Electric Arc Furnace Process | Electric Arc Furnace Steel Making | Carbon Arc Furnace

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.

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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.

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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.

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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.

Crucible Melting Furnace | Metal Melting Furnace | Tilting Crucible Furnace

Crucible furnaces

These furnaces melt the metal without direct contact with a burning fuel mixture. For this reason, they are sometimes called indirect fuel-fired furnaces.

Crucible furnaces are mostly used for melting non ferrous metals and alloys. The metal is melted in a crucible which is a refractory vessel made of silicon carbide, Graphite or some other refractory material.

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Three types of crucible furnace are:

1. lift out type

2. Stationary

3. Tilting

They all utilize a container (the crucible) made out of a suitable refractory material (for example, a clay-graphite mixture) or high-temperature steel alloy to hold the charge. In the lift-out crucible furnace, the crucible is placed in a furnace and heated sufficiently to melt the metal charge.

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Typical Fuels are:

· Oil,

· Gas and

· Powered coal.

When the metal is melted, the crucible is lifted out of the furnace and used as a pouring ladle. The other two types, sometimes referred to as pot furnaces, have the heating furnace and containers as one integral unit.

In the stationary pot furnace, the furnace is stationary and the molten metal is ladled out of the container. In the tilting pot furnace, the entire assembly can be tilted for pouring. Crucible furnaces are used for nonferrous metals such as bronze, brass and alloys of zinc and aluminium. Furnace capabilities are generally limited to several hundred pounds.

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The metal in the crucible is ordinarily exposed to the furnace atmospheres. It must be mentioned that most non ferrous metals and alloys absorb gases, oxidize and form dross readily when melted. Steps should be taken to reduce the contamination by fluxing, degassing and reducing the time in melting. The thermal efficiency of crucible furnaces rarely exceeds 10 percent.

In the stationary pot furnace, the furnace is stationary and the molten metal is ladled out of the container. In the tilting-pot furnace, the entire assembly can be tilted for pouring; Crucible furnaces are used for nonferrous metals such as bronze, brass and alloys of zinc and aluminium. Furnace capacities are generally limited to several hundred pounds.

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