An experimental and theoretical study of nitrogen flux in stainless steel TIG welds

Welding of nitrogen-alloyed stainless steels is today performed both in production and on-site. However achievement of the correct nitrogen balance during welding is delicate, and when the process is disturbed, it is not always clear which corrective measures should be made. In order to make rational decisions and allow for a reliable and robust process control, the mechanisms controlling the net nitrogen content of the weld metal must be identified, understood and mastered. The paper illustrates the present situation and illuminates the factors governing the nitrogen exchange between the weld pool, plasma and shielding gas in TIG welding. A factor of major importance is the nitriding capacity of plasma which is strongly enhanced compared with a gas with corresponding nitrogen content. Deviations from the ideal behaviour of a molecular diatomic gas are thus observed for the plasma and the Siewerts law is consequently not obeyed. Techniques applicable to the study of the nitriding capacity of plasma and the rate controlling reactions of nitrogen absorption and desorption from liquid steel in contact with plasma have been examined. Experiments using stationary TIG are and real welds with travelling are are presented, both were performed using different materials and shielding gases. A model was derived that accounts for several of the most important welding parameters and the alloying levels of the material allowing for predictions of how the individual parameters influence the nitrogen content.