Degradation mechanisms of magnesia-chromite refractories in vacumm-oxygen decarburisation ladles during production of stainless steel

Abstract

Magnesia–chromite bricks are used as refractories for the refining of stainless steel in vacuum–oxygen decarburisation (VOD) ladles. Refractory wear is not uniform. In the present work, worn bricks from different zones in the ladle have been analysed, and a set of interdependent degradation mechanisms is proposed. Refractory wear as a function of position in the ladle is discussed. Slag infiltration and MgO dissolution from the refractory were observed in all samples, whereas FeO_x decomposition was seen at two levels in the high wear samples. First, partial decomposition of primary chromite crystals ( (Mg) [Fe³⁺, Cr, Al]₂ O₄ ) occurred at the hot face of the brick. Three layers were distinguished in the reacted chromite crystals and a reaction mechanism is presented. Second, a decrease in the FeO_x content of the magnesia phase occurred at the hot face of the brick. The negative effect of the presence of FeO_x in magnesia–chromite refractories is discussed and the influence of the ferrostatic pressure is demonstrated. Finally, the consequences of the following phenomena are discussed: increase in brick porosity, slag infiltration, corrosion, erosion of the partially liquid bonded refractory system, and spalling and cracking.