Microstructure and residual stress formation in induction-assisted laser welding of the steel S690QL

Steels of high mechanical strength combined with high toughness, such as those in quenched and tempered condition are required to reduce weight in industrial machinery. Their mechanical performance is impaired by welding operations which often cause a reduction of toughness and increase the probability for cold cracking due to martensite formation in the weld seam. The limited weldability of high-strength steels therefore demands appropriate joining procedures to increase their use in industrial construction and reduce reworking costs. Induction heating is capable of directly producing heat inside a work piece. This enables the integration of induction heat-treatments into serial welding processes. In this work, the effect of induction-assisted laser welding on the microstructure and residual stresses in S690QL butt joints was investigated. The results reveal that conventional laser welding causes strong martensite formation in the weld seam and the heat-affected zone. This leads to prohibitive hardness values. Induction heat-treatments result in an efficient reduction of hardness in the fusion zone. However, the efficiency decreases with increasing sheet thicknesses. The residual stress distributions after laser welding with and without induction heating are typical of fusion welding. Although an effective reduction of hardness is achieved by induction-assisted laser welding, the residual stresses remain significantly high.