Fusion welding of high-strength quenched and tempered steels, such as S690QL, requires careful control of heat input due to the sensitivity of microstructural transformations in different regions of the weld joint. In this study, a single-pass twin-wire MAG welding process was employed to produce lap fillet welds between S690QL steel plates. The welded joints were evaluated in terms of macrostructural integrity, microstructural evolution, microhardness, tensile behaviour, fractography, and welding efficiency. Macrostructural inspection confirmed uniform weld geometry, complete fusion, and the absence of surface or subsurface defects. Optical microscopy and SEM analysis revealed that the base metal retained its tempered martensitic structure, whereas the weld metal exhibited a heterogeneous martensitic–bainitic microstructure influenced by local thermal gradients and cooling rates. The heat-affected zone displayed distinct subzones, coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), and inter-critical HAZ (ICHAZ), with localized secondary martensite and carbide precipitation. Microhardness mapping indicated the highest hardness in the HAZ, consistent with these microstructural variations. Tensile testing showed a yield strength of 698.31 MPa and an ultimate tensile strength of 796.13 MPa for the welded joint, comparable to the base metal (UTS = 770.74 MPa). Fractography confirmed ductile failure via microvoid coalescence, with measured microvoid diameters of 7–10 μm in the weld metal and 6–8 μm in the base metal. The twin-wire MAG process reduced arc-on time by 62% relative to conventional single-wire MAG welding for the same joint length, demonstrating significant productivity improvement.
Gudapati et al. (Sun,) studied this question.