Electric arc furnaces are used for melting of steel. Direct arc electric furnaces are very popular for the melting of alloy steels and range in size from a few kilograms, for laboratory units, to in excess of 100 tonnes per batch. Typically units found in foundries are in the range of 1 to 10 tonnes. The furnace generally consists of a cylindrical steel shell (Spherical bottom) which is lined with acid or basic refractoriness.
The roof which can normally swing away to facilitate charging, generally contains three carbon electrodes (or water cooled graphite electrodes) operating on a high tension three-phase power supply. These electrodes protrude vertically through the roof and an electric current passes directly through them and into the metal bath. The distance between the electrodes and the metal bath is automatically controlled and determines the power input into the bath. So that it produces a stable arc. Temperature of the melt can be easily controlled and regulated.
Expensive alloying elements such as Chromium, Nickel, and Tungsten etc. can be easily added without any loss by oxidation.
There is complete absence of fumes and gases which are present in fuel fired gases. This ensures excellent control on the quality of the melt and leads to production of very high quality castings.
These furnaces generally have a door at the back for alloying, oxygen lancing and slag removal purposes, and a pouring spout at the front. The entire unit is capable of being tilted for discharge of the melt through the pouring spout. Some of the advantages of direct arc furnaces include high melt rates, high pouring temperatures and excellent control of melt chemistry.
1. High cost of electricity and cost of equipment.
Indirect arc furnaces
Generally consist of a horizontal barrel shape steel shell lined with refractories. Melting is effected by the arcing between two horizontally opposed carbon electrodes or graphite electrodes. Heating is via radiation from the arc to the charge.
The barrel shaped shell is designed to rotate and reverse through approximately 180°C in order to avoid excessive heating of the refractories above the melt level and to increase the melting efficiency of the unit. The furnace is mounted on the rollers and the rollers can be driven to rock the furnace.
The metal melts because of the heat radiated from the arc and heat from the hot refractory lining. Indirect arc furnaces are suitable for melting a wide range of alloys but are particularly popular for the production of copper base alloys.
The units operate on a single-phase power supply and hence the size is usually limited to relatively small units.