Kinetics of decomposition and re-oxidation resistance of θ- and χ-iron carbides at elevated temperatures and influence of their formation conditions

The rate of decomposition of theta-iron carbide (Fe3C) and chi-iron carbide (Fe5C2), which were produced by carburizing reduced iron from a hematite ore and a limonite ore with 80%CO-H-2-H2S mixture of a(s)=0.5, was determined from their compositional change. Their decomposition starts from about 873 K with Fe5C2 and about 973 K with Fe3C. To mass fraction variation curves of theta- and chi-iron carbide, the integrated rate equation for the first order reaction was applied to obtain the rate constant, k. The value of k for Fe5C2 is much greater than that for Fe3C. Fe5C2 firstly decomposes to Fe3C plus carbon and secondly the formed Fe3C decomposes to metallic iron plus carbon. Microscopically, the metallic iron grows as if the flat interface between Fe3C and metallic iron propagates through the grain of Fe3C. And the iron carbide and formed metallic iron are always in front of pore. Therefore, the gas molecules in the atmosphere can react with the carbide, metallic iron and carbon throughout the reaction. The re-oxidation in dry air accelerated by the exothermic reaction for both iron carbides begins from about 623 K. The reason why the re-oxidation starts at the lower temperature than the decomposition does is direct oxidation of Fe5C2 and Fe3C to iron oxides and CO2 before their decomposition. The re-oxidation starting temperature is raised by the increase of the temperature of reduction and carburization, but the improvement is not very large.