Plasma Furnace

an electric furnace for heating, melting, and metallurgical processing of metals and alloys, in which the source of heat is plasma generated by plasmatrons. A distinction is made between plasma arc furnaces and high-frequency plasma furnaces.

Two main types of plasmare furnaces are known: (1) hearth or crucible batch furnaces and (2) furnaces of semicontinuous action with crystallizers. The hearth plasmare furnace is identical in hearth design and type of lining materials to an ordinary arc furnace used for similar purposes. The dome or body of the furnace has one or more openings with water-cooled covers for taking samples during smelting, for measuring the temperature of the metal, and for the addition of alloying additives, deoxidiz-ers, and slag-forming materials. Seals around the openings make possible the maintenance of increased pressure of the plasma-forming gas in the furnace. The cathodes for arc discharge in plasmare furnaces are the cathodes of several plasmatrons and are usually made of tungsten or a special refractory alloy; the anodes are the metal in the hearth of the furnace. The current passing through the metal is conducted out of the furnace by a hearth electrode, usually water-cooled, in the hearth of the furnace. The arc in a plasmare furnace is blown by a direct or vortex flow of an inert gas, usually argon; this stabilizes the arc and increases its temperature to 10,000°-20,000°K, and also generates an inert atmosphere above the metal being melted. Piasmarc furnaces are used in the production of high-performance steels and special alloys.

In plasmare furnaces with crystallizers, the billets to be processed are positioned vertically, according to the scheme of the Institute of Electric Welding of the Academy of Sciences of the Ukrainian SSR, or horizontally, according to the scheme of the Institute of Metallurgy of the Academy of Sciences of the USSR, with a supplemental supply of alternating current to the billets if required. Instead of solid billets, the charge may consist of finely divided material. An excess pressure is maintained in the furnace cavity. The pressure is usually moderate, but it may be increased to several tens of atmospheres. Because of separate control of the rate of melting and the heat flux of the arc, the process of crystallization of the melted material in plasmare furnaces may be controlled within wider limits than in vacuum arc and electroslag furnaces.

Low-pressure plasmare furnaces (10³-0.10 newtons per sq m, or 10^–2-10^–6 kilograms-force per sq cm) are used instead of the more costly and complex electron-beam furnaces for smelting and degassing of gas-saturated materials.

In high-frequency plasma furnaces, because of the special design feature of the plasmatron, the plasma does not contain particles of the electrode material and is thus of higher purity. Therefore, furnaces of this type are more often used for growing single crystals and for the processing of pure materials.