Vacuum induction

Fig. 2. Schematic arrangement of a furnace in a vacuum chamber equipped with charging and mold locks for vacuum induction melting (1) (a) front cross...

The magnesium-reduced beryllium pebbles generally assay 96% beryllium and are always associated with residual magnesium and slag. These pebbles are purified to about 99.5% by vacuum induction melting in beryllia crucibles at temperatures of about 1400 °C. The ingots are machined and machined scarf is milled to produce beryllium powder. The ground metal powder is pressed and sintered under vacuum. The product is called vacuum hot-pressed beryllium, and this is machined for component manufacture. 

Vitreous silica is used for gas-heated or electrically heated devices in various shapes, eg, as a tube or muffle because of its electrical resistivity, impermeability, and low expansion. In its simplest form., an electric-resistance furnace consists of a vitreous silica tube or pipe on which the resistance element is wound (see Furnaces, ELECTRIC). Because of its indifference to temperature gradients, a tubular furnace of vitreous silica may be made to operate at different temperatures at various portions of the tube, either by arrangement of the heating elements or by cooling sections of the tube with water. Vitreous silica pipes may be employed in vacuum-induction and gas-fired furnaces (see VACUUM TECHNOLOGY) (221). 

For preparation of alloys nickel by cleanliness of 99.99 %, magnesium by cleanliness of 99.95 %, lanthanum by cleanliness of 99.79 %, and mishmetall (industrial mixture of rare-earth metals (REM) Ce - 50, La - 27, Nd - 16, Pr - 5, others REM - 2wt. %) were used. The melting of metal charge was carried out in the vacuum-induction furnace under fluxing agent from eutectic melt LiCl-KCl. The composition of alloys was supervised by the chemical analysis and the X-ray testing. 

Chemical compositions plus processing and heat treatment schedules relative to Inconel 718 and Udimet 718 are presented in Tables I and II. Hardness and ASTM grain size (average numbers) are included in Table II. Both the Inconel 718 and Udimet 718 superalloys were obtained as large-diameter (29 cm and 26 cm, respectively) hot-finished forging billets, vacuum-induction-melted and vacuum-arc-remelted (VIM-VAR). The two alloys have comparable chemical compositions, but the Inconel 718 has a significantly higher silicon content. 

Process code HIP, hot isostatically pressed at 1200 C, 69 MPa for 2 hr. VIM, vacuum induction melted. VAR, vacuum arc remelted. EFR, electroflux remelted. 

Some superalloys, particularly Ni-Fe and Co based alloys, are directly melted in electric furnaces by classical methods usually applicable to stainless steels. However for Ni and special Ni-Fe superalloys, vacuum induction melting is required in order to reduce the content of interstitial gases (O, H, N) to a very low level. This enables foundries to achieve high and controlled contents of oxidisable elements such as Ti or Al. 

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