Microstructure and mechanical properties of heat affected zone of laser-MAG hybrid welded low carbon bainitic steel joints

In the study, laser-MAG hybrid welding of low carbon bainitic steel was conducted. The weld formation, microstructure and mechanical properties were investigated. The thermal simulation technology was used to investigate the microstructure evolution of the coarse grain heat affected zone (CGHAZ) under different t(8/5) conditions. Meanwhile, the simulated heat affected zone continuous cooling transformation (SHCCT) diagram of low carbon bainitic steel was developed and the optimized heat input range (6-18 kJ/cm) was obtained. The microstructure in the CGHAZ were lath bainites (LB) with a little granular bainites (GB), and M-A constituents. The average tensile strength of the joint was 714 MPa, reached 92.5% of the base metal. The impact energy of the HAZ was 33.8 J, which was 66.0% of the base metal. The reduction of impact toughness in the HAZ attributed to easier propagation of crack leaded by the M-A constituents and parallel slats within LB.