Influence of N₂ protection on microstructure and properties of CMT-P composite arc welded joints

Taking UNS S32750 super duplex stainless steel as research object, cold metal transfer pulse (CMT-P) composite arc welding technology was adopted. Using optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron probe microstructure characterisation tools, microhardness, low temperature impact toughness properties test methods, this paper comparatively studied the influence mechanism of pure Ar and Ar + 2%N₂ as shielding gas on the microstructure, micro-hardness and low temperature toughness of welded joint. The result showed that compared with pure Ar in shielding gas, the welding process with 2%N₂ protection has less spatter, the weld metal is flat, straight and the fish scale pattern is more detailed and tight. In addition, the heat affected zone (HAZ) is mainly composed of excess ferrite and a small amount of austenite, accompanied by some harmful precipitation of Cr₂N. Therefore, compared with other areas of CMT-P composite arc welded joints, HAZ is with higher microhardness and lower toughness. The addition of 2%N₂ increases the austenite content in the weld metal and HAZ, the solid solution amount of N atoms in the ferrite and austenite, thereby improves the low-temperature toughness of the welded joints.