The fluid flows in molten pools during arc welding are important factors. These in turn influence in overall heat and mass transfer, which determine the mechanical properties and quality of the weld fusion zone. Here, modelling results are presented concerning the time dependent weld pool flow and temperature in gas tungsten arc welding (GTA) of the difference type of stainless steels. It is proved that the temperature fields are strongly affected by the convection at the weld pool’s surfaces. With the stainless steel type 304 (low sulfur content 0.0005 weight % and high sulfur content 0.0139 weight %), the actual chaotic time dependent melt flow is obtained with a fully time dependent model. In those cases, the fluid flow in the weld pool is highly complex and it influenced the weld pool`s depth and width. For the 645 SMO steel, which has an extremely low sulfur content and low conductivity, the chaotic fluid flows did not appear. The calculated geometry of the weld fusion zone and heat affected zone were in good agreement with the experimental results, both with or without chaotic fluid flows.