The nitrogen content of a stainless steel weld has a strong influence on microstructure and the material properties. In particular nitrogen will favorably influence the strength and corrosion resistance. In order to maintain these beneficial effects after welding is it crucial to avoid nitrogen losses during welding. One way to avoid such losses is to utilize shielding gases containing a balanced amount of nitrogen. The reproducibility of the N content on welding of nitrogen-alloyed stainless steels is, however, frequently reported to be fairly low. To manage this, an increased knowledge of the mechanisms controlling the nitrogen content of the weld metal is required.

Three different duplex stainless steels were investigated: SAF 2304 (UNS S32304), 2205 (UNS S31803) and SAF 2507 (UNS S32750). The steels were TIG welded with a stationary (non-travelling) arc, without filler and for times up to twenty seconds. Shielding gases containing argon with nitrogen additions 2.5–10% were employed. Based on a Fick’s law type of expression, an equation describing the flux of nitrogen in a stationary TIG weld was derived. The model is able to describe the flux of nitrogen from the plasma into the molten metal at short times and the escape of nitrogen as the weld pool cap size increases with time. The input parameters are arc time, weld pool geometry, plasma area, nitrogen activity and nitrogen content of the shielding gas. The output parameter is the nitrogen content of the weld metal.

Systematic variations of parameters such as weld geometry, nitrogen content of the shielding gas and nitrogen activity coefficient are performed to demonstrate the predictive capability of the model. An extension of the model was used to simulate nitrogen flux on autogenous bead on plate TIG welding with a travelling arc.