Kinetics of Decomposition and Re-oxidation Resistance of .THETA.- and .CHI.-Iron Carbides at Elevated Temperatures and Influence of Their Formation Conditions

Abstract

The rate of decomposition of 0-iron carbide (Fe 3 C) and X-iron carbide (Fe 5 C 2 ), which were produced by carburizing reduced iron from a hematite ore and a limonite ore with 80%CO-H 2 -H 2 S mixture of a s =0.5, was determined from their compositional change. Their decomposition starts from about 873 K with Fe 5 C 2 and about 973 K with Fe 3 C. To mass fraction variation curves of θ- and X-iron carbide, the integrated rate equation for the first order reaction was applied to obtain the rate constant, k. The value of k for Fe 5 C 2 is much greater than that for Fe 3 C. Fe 5 C 2 firstly decomposes to Fe 3 C plus carbon and secondly the formed Fe 3 C decomposes to metallic iron plus carbon. Microscopically, the metallic iron grows as if the flat interface between Fe 3 C and metallic iron propagates through the grain of Fe 3 C. 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 Fe 5 C 2 and Fe 3 C to iron oxides and CO 2 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.