Effect of Hardness Distribution on Strength of Narrow-Gap Hot-Wire Laser-Welded Joint for High-Tensile Strength Steel

Application of high-heat input welding on high-tensile strength steels causes deterioration of mechanical properties of the welded joint, due to softening and grain coarsening in the heat-affected zone (HAZ). In this study, low-heat input narrow-gap hot-wire laser welding was applied to 12 mm thick 780 MPa-class high-tensile strength steel plate. Conditions were optimized based on microstructural observations of joints produced at various welding speeds. Heat input was estimated from measured grain size. Evaluation of properties of joints welded at 0.5 m/min revealed sound toughness, tensile strength, and elongation. The effect of undermatched weld metal width on joint strength was analyzed using a finite element method. When the width of undermatched weld metal was 2.5 mm, the joint strength was 99% of the base metal strength; when it was 7.5 mm, the strength dropped to 95%. The effect of HAZ softening width on joint strength with even-matched weld metals was similarly analyzed, showing that even when the HAZ softening width was 2.0 mm, the joint strength was 98% of the base metal strength. The results of this study suggest that narrow-gap hot-wire laser welding can efficiently reduce heat input and the HAZ softening zone, thereby achieving both high strength and high toughness.