Based on several recent evaluations of the plasma keyhole process, its potential for application to duplex stainless steel welding at hyperbaric underwater environments has been investigated.

With the background of basic research work being carried out in advance, the effects of increasing pressure of the environment and of various nitrogen partial pressures in the plasma gas argon on nitrogen pick up to the plasma keyhole weld metal was established, at operational parameters providing sound welds, up to 31 bar. In accordance with other literature, the nitrogen contents were reduced at increasing ambient pressure and constant partial pressures of nitrogen in the plasma gas flow. Increasing partial nitrogen pressures were providing higher nitrogen contents in the weld metal, which, however, should not be interpreted in terms of a square root relationship. The limits of porosity were discussed and evaluated, the present tests revealing no porosity. The increase of nitrogen in the weld metals resulted in reduced ferrite contents and slightly increased pitting potentials in artificial seawater at RT. These results conformed to basic previous investigations on the effect of nitrogen and cooling regimes. It was shown, that the base materials should contain about the same nitrogen contents as the weld metals, i.e. about 0,25–0,27%N in order to provide for the same microstructures in the heat affected zones as compared to the weld metals. For the not heat affected base material, this would require a slightly increased temperature for solution heat treating and subsequent quenching, in order to arrive at the required ferrite/austenite ratio and properties outside the heat affected zone.