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Mass transfer aspects of A.C. electroslag remelting Fraser, Michael E.

Abstract

Previous attempts at a quantitative description of alloy losses in electroslag remelting (ESR) operations have not adequately reflected the true nature of this process. In the current work a mass transfer model was developed which takes into account four reaction sites and mass transfer of all species at each site. An experimental program was subsequently devised to test extensively the provisions of the model. The overall system was simplified by avoiding complex alloys and multicomponent slags and also by excluding air. Care was taken to deal only with species whose thermochemistry was known in,both slag and metal phases. The electrode material used was mild steel in which manganese was the onlyoxidizable species. Slags used throughout the melt program were CaV - 20% CaO, for which system the activity data for FeO, MnO were available. The melt program provided data on Mn losses in a variety of ESR operating conditions including steady and unsteady state, live and insulated mold configurations. The effects of melting in air and of direct current operation were also investigated. Several melts undertaken with Armco iron were designed to permit direct calculation of certain mass transfer coefficients. The mass transfer model was applied to each melt and the predictions compared to the experimental data. There was generally good agreement between the two, confirming that the manganese losses were controlled entirely by mass transfer effects. Further study of the model showed mass transfer of species in the slag phase to be the predominant rate controlling step. The relative roles of the various reaction sites in contributing to the overall mass transfer were also elaborated by this analysis. In order to assess the full potential of this mass transfer approach, the model was extended to the manganese, sulphur reaction system and to larger, commercial-sized ESR furnaces. Projections of alloy losses based on this scale-up study appeared to be consistent with the sparse information that is currently available.

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