Development of gas shielded arc welding process to achieve a very low diffusible hydrogen content in weld metals

High tensile strength steels (HTSS) are widely used for welded structures, and their strength becomes increasingly high. However, the susceptibility of cold cracking increases as the strength of the steel increases. To avoid cold cracking, low hydrogen welding consumables is used and preheat and interpass temperature is controlled. To lower preheat temperature or to weld without preheat, lower hydrogen welding consumables and process are required. Hydrogen absorption mechanism in gas shielded arc welding is discussed in this paper, and a new developed welding torch for ultra-low hydrogen welding is introduced. Hydrogen in welding wire is present as moisture in flux and lubricating oil at the wire surface. In arc welding, their temperature is increased by Joule heat and then they evaporate into shielding gas. Consequently, shielding gas near welding wire includes much more hydrogen than shielding gas in the gas tanks. This "wet shielding gas" carries hydrogen into arc, and some of hydrogen remains as diffusible hydrogen in the weld metal. "Wet shielding gas" is evacuated and kept away from the arc by using developed torch, resulting in a decrease of the diffusible hydrogen. Evacuating 5 l/min "wet shielding gas" reduces diffusible hydrogen by 50 %.