Mechanisms of carburization and melting of iron by CO gas

Mechanisms of carburizing and melting iron in CO atmosphere with carbon activity of unity have been studied at a temperature range of 1 486-1 664 K, i.e. above the eutectic temperature of the Fe-C system. The behavior of melting iron was observed in situ by a confocal scanning laser microscope incorporated with an infrared image furnace. Carburization to gamma -iron is controlled by the mixed rate-determining step, which consists of the carbon diffusion in gamma -iron and chemical reaction at the gas-solid interface. The diffusion coefficient of carbon in gamma -iron was determined from the carbon concentration profiles as follows, D/m(2) s(-1) =2.0x10(-6) exp(-112.0x10(3)/RT) (1 486-1 664 K) where R is the gas constant, 8.314J K-1 mol(-1). The mechanism of iron melting by carburization is discussed according to the analogous treatment to the heterogeneous nucleation theory. An incubation time is necessary for melting iron, where the incubation time is the time required for the carbon concentration to reach the supersaturated concentration from the solidus concentration of carbon.