Characterization of induction-assisted welds in high-strength steel grades

High-strength steel grades with high wear resistance and high toughness are potential candidates to reduce weight in heavy construction, agricultural machinery and shipbuilding. Weldability of HSS grades has limitations due to the loss of material properties caused by the complex thermomechanical treatments (i.e. increase of hardness and reduction of toughness caused by martensite formation within the weld seam and the resulting increment of the cold crack probability). The existing limitations can be overcome by the use of appropriate welding methods, such as laser welding combined with induction heating. In the present work, the effect of inductive heating on the microstructure and mechanical properties of fusion and heat-affected zones in butt-joints of the structural steel S690QL (3.0-6.0mm sheet thicknesses) produced by CO(ind 2) laser welding is studied by optical microscopy and hardness measurements. The results reveal that laser welding without induction heating causes heavy martensite formation in the fusion and heat affected zones, which results in a strong increase of hardness when compared with the parent material. Inductive annealing produces tempered martensite in the fusion zone, thus, leading to a significant and homogeneous reduction of hardness. The effect decreases with increasing sheet thickness. The most promising results are obtained by induction pre- and post-heat-treatments, where the hardness in the fusion zone is similar to that found in the parent material for both thin and thick sheets. However, the hardness distribution is not homogeneous and thermal martensite is formed in the heat-affected zone due to still fast cooling times