The Thermodynamics and Kinetics of Gas Dissolution and Evolution from Iron Alloys

Abstract

Evolution of hydrogen and nitrogen during the solidification of cast iron may be a cause of pinholes or porosity in the casting. The fundamental thermodynamics and kinetics of the absorption and desorption of the gases are reviewed. Recent experimental results on the effect of S, P, Bi, Sn, Pb and Te on the rate of the nitrogen reaction with carbon saturated iron are presented. Several of these elements are apparently surface active on the melts and retard the rate of the chemical reaction significantly. The nitrogen in the liquid metal may come out of solution during solidification causing porosity. Even though the solubility of nitrogen is higher in austenite than in the Fe-C melts containing more than 2% C, nitrogen may evolve during solidification because the carbon enriched liquid has a lower solubility for nitrogen. Hydrogen can contribute to the total gas pressure and the thermodynamic pressure for the combined effect of hydrogen and nitrogen is given. The possibility of removing nitrogen by argon bubbling was explored using a mixed control model and the rate of removal is estimated to be slow. However, it may be possible to remove hydrogen by this method.