Effect of droplet transition on arc morphology, Mn evaporation and microstructure during the CMT welding with high nitrogen Cr-Mn steel

The instability of arc morphology and severe spatters caused by the droplet burst leads to a great challenge in cold metal transfer (CMT) welding with high nitrogen steels (HNSs). During this process, the effusion of evaporative Mn will intensify the loss of nitrogen and change the solidification models. This paper focuses on the influence of droplet transition on arc morphology, Mn evaporation and the additive seam characteristics under various parameters. The results show that four typical stages of the droplet were found as the growth, parking, expansion and bursting stages. A stable arc and droplet transition process can be achieved by using a lower peak current (140 similar to 240 A) along with less peak-current duration (3-5 ms) which also avoids the spatter accumulation zone at the weld toe area. The welding fume is mainly composed of the MnO and Mn3O4 oxides and varies with the peak current. The loss of Mn was more sensitive to the peak current than the peak current durations (<= 7 ms); and a maximum value of 1.83 wt% was found at a peak current of 300 A. Once the loss of Mn was higher than 1 %, the loss of nitrogen will exceed 20 % resulting in an austenite-ferrite dual-phase microstructure.