Arc Vacuum Furnace

an electric furnace for smelting metals in a vacuum by means of the energy of an electric arc. An arc vacuum furnace is a gas-discharge system in which the arc exists on the vapors of the metal being smelted. A distinction is made between arc vacuum furnaces for smelting ingots (mainly of titanium and steel) in water-cooled copper molds and for making shaped castings of highly reactive and refractory metals (mainly titanium and niobium) by so-called smelting in a lining.

Arc vacuum furnaces operate with consumable and non-consumable electrodes; the former type is more common in industry. The consumable metallic electrode consists of the material being remelted, and its chemical composition generally corresponds to that of the alloy being produced. An electric arc develops between the electrode and the dummy bar when a direct current is supplied. The heat emitted melts the electrode, and the molten metal formed flows either into a mold or into a crucible (in the case of smelting in a lining). In an arc vacuum furnace with a nonconsumable electrode made of tungsten or graphite, a hard charge is supplied to the smelting zone. The power of the electric arc is selected to ensure a dense ingot macrostructure, without defects. The pressure in the arc gap during smelting is determined by the pressure of the metal vapors above the melt; for steel it is 0.1–1.0 newtons per sq m (N/m²), for titanium 1–10 N/m², and for molybdenum 0.01-0.1 N/m². The required pressure is maintained by vacuum pumps.

The metal produced in an arc vacuum furnace with cooled mold has high mechanical properties and also a low amount of gaseous impurities and nonmetallic inclusions. Thus, when steel is resmelted in an arc vacuum furnace, the quantity of nonmetallic inclusions in the metal drops by a factor of 2–3, and large inclusions (more than 15–20 microns) are eliminated entirely as a result of the resmelting process. The concentration of nitrogen is reduced by 30–35 percent, and that of oxygen by a factor of 2–3; the sulfur content is reduced by 20 percent. The remelted metal has high viscosity and plasticity over a wide range of temperatures, increased resistance to fatigue, and high isotropicity of mechanical properties.

Cooled graphite and metal crucibles are used for smelting in an arc vacuum furnace in a lining. The thickness of the lining and the course of the smelting process are kept constant by regulating the power of the electric arc. In smelting in a lining, the required amount of molten metal is smelted in a crucible and is then poured into a form. Permanent molds, as well as forms made of graphite or magnesite, which are mounted on the stand of a centrifugal casting machine that is part of the furnace, thus improving the filling of the forms, are used for shaped castings of titanium. Castings of titanium alloys produced in an arc vacuum furnace by smelting in a lining have good mechanical properties. Work is under way to produce an AC arc vacuum furnace using easily ionized additives introduced into the electrodes.