22SiMn2TiB Steel Weld Joint With High Nitrogen Achieved By PMIG Automatic Welding

In this dissertation,307Mo and high nitrogen austenitic stainless steel wire were used as a welding filler. PMIG robotic automation welding technology was carried out on the 6mm thick super-strength steel, and the factors of welded joints of solid solution nitrogen content were investigated. Meanwhile,the mechanics performance analysis and research of the welded joints were also implemented.By adjusting the welding current, voltage and other parameters,the effect of welding heat input on weld quality and welding quality had been studied. The results showed that within an appropriate range of welding parameters, small weld welding parameters are more favorable, resulting in less welding defects, high content of solid solution nitrogen and uniform weld microstructure.The influence of the active O element on the welded joints was also clarified. After adding 2.5% O2 to Ar, better appearance of weld is gained, and weld solution nitrogen content increased. With the increase of N2 ratio in the process of welding, the content of the weld solution nitrogen first increased and then decreased. While the proportion of nitrogen reaches 40%, using 307Mo and high nitrogen welding wire separately as the welding filler wire, joints have reached the maximum content of the solid solution nitrogen(0.35% and 0.87% respectively). The high nitridation of welded joints has realized bascially.In the same heat input conditions, solidification mode and microstructure state of the weld metal is affected by the weld nitrogen content. The microstructure of the weld is the austenite matrix and ferrite structure, which deviates from theoretical value. And with the increase of the proportion of nitrogen, austenite dendrites coarsen, the amount of ferrite reduces. It shows that weld is an unbalanced microstructure, and too fast solidification leads to residues of ferrite.The XRD results showed that the weld microstructure formed by two kinds of welding filler metal are austenite and nitrides. The addition of oxygen element makes the diffraction peaks appear a certain angle deviation. With an increase in the content of solid solution nitrogen, the nitrogen occupies the lattice gap of the face centered cubic lattice, causing a certain lattice distortion. The image of scanning electron microscopy showed that the homogenization of the weld zone microstructure is realized basically, and a good element transition between the weld and the base metal is also achieved.The mechanical property results showed that the weld strength is proportional to the nitrogen content, and the oxygen makes the mechanical properties of the welded seam uniform, but how to avoid the formation of pores in the welding process will be a direction of future research.