Optimizing deoxidation and desuIphurization during vacuum induction melting of alloy 718

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Abstract
Work that has helped to improve vacuum induction melting (VIM) procedures for alloy 718 is described. Three aspects of melting in a 5 t industrial VIM furnace are considered: oxygen control by carbon deoxidation; oxygen and sulphur control by alkaline earth and rare earth additions; and the effect of these additions on melt cleanness, with respect to virgin and remelt charge materials. Measurements of total oxygen content and oxygen activity have been carried out on both virgin- and remelt-charged heats. Measured and calculated activity coefficients are compared. In the absence of aluminium, good agreement is found between measured and calculated values. The fact that measured activity coefficients for a complex nickel-based superalloy agree well with predictions based on data for dilute iron alloys could indicate that such data may be useful for predicting oxygen behaviour in a wide range of iron-based and, to some extent, nickel-based alloys. A method for evaluating alkaline earth and rare earth additives in relation to bath sulphur content is given, together with the rate law for the removal of excess residual additives. The effect of these procedures on inclusion density and morphology is discussed. MST/91

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