This study deals with the selection of appropriate welding parameters during autogenous arc welding of duplex stainless steels in order to achieve an optimum phase balance of austenite and ferrite in the as-welded microstructure. Specimens of duplex stainless steel 2205 with dimensions (40 × 40 × 10) mm3 were welded using autogenous arc welding under 95% Ar + 5% vol. N2 atmosphere. The weld pool temperature was measured by non-contact infrared temperature measurement, the weld bead dimensions were determined using scanning electron micrographs, and the final nitrogen concentration was evaluated by optical emission spectroscopy. The kinetics of nitrogen absorption and desorption in molten duplex stainless steel was discussed and all the relevant variables were presented. The effect of welding current and speed on the final nitrogen concentration was also discussed. Finally, based on this analysis, a method was set up which can be used to optimize the phase balance by using predictive methods of the Ferrite Number, such as the Welding Research Council (WRC)-92 diagram.