Minimization of heat-affected zone size in welded ultra-fine grained steel under cooling by liquid nitrogen during laser welding

Ultra-fine grained steel (UFGS) with an average grain size of less than 1 μm has been developed and is expected to demonstrate superior mechanical and chemical properties. However, its welded heat-affected zones, HAZ, substantially affecting the strength of a welded joint, will be easily softened after welding. Therefore, minimization of UFGS's HAZ size during laser welding was carried out using the cooling conductor liquid nitrogen. It was found that a shielding gas, with adequate flow rate for the liquid nitrogen depth, was used to remove nitrogen from the area of laser beam irradiation to stabilize the weld bead. Also, the YAG laser system was mainly used because it has a lower temperature, which results in a decreased occurrence of pits and blowholes, of laser induced plasma or plume. HAZ size minimization strongly depends on the initial plate temperature. Reduced initial plate temperature shrinks the specific heated temperature range in which softening occurs. However, under room temperature, due possibly to decreasing thermal conductivity that prevents heat removal from HAZ, the benefit of reducing the initial plate temperature is limited. The optimal initial temperature to minimize the HAZ size was found to be 123 K.