ステンレス鋼のプラズマアーク溶接におけるバックシールドガスの影響

Abstract

In the plasma welding, the welding conditions, that is nozzle diameter, composition and flow rate of plasma gas and cover shield gas, welding current and travelling speed, must be selected according to material and thickness of the work. In the plasma arc welding of stainless steel, we discovered that the range of proper welding condition is remarkably affected by the quality of the back. shield gas.To study the effect of the composition of back shield gases on the forming of back beads, we examined plasma arc welding of 304 stainless steel (t=9 mm), with various back shield gases of Ar, N2, CO2, Ar mixed CO2, Ar mixed O2, N2 mixed CO2, and N2 mixed O2.A summary of the results is shown below.(1) When O2 or CO2 is mixed to inert gas as back shield gas, the length of back plasma flame is shorter than in the case of pure inert gas. There is a tendency of the flame be coming shorter according to the amount of O2 or CO2 mixed to inert gas.(2) When we used inert gas that is Ar or N2 as back shield gas, the range of proper welding condition is very narrow. Therefore welding work is very difficult. But the range of proper welding condition is remarkably expanded by mixing of oxidizing gas that is O2 or CO2 to inert gas.(3) By mixing of 10% O2 or 30% CO2 with Ar, the range of proper welding condition becomes maximum. With mixing of over 50% O2, the back bead is severely oxidized, spatter increase, undercut and concave occur at the back bead.(4) Indifferently to the composition of back shield gas, the cross section of welded metal appeared wine cup. And microstructure, hardness and corrosion resistibility of the welded metal were not affected by the composition of back shield gas.