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The electric furnace has proved to be an ideal melting and refining unit for the steel industry. Its advantages include the following: the nonoxidizing condition of the carbon arc, which is pure heat, makes possible a tightly closed furnace and permits maintenance of reducing atmospheres; the temperature attainable is limited only by the refractory nature of the furnace lining; close limits of regulation and control are obtainable; the efficiency of the unit is extremely high; refining and alloying are readily accomplished and controlled. The steel industry has at its command a method of producing steel, which in competent hands, can operate on a tonnage basis and meet the most exacting specifications.
Arc-Type Electric Furnace
In most electric furnaces the heat is produced by means of an arc either above the bath, or by means of arcs between the slag and electrodes suspended above the bath, the latter being the more satisfactory and common method. There are generally three carbon or graphite electrodes suspended from the roof. Each of these electrodes is connected to one phase of a three-phase current and lowered into the bath in such a manner that an arc is formed between the slag and each electrode. The modern arc-type electric furnace is cylindrical in shape and of the standard three phase, three electrode service arc type as it was originally developed. Basic differences of construction lie in the mechanical design. Some of these include different methods of filling, various methods of applying removable roofs to medium-sized furnaces, and arrangement and application of electrical equipment. The lining of the basic electric furnace is of extreme importance and one of the principal features of the furnace. The hearth must be made from a magnesite or burned dolomite material to withstand the lime slags used in the basic process. The walls and roof may be made from a silica brick and need not be basic, but must withstand extremely high temperatures. In making up a heat of steel, the bulk of the charge is made up of carefully selected steel scrap of such a composition that, upon melting down, the bath will contain a smaller amount of the alloying elements than is required in the finished steel. The practice permits the addition of suitable ferroalloys and carbon for proper adjustment of the chemical analysis of the steel charge. Other melting methods such as open-hearth, may use pig-iron as a raw charge, whereas the electric furnace never starts out with a charge of pig-iron. Through the use of carefully selected raw materials a greater economy may be had in this process. The electric furnace may start with cold charge, and the steel made this way is spoken of as cold-melt electric furnace steel to distinguish it from that resulting from the practice of using a hot or molten charge previously melted in an open-hearth furnace and transferred to the electric furnace. In the manufacture of high grade steel for tools, the slower and more expensive cold-melt method is almost always used. When the heat of steel has been properly deoxidized or killed and is of proper chemical composition, it is ready to tap into the ladle. The melt is tapped into the ladle and subsequently poured or teemed into a mold made from cast iron. The resulting casting is called an ingot, and the mold used is the ingot mold. The tapping and pouring of the steel into the ingot molds is carried out with the greatest care. It is important that the cast ingot be as free from blowholes, shrinkage holes, segregation, etc., as is possible to insure a good ingot. No steel from an ingot is sounder than the ingot from which it was made.
Induction Furnace
A radically different type of electric furnace, sometimes used in making small tonnages of steel or special alloys, is the high frequency induction furnace. In this, the melting method consists of a crucible furnace into which the charge is placed. An electric current is induced within the metal charge from a coil of copper tubing carrying a high frequency current, which surrounds the crucible. The induced current is forced to flow directly through the charge, causing very rapid heating. A fortunate phenomenon of induction furnaces insures a thorough mixing of the molten metal. This is a natural stirring action which takes place in the liquid charge as soon as the molten state of the charge is reached. By the induction method high melting temperatures are obtained, as well as exact and controllable melting conditions. It is more economical than the electric-arc type of furnace; in some instances it may replace the open-hearth furnace, particularly when small quantities of special alloys are required.
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