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Refractories in vacuum induction melting Da Costa e Silva, Andre Luiz V.

Abstract

The literature in Vacuum Induction Melting is briefly reviewed, with especial emphasis on refractory practices and refractory-metal interactions. Since it was determined that in both steel and superalloy vacuum melting in large furnaces lining failures are associated with attack on the cement joints, tests were performed to characterize this attack and determine the most suitable cements from this standpoint. The presence of low-stability oxides (SiO₂, P₂O₅,...) was shown to be the main reason for the cement's low resistance to metal attack. It is suggested that in the case of steels, diffusion of the less stable oxides may be the rate controlling step in the corrosion process. In the case of superalloys, the high interaction between oxygen and the elements present in the alloy (Ti, Al, Cr) causes an extreme depression of the oxygen activity in the melt, hence enhancing the dissolution of all refractory oxides. In order to produce cements with a very small content of the low-stability oxides, the use of fluorides as fluxes was attempted. The cements so produced performed well, as far as resistance to attack, adherence to bricks and technological properties were concerned. To verify the validity (on a large scale) of the mechanisms observed and proposed in the tests, samples from industrial Vacuum Furnaces were examined. It was concluded that the processes occurring in a large furnace can be rationalized based on the test observations. Also comments were made on the need for improved pouring facilities, if the products of the metal-refractory interaction are to be kept out of the final material produced. This is because in the present state of the refractory technology and practice, these interactions cannot be avoided.

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