Welding experiences are demonstrated, which were developed on 2205 type duplex and 2509 type superduplex stainless steels (DSS and SDSS). The welded joints were produced by activated TIG welding and laser beam welding. For the ATIG process, which is a special kind of the TIG-welding, silica powder was used as activating flux. Microstructural characterizations were applied for the measurement of ferrite/austenite phase ratio in the weldments. The welded joints were thermally aged at temperature of 400 °C. The phase transformations produced during thermal ageing were investigated by optical and scanning electron microscopy and thermoelectric power measurement. Investigations show that the thermal ageing induced phase transformations are similar at ATIG-welded weldment and DSS base material, but the incubation time is significantly shorter in the weldment than in the base material when laser welding is applied.