Oxidation resistance and microstructural evolution in MgO–C refractories with expanded graphite

Abstract MgO–C refractory system is the most important one in various stages of steel melting operations. Flaky graphite is a key ingredient in these refractories that imparts high thermal shock resistance and excellent corrosion resistance against molten metal and slag. However, the proneness of graphite to high temperature oxidation renders these refractories porous; thus the materials recession leads to deterioration in thermomechanical properties. In our earlier work, we had shown that the partial replacement of the graphite phase by expanded graphite (EG) in a 5% graphite containing MgO–C refractory system enhances the hot strength and thermal shock resistance. In this work, we have evaluated the high temperature oxidation resistance of similar MgO–C refractories where 0.2, 0.5 and 0.8 wt% of graphite has been replaced by EG. One of thus modified systems exhibited excellent oxidation resistance as compared to the standard refractories. Detailed microstructural investigation of the refractories exhibited presence of all theoretically proposed phases. Mercury intrusion porosimetry indicated that the pores of higher size range (10–100 µm) were responsible for the aggressive oxidation in the unmodified compositions. Hierarchical packing of the refractory constituents and the in-situ homogeneous formation of high temperature ceramic phases were responsible for such beneficial thermomechanical properties.