A study of slag-infiltrated magnesia-chromite refractories using hybrid microwave heating

Rebonded magnesia-chromite refractories are used in vacuum oxygen decarburisation ladles for the secondary refining of stainless steel. They suffer from acute wear due to the stringent chemical, thermal and mechanical conditions imposed. Although post-mortem investigation of worn refractories is indispensable to study the degradation mechanisms, its application is often hampered by evaluation difficulties due to crystallisation phenomena occurring during in situ cooling. To study the actual microstructures at elevated temperatures, industrially worn and virgin refractory samples were reheated and quenched in a cylindrical single-mode microwave furnace. Usage of a tubular susceptor allowed reproducible hybrid heating of the samples up to 1800°C. The quenched samples were analysed with SEM and EPMA-EDS. Concurrently, elemental line scans, X-ray mappings and quantitative image analyses were performed. This allowed the description of the ‘high-temperature inactivation’ mechanisms of the secondary chromite bonding phase: dissolution (1) in the periclase phase and (2) in the liquid slag, with (2) being predominant. Conclusions are drawn with respect to the refractoriness of the contributors to the spinel bonding phase: MgO.Cr2O3 (magnesiochromite)>MgO.Al2O3 (spinel)≫MgO.Fe2O3 (magnesioferrite). Hybrid microwave heating is shown to be an interesting alternative for conventional furnace experiments on refractory samples.