Physical modeling of nitrogen partition between the weld metal and its plasma environment

When pure iron is exposed to diatomic nitrogen, the equilibrium concentration of nitrogen in iron can be determined from Sieverts' Law. However, during many fusion welding processes, a plasma, consisting of various excited species, atoms, ions, and electrons, is present near the weld pool and affects the dissolution process. Sieverts' Law, therefore, cannot be applied to understand the dissolution of nitrogen during welding of steels. The work reported here is aimed at understanding the basic laws governing the partition of nitrogen between the weld metal and its plasma environment. To avoid temperature gradients on the weld pool surface and spatially variable properties of the plasma in the gas phase, small high-purity iron samples, maintained at a constant temperature, were exposed to a number of low-pressure nitrogen plasmas and analyzed for the resulting nitrogen contents. Throughout the range of experimental conditions, the concentration of nitrogen in each iron sample was significantly higher than the Sieverts' Law predictions. Nitrogen-containing plasmas were characterized using optical emission spectroscopy to examine the species present and determine electron temperature. Thermodynamic calculations show that a trace amount of monatomic nitrogen in the plasma causes a many-fold increase in the nitrogen solubility above that predicted by Sieverts' Law. The experimental results are explained on the basis of a model involving the dissociation of diatomic nitrogen in the plasma and subsequent dissolution of atomic nitrogen in iron. The applicability of the results of the physical modeling experiments to GTA welding of iron is demonstrated.