Microstructure and mechanical properties of heat-affected zone of high nitrogen steel simulated for laser welding conditions

In order to obtain some information on the weldability of high nitrogen steel (HNS), the microstructure and mechanical properties of the heat-affected zone (HAZ) of HNS under laser welding conditions were investigated by using thermo-simulation technique. The experimental results indicate that the microstructure in the simulated HAZ of HNS consists of austenite and delta-ferrite that occurs in the grain boundary of austenite. The hardness of the simulated coarse-grained heat-affected zone (CGHAZ) increases when the cooling rate increases, and that of the simulated HAZ decreases while the peak temperature decreases. The results also show that the hardness of the simulated HAZ is higher than that of the base metal, indicating no softening of the HAZ under appropriate welding conditions. The impact toughness of simulated CGHAZ increases at first and then decreases with the increase of the cooling rate, whereas two brittle zones exist in the HAZ.