Strength of Direct Reduced Iron Following Gas-Based Reduction and Carburization

The effects of gas composition and the extent of reduction on the compressive strength of direct reduced iron (DRI) were investigated. Gas mixtures containing different proportions of hydrogen, carbon monoxide, water vapor, carbon dioxide and methane were tested. The structure of laboratory and industrial DRI was examined and correlated with gas composition and mechanical properties. After reduction, structural changes accompanying reduction and carburization were investigated with scanning electron microscopy, swelling was measured, and phases were quantified by X-ray diffraction. The major strength loss occurred in the first reduction step, when hematite grains transformed to porous magnetite. Carburization after reduction had a minor effect on strength, except when extensive precipitation of elemental carbon caused lower strength, similar to the metal dusting corrosion mechanism. Higher strengths were obtained when water vapor was added to the reducing gas.