Effect of mixed powder proportion on microstructure and defects of high-nitrogen steel welded joints

The lack of Mn content and the importance of Mn to nitrogen content had been confirmed in this study and the welding of high-nitrogen steel with different proportions of MnN/CrN mixed powders was investigated. The distribution of pore defects in the welds and welding stability were analyzed by the synchronous acquisition of photographs using a high-speed camera, current–voltage curves, and plasma spectra. The nitrogen conversion process from powder to molten pool was studied, and the optimized MnN ratio was obtained by observing the microstructure changes in the weld. CrN powder did not produce intermediate products after melting, and it mixed well with the molten pool after melting/decomposition. Cr of the powder significantly improved nitrogen solubility in the austenite, thereby inhibiting micro pores; no CrN/Cr₂N precipitates were found in the welds. The addition of Mn increased the austenite ratio; however, the MnN powder started decomposing at 800 ℃ and produced intermediate products Mn₄N, Mn₂N and N₂, forming an unstable arc and keyhole and resulting in more pore defects. Thus, a defect-free welding joint with high tensile strength and toughness was obtained by controlling the proportion of MnN to < 20 wt%. When the proportion of MnN was > 30 wt%, pores were formed, which deteriorated the mechanical properties of the joint.