Abstract:Work which has helped to improve vacuum induction melting (VIM) procedures (radically) for alloy 718 is described. Three aspects of melting in a 5Tonne 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, 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 removal of excess residual additives. The effect of these procedures on inclusion density and morphology is discussed.
Such materials are typically poured into 500mm diam electrodes and remelted by Vacuum Arc Remelting (VAR) or Electro-Slag Remelting (ESR) This is known within the industry as VIM+VAR or VIM +ESR processing.
Naturally all remelted materials were fully analysed and chemically, macro- and micro-structurally and examined. Ultrasonics are typically performed on forged bars. Both virgin & remelt routes were qualified for aero-engine applications - a first within the aeronautic and space-industries.
More information, if necessary, may be obtained upon request.
I would naturally be pleased to hear of evolutions, from raw material prices & melting (increased furnace size & reduction of unit costs) to final applications such as in aero-engines (CFM56, RR, Pratt & Whitney families ...) all possible realisations based upon this and similar work.