Microstructure evolution of deformed hematite during direct reduction by hydrogen—the role of deformation twinning

Iron ore reduced in shaft furnaces is subjected to high temperatures and compressive stresses throughout the process. To reproduce these conditions and understand the influence of deformation on the reduction behaviour, in this study we deform dense hematite (Fe₂O₃) crystals under uniaxial compression, introducing cracks and twin defects, and then reduce the samples at 700°C in pure hydrogen gas in a thermogravimetric analysis setup until partial or full reduction. Upon full reduction (>95% metallic iron) we compare the reduction kinetics of the sample with those of an undeformed crystal, and we find that deformed hematite maintains higher reduction rates as a function of reduction degree throughout the process. We then use electron microscopy to characterise the microstructure of a partially reduced sample in order to investigate the influence of the defects introduced during deformation on the reduction behaviour, focusing particularly on the conversion of hematite into magnetite (Fe₃O₄) and the hematite/magnetite interface. We find that hematite grains containing deformation twins present different interface morphology and hematite/magnetite orientation relationship than twin-free grains, and we elucidate their effect on reduction rate.