Ferrite fraction in duplex stainless steel welded with a novel plasma arc torch

Duplex stainless steels generally solidify with a ferritic microstructure, followed by a partial transformation to austenite during cooling. Rapid cooling may render the transformation incomplete. The too-low or too-high cooling rate can also result in the formation of intermetallic phases and precipitates which can have a detrimental effect on the pitting corrosion resistance. The possibilities of achieving a microstructure with 30-70% ferrite in duplex stainless steel (1.4462, DX 2205) autogenously welded with a novel plasma welding equipment were investigated. Welding was performed as a square butt joint, with a 0-mm nominal gap size, on a 6-mm plate thickness, without the use of filler metal. The plasma torch uses a focus gas that allows it to constrict the arc in addition to the conventional constriction by the plasma nozzle. The three separated gas flows (plasma, focus and shielding gas) allow the gas composition to be tailored in the different gas flows. The effect of argon, nitrogen and helium content on the phase balance related to the gas flow channels was studied. The presence of nitrides as well as the sigma phase was investigated, and the joint quality was evaluated. The microstructure of the welded samples, investigated by image analysis, was correlated to pitting corrosion resistance. It was shown that high joint quality, quality level B, with favourable ferrite/austenite phase balance can be achieved through autogenous single-pass welding of duplex stainless steel 1.4462, DX 2205 (6.0 mm) using the novel plasma torch. With N-2 additions in, most notably, the plasma gas and the shielding gas, it was possible to get a ferrite phase fraction in the weld metal close to 50%.

Automated summary

This study investigated the possibility of achieving a microstructure with 30-70% ferrite in duplex stainless steel (1.4462, DX 2205) through autogenous welding with a novel plasma welding equipment. The effect of gas composition on the phase balance and joint quality was studied. It was found that high joint quality with a favorable ferrite/austenite phase balance can be achieved using the novel plasma torch.