Effect of the extent of pre-reduction of γ-Fe2O3 by H2 on the synthesis of Fe3C powder from CH4

• Thermodynamic analysis carried out in Fe-O-C-H system predicts Fe3C formation.
• Pre-reduced powder (12.36 wt.% oxygen) yields no Fe3C owing to poor CH4 pyrolysis.
• Pre-reduced powders (3.26 and 0.59 wt.% oxygen) yield single Fe3C phase.
• Carburization consists of reduction of remaining oxides followed by Fe3C formation.
• Presence of reduction product, water vapor, in powder bed inhibits Fe3C formation.

The present study aims to investigate effect of the extent of pre-reduction of γ-Fe2O3 (maghemite) on the synthesis of Fe3C (cementite). Initially, pre-reduced powders with oxygen contents of 12.36, 3.26 and 0.59 wt.% were obtained at 800 K in H2 atmosphere for 2.5, 5 and 10 min, respectively. Carburization behaviors of these pre-reduced powders were then explored using undiluted CH4 at 800 K. Mass change measurements, XRD and SEM analyses were used to characterize the samples. It was found that single Fe3C phase was obtained from the pre-reduced powders containing 3.26 and 0.59 wt.% oxygen for carburization periods of 15 and 30 min. The kinetic data for the carburization under pure CH4 indicates that there are essentially two successive distinct regimes: reduction of the remaining oxides and Fe3C formation. When the pre-reduced powder with oxygen contents of 3.26 and 0.59 wt.% are used, CH4 pyrolysis rate suffices for the reduction of the remaining oxides and carburization. The pre-reduced powder with the highest oxygen (12.36 wt.% oxygen) did not yield Fe3C, but iron and iron oxides, because the extent of the reduction by CH4 is found to be too small for such high oxygen content. It was postulated that the reduction product H2O vapor hinders the formation of Fe3C. The carburization process was discussed in terms of reaction pathways using equilibrium thermodynamic analysis and reaction kinetics.