Electric Slag Remelting Furnace
The ESR process involves the gradual melting of the lower end of a cast or forged electrode through a layer of molten metallurgically active slag into a water-cooled copper mold. The slag is usually calcium fluoride with the addition of lime, magnesia, alumina and other oxides. In its molten state the slag is electrically conductive and acts like a resistance heating element when a high current is passed between the electrode and the mold. With the continued passage of electric current the slag becomes superheated. The electrode tip is immersed in this superheated slag and begins to melt. The immersion of the electrode in the slag is precisely maintained and a controlled melt rate is established. An advanced computer control system maintains the melt rate and electrode immersion utilizing custom designed algorithms taking into account measurements from the load beam weighing system, measurements of the furnace voltage (or resistance), and measurements of voltage (or resistance) swing. The molten metal droplets sink through the lower density molten slag and re-solidify in the water-cooled copper mold. Through this process the slag-metal reactions dissolve inclusions and purify the metal as it falls through the molten slag. After the process has continued for some time, a purified ingot having a controlled solidification structure is built up in the mold. The slag floating on top of the liquid metal pool also acts to prevent oxidation of the molten metal when melting in air. It is also common for the ESR process to take place in an inert gas environment to further prevent oxidation of the metal and to prevent oxidation of the slag. Toward the end of the process, the power is gradually reduced providing a controlled hot top. The resulting ingot is structurally sound, chemically homogeneous, and provides a high yield of useful product.