Decarburization and Denitrogenization of Iron and Iron-Chromium Alloys by Plasma Jet of Hydrogen-Argon Gas Mixture

Abstract

Trials to enhance the decarburization and the denitrogenization of molten Fe-Cr alloys were made by blowing plasma jet of argon-hydrogen gas mixture on to the surface of molten metals. The weight of melts was 1 to 3kg, the gas blowing rate 8 t/min and the electric power consumption 15 to 30kVA. The final concentrations of carbon and nitrogen obtained are as follows: C<0.002% for Fe, 18-8 stainless steel, 60% Cr-Fe alloy and Cr, N=0.0015% for Fe and N=0.0065% for 25% Cr-Fe alloy. This satisfies the conditions of intergranular corrosion resistance and ductility, C+N<100-130ppm, for ferritic stainless steel. By estimating the dissociation constant of H2 in plasma jet at the metal surface, it is indicated that hydrogen atoms in the plasma which are not thermally recombined to molecules react with carbon and nitrogen in molten metals. The mechanisms of decarburization and denitrogenization were also discussed.