Nitrogen Dissolution Calculations During the GTA Welding of Steels

The presence of nitrogen concentrations at levels much higher than Sieverts' Law predictions in iron and steel weldments is well established. Since there is no commonly accepted methodology to calculate this concentration, a computational methodology is developed to determine the nature and concentration of various species in the plasma phase above the weld pool surface. Based on this methodology, a comprehensive thermodynamic analysis of the plasma phase of a GTA welding arc containing nitrogen is performed. It shows that ionized species dominate close to the electrode, while neutral monatomic and diatomic nitrogen are the primary species near the metal surface. Furthermore, the equilibrium monatomic nitrogen partial pressure at even low plasma temperatures is high enough to cause nitrogen saturation in the weld metal. When oxygen is added to a nitrogen-containing plasma, the resulting nitrogen concentration in the weld metal is further enhanced. Definitive proof is provided here for a mechanism in which nitrogen and oxygen species interact in the plasma phase resulting in a significant increase in the concentration of monatomic nitrogen at plasma temperatures below 6000K. Emission spectroscopic analysis of glow discharge plasmas validates the presence of NO in nitrogen and oxygen containing plasmas.