Characterization of oxides (scale) growth of low carbon steel during reheating

This study involved characterizing the oxidation behaviour of low carbon steel in various gas mixtures of O$ rm sb2, CO sb2, H sb2O$ and N$ sb2,$ at different oxidation temperatures. Rate measurements for the oxidation of the low carbon steel during reheating in a walking-beam steel reheat furnace were also carried out. Scaling rates during reheating were predicted from oxidation rates obtained in the laboratory.In-situ characterization of the phase compositions of the iron oxides, "scale", that form on low carbon steels during their oxidation at elevated temperatures was carried out using a neutron diffraction technique. Growths in the intensities of the diffraction peaks associated with characteristic crystal planes of the various oxides (Fe$ rm sb{x}$O, Fe$ rm sb3O sb4$ and Fe$ rm sb2O sb3)$ were monitored on-line. The volume fractions of the oxides in the developing scale were calculated on the basis of ideal structure factors and measured relative intensities of diffraction peaks.Oxidation in binary gas mixtures of oxygen and nitrogen in the temperature range from 1000 to 1250$ sp circ$C was carried out. Sample weight gain versus time data were analyzed, along with measurements and calculations of sample heating rates due to exothermic reactions at the sample surface. It was found that initial rates of oxidation depended on oxygen content in the gas mixture and that these rates were linear up to oxide thicknesses between 0.4 and 0.5 mm. The linear rates of oxidation were found to be controlled by the mass transport of oxygen from the gas phase, to the reaction surface, through a gas boundary layer. Subsequent oxidation rates followed a parabolic oxidation mechanism.Oxidation experiments were also carried out in binary, ternary and quaternary gas mixtures of O$ rm sb2, CO sb2, H sb2O$ and N$ sb2$ at various temperatures. Reaction rates during oxidation in binary gas mixtures of CO$ sb2$-N$ sb2$ and H$ sb2$O-N$ sb2$ followed a linear rate law and were found to be proportional to the partial pressures of CO$ sb2$ and H$ sb2$O in the gas mixtures. The oxidation rates showed a strong dependency on temperature. Oxidation in oxygen containing atmospheres showed that the main oxidizing agent was free O$ sb2.$ Additions of CO$ sb2$ and H$ sb2$O had little effect on the magnitude of oxidation rates. Oxidation in these atmospheres exhibited an initial linear rate law which gradually transformed to a parabolic rate law.During reheating in a walking-beam steel reheat furnace, it was observed that scaling rates can be reduced by lowering input air/fuel ratios to the furnace, which resulted in lowering concentrations of free oxygen in the combustion products from about 3% to about 1.5%. The predicted scaling rates during reheating using isothermal oxidation rate constants suggested that rates of oxidation during reheating in the industrial reheat furnace followed a combination of linear and parabolic rate laws, with the components of the linear oxidation rates being predominant. The observed reduction in scaling rates was a result of the decrease in the free oxygen within the furnace atmosphere