Laser-arc hybrid welding process: effect of welding speed on softening behavior of 700L beam steel

The lightweighting of automotive structures demands chassis steel strength upgraded from 500–600 MPa to 700–800 MPa. 700L beam steel has a ferritic–pearlitic matrix with grain-boundary dispersed cementite (Fe₃C), offering 800 MPa tensile strength and a fatigue ratio of 0.75. Conventional arc welding induces severe heat-affected zone (HAZ) softening. This study investigates laser-arc hybrid welding of this steel. Welding thermal cycling dissolves grain-boundary Fe₃C and triggers HAZ softening, causing fracture in softened zones during tensile and fatigue tests. At 2.0 m/min welding speed, HAZ width decreases to 450 μm; all tensile specimens fail in base metal. Within 1.0–2.0 m/min, joints achieve >107 high-cycle fatigue cycles at 550 MPa cyclic stress. Weld metal and HAZ show higher impact absorbed energy than base metal.