Oxide reduction and sintering of Fe–Cr alloy honeycombs

H₂ reduction and densification of extruded Fe₂O₃–Cr₂O₃ mixtures with honeycomb geometry (0.3-mm wall thickness) were studied using dilatometry, x-ray diffraction, and scanning electron microscopy. Reduction of pure Fe₂O₃ heated at 5 °C/min was completed by approximately 525 °C. Two separate sintering steps ensued, well separated in temperature from reduction, one associated with elimination of pores between Fe agglomerates (750–900 °C), and the other associated with sintering of these agglomerates (995–1275 °C). With increasing Cr2O3 content, sintering was increasingly delayed until initiation of Cr₂O₃ reduction above 1000 °C. The α → γ iron phase transformations were consistent with the Fe–Cr phase diagram, but the γ → α transformations at higher temperatures deviated from the phase diagram because of the influence of impurities (Si, Ca) in the solid solution. Pure Cr₂O₃ honeycomb formed an approximately 35-μm-thick porous Cr coating that imposed a diffusion barrier to further reduction. The high vapor pressure of chromium prohibited compositions in excess of 25 wt.% Cr₂O₃ from full reduction and densification.