Electric-arc furnaces
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.
Disadvantages:
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.
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