| Polished and thin sections from used graphite-bonded CaO-stabilized zirconia submerged entry nozzles were observed under the optical microscopes and by means of the electron optics. A new mechanism for the excessive slagline attack is advanced as follows: Once graphite dissolves in the metal, the slag comes in contact with zirconia particles embedded in the graphite bond which have been receded in contour. The graphite bond reduces SiO₂, Na₂O, MgO and probably B₂O₃ in the mold flux to SiO, Na, Mg and B. The gaseous products diffuse into ZrO₂ particles through their pores and oxidize to form low-melting compounds with the CaO, thus destabilizing the zirconia. In addition, the compounds etch grain boundaries, resulting in loose particles with well-defined roundness. Fresh slag penetrates the weakened particles and breaks them down into pieces, which are, in turn, dispersed into the slag. When the slag reaches the graphite-rich region, the metal replaces the slag. The process then repeats itself. |
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