Induction heat treatment of laser welds

In this paper, a new approach based on induction heat-treatment of flat laser welded sheets is presented. With this new concept, the ductility of high strength steels GA260 with a thickness of 1.8 mm and CMn with a thickness of 2.13 mm is believed to be improved by prolonging the cooling time from 750º to 450º C. Initially, a simple analytical model was used to calculate the ideal energy contributions from a CO2 high power laser source together with an induction heat source such that the temperature can be kept at 600º C for 2.5 seconds. This knowledge was then used for the design of an induction coil. A number of systematic laboratory tests were then performed in order to study the effects of the coil on bead-on-plate laser welded samples. In these tests, important parameters such as coil current and distance between coil and sample were varied. Temperature measurements were made at distances of 0, 2 and 4 mm away from the virtual weld line for the induction heating process and at distances of 2 and 4 mm for the laser welding with induction heating process. For practical reasons during performance of these laboratory experiments, the induction coil was placed below the sheets, trailing the laser beam as close as possible. After welding, the samples were quality assessed according to ISO 13.919-1 and tested for hardness. The metallurgical phases are analysed and briefly described. A comparison between purely laser welded samples and induction heat-treated laser welded samples is made. The temperature measurements show that, with the chosen mechanical set-up of laser and induction coil, it is difficult to obtain a quick increase to around 300º C, the temperatures required for efficient heat-treatment with the induction coil alone. Despite this, a reduction in the hardness values of both GA260 and CMn were recorded when laser welded samples were induction heat-treated. The reduction was 6 to 8 % for GA260 and 41 to 45 % for CMn, respectively. GA260 displays a ferrite structure in basic and welded form, whereas CMn, with ten times more carbon, shows a ferrite and bainite structure when laser welded. Thus, when induction heat-treating, the carbon was better dispersed in the grains of CMn.