The level of ferritization in the high temperature heat-affected zone (HT-HAZ) affects the weldability of duplex stainless steels. The modern grades show different sensitivity and especially the nitrogen content has been proposed to control the grain growth and austenite formation. In this work, 3-, 4- and 5-mm thick UNS S32101, UNS S32304, UNS S32205 and UNS S32750 with a nitrogen range of 0.09–0.28 wt.-% were considered. Gas tungsten arc welding (GTAW) was conducted bead-on-plate with pure argon (Ar) and Ar + 2% N2 as shielding gas. Measurements of the HT-HAZ width and ferrite content were compared with values reported for welds and thermo-mechanical Gleeble® simulations in existing literature. The HT-HAZ width decreased with the material thickness and the nitrogen content in the base metal, while only the nitrogen alloying had a clear effect on the phase balance. Nitrogen additions to the shielding gas efficiently counteracted weld metal nitrogen loss and improved the austenite formation as compared to 100% Ar but had no clear effect on the HT-HAZ width nor ferrite content. A positive influence of the base material nitrogen content on the phase balance could be validated with thermodynamic simulations using Thermo-Calc and DICTRA, but the negative effect of high cooling rates was underestimated.