain coarsening. Such microstructural changes resulted in a gradual deterioration of the mechanical properties, including ultimate tensile strength and impact toughness, with increasing heat input.