Investigation of the effects of mechanical properties and carbon content on cold cracking in laser welds of high-strength thin steel sheets

This study reports a case of cold cracking along welds, which arises from solidification cracking within the crater during the laser welding of high-strength steel sheets. In this investigation, we aimed to delineate the factors influencing cold cracking that originates from solidification cracking in the crater. This was achieved by using steel sheets whose mechanical properties (tensile strength: 0.6 to 1.5 GPa) and chemical composition (carbon content: 0.20 to 0.55%) were individually adjusted. The evaluation method involved performing laser welding in a stitch pattern on an oiled steel sheet, with variations in welding length. The evaluation focused on the maximum welding length at which cold cracking occurred (L_MAX). The results indicated that while a high tensile strength of the steel sheet marginally increased the L_MAX, the impact remained limited. Conversely, the carbon content of the steel sheet significantly influenced cold cracking; the L_MAX for carbon contents of 0.30% and 0.45% was substantially greater than that for 0.20%. However, an unusual behavior was observed at a carbon content of 0.55%, where the L_MAX was smaller than that for 0.45%, despite the significant hardening of the weld metal. This phenomenon was hypothesized to occur because the tensile residual stresses in the welds decreased as martensitic transformation starting temperature lowered and the expansion strain during the transformation increased with higher carbon content.