Thermodynamic aspects and kinetic modeling of decarburization of steels in walking-beam furnaces

An open-fired walking-beam furnace is considered as a reactor with enthalpy-controlled stages and counter-current flows of gaseous and solid phases. The model is initially tuned using the basic characteristics of the real system: the number of heating zones; the temperature profile; the chemical composition of the fuel; the air:fuel ratio and the fuel consumption in each zone; the amount of steel loaded into the furnace per unit time, etc. Heat losses are evaluated, and the model then becomes capable of mapping the carbon activity and the detailed chemical composition profiles along the furnace for various control settings. Computational aspects and practical applications are discussed. It was found that for reasonable air:fuel ratios and for medium-carbon steels, a driving force always exists for decarburization. Nevertheless, some grades are prone to carbon loss, while others are not. In our experiments, the susceptibility to decarburization was found to depend on the rate of heating, which implies a kinetic nature of the phenomenon. A diffusion model is developed to explain these experimental findings and the different behavior of the various grades.