The  electric   furnace

 

Where the cost of electrical energy is high, the electric process is more expensive in operation than the Bessemer or open hearth process. In this case only the finest grades of steel are produced by the electric furnace method. Stain�less and heat resistant steels are made almost exclusively by   that   process.

Electricity is used only for the production of heat and does not of itself impart any superior quality to steel. Never�theless, the electric furnace method confers certain advantages impossible of attainment in other steel melting proc�esses. The electric furnace generates extremely high tem�peratures (up to 3500� F.) very rapidly. The temperature is at all times under precise control and is easily regulated.

Furthermore, the production of heat by electricity is unique in that oxygen is not necessary to support com�bustion and the atmosphere within an electric furnace, may be regulated at will. The quantity of oxygen entering the furnace can be precisely controlled, which is not the case in the open hearth and Bessemer processes. Thus, the pre�sence of oxides, compounds of oxygen with other elements, or other impurities undesirable in fine steels can be mater�ially reduced. Sulphur is almost completely eliminated in the basic electric furnace. Besides that the electric furnace process permits the addition of expensive alloying elements to molten steel without loss by oxidation. For all of these reasons the metallurgist can control the composition of the metal more closely.

The electric furnace is a circular steel shell resembling a huge tea-kettle in general appearance. It is mounted on rockers so that the furnace can be tilted to pour off molten metal and slag. The bottom of the furnace consists of a layer of heat resistant materials below which it is lined with re�fractory bricks. The side walls which are also lined with refractory brick contain three or more openings: a clay lined spout for tapping off the molten metal and slag and doors for charging raw materials. The doors which are operated mechanically are in some cases watercooled.

The roof of the furnace is lined with 250 mm or more of refractory brick and is shaped like a flat dome. Through this dome great columns of carbon reach into the furnace. These are the electrodes which carry the current to the steel charge. They are 75�700 mm in diameter and about 1800 mm or more long. The flow of current is regulated by raising and lowering the electrodes each of which may be adjusted independently of the others.

The electrodes are spaced far enough apart so that no arc can occur between one and another. They project to within 25�30 mm of the layer of the molten slag which floats on top of the steel. The current jumps the gas-gap between the electrode and the slag, passes through the slag into the metal to the foot of the next electrode striking anoth�er arc. All the heat produced in the furnace is generated by the arcs.

Because the metal immediately below the electrodes is hotter than that near the walls of the furnace, the molten metal   is   in   slight   constant   motion.