CO and CO2 are important in atmospheres used in processing steel. At austenitizing temperatures, carbon dioxide reacts with surface carbon in a steel surface to produce carbon mon
oxide:
(C) + CO2 ↔ 2CO
in which (C) represents carbon dissolved in austenite. The reaction continues until there is no CO2 or until the steel surface is completely free of carbon, at which point, if there is a continuing supply of CO2, iron and ferrous oxide will be oxidized by the reactions:
Fe + CO2 ↔ FeO + CO
3FeO + CO2 ↔ Fe3O4 + CO
Ferrous oxide (FeO) is the stable oxide formed above 555 °C (1030 °F), whereas the magnetic oxide (Fe3O4) is formed below 555 °C. The reactions will proceed until equilibrium is established, and they progress at a rate depending on time, temperature, and pressure of the system.
oxide:(C) + CO2 ↔ 2CO
in which (C) represents carbon dissolved in austenite. The reaction continues until there is no CO2 or until the steel surface is completely free of carbon, at which point, if there is a continuing supply of CO2, iron and ferrous oxide will be oxidized by the reactions:
Fe + CO2 ↔ FeO + CO
3FeO + CO2 ↔ Fe3O4 + CO
Ferrous oxide (FeO) is the stable oxide formed above 555 °C (1030 °F), whereas the magnetic oxide (Fe3O4) is formed below 555 °C. The reactions will proceed until equilibrium is established, and they progress at a rate depending on time, temperature, and pressure of the system.
Equilibrium curves for the formation of scale (FeO and Fe3O4) when heating iron in a CO-CO2 atmosphere at different temperatures. Source: ASM Handbook, Volume 4: Heat Treating, ASM International, p 543, 1991.
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