High nitrogen steel (HNS) has an excellent tensile strength and impact property due to the solid solution strengthening of nitrogen, which provides a good application prospect in many fields. Fusion welding is one of the main processing methods of HNS, but the process is prone to spatters, serious nitrogen losses, N₂ porosities, and poor performances of joints, resulting in the failure of large-scale engineering applications of HNS. In this work, the development of HNS welding is reviewed, and the problems including the droplet transfer instability, N₂ porosities, nitrogen losses and poor mechanical properties due to high nitrogen content are discussed. According to previous welding experiences, the adoption of a welding method with a low heat input is proposed, which utilizes powders instead of wires, optimizes compositions in the shielding gas and feeding materials in order to solve the above problems, and improves the mechanical properties of the weld.

In this work, the effects of preheating temperatures on martensitic transformations in a laser beam-welded AH36 steel joint were observed using a numerical study. In the same weld, the martensitic contents increased slightly from the upper area, the middle area to the lower area, and simulated martensite contents in the fusion zone were slightly lower than that in the HAZ (Heat Affected Zone). Under different preheating temperatures, simulated martensitic contents decrease with the increase of the preheating temperature. According to the simulated results, the average cooling rate and the CCT (Continuous Cooling Transformation) diagram were drawn to analyze the relationships between preheating temperatures and martensitic transformations. Simulated martensitic contents agreed well with the experimental metallographic microstructures. Moreover, the measured microhardness was reduced with the increasing preheating temperature, and measured microhardness in HAZ was higher than that in the fusion zone. The accuracy of the simulation results was further confirmed. The main significance of this work is to provide a numerical model to design martensitic contents in order to control the performances of the weld, avoiding many tests.