Microstructure and mechanical properties of laser welded joint of NULCB steel

In order to investigate the weldability of a new ultra-low carbon bainitic(NULCB) steel,the steel is welded with a 3kW CO2 laser and the microstructure and mechanical properties of the weld joint are investigated.The experimental results indicate that the hardness of welded joint is higher than that of the base metal while the laser heat input is between 120J/mm and 600J/mm,and no obvious zone is softened after laser welding.The microstructure of the weld metal and the coarse-grained heat-affected zone are both granular bainite that consists of the bainite lath and the M-A constituent.When the heat input increases from 120J/mm to 600J/mm,the average width,gross and shape parameter of M-A increases,but the line density decreases.As the heat input of laser welding increases,the impact energy of the weld metal increases at first and then decreases,and it is higher than that of the base metal under appropriate laser welding conditions.