List of Heat Treatment Furnaces | Metallurgy

The description of some important heat treatment furnaces is given below:

1. Oil and Gas Fired Furnaces:

Oil-fired or gas-fired furnaces may be designed as direct fired, semi-muffle, or muffle.

a. The ‘direct-fired furnaces’ allow the steel to come into direct contact with the hot gases of combustion and are mainly used for heating steel for forging operations.

b. The ‘semi-muffle’ type is so designed that the gases of combustion are deflected and do not impinge directly upon the steel being heated, although they circulate around it. This affords better control of heat, which results in a more uniform product.

c. The full-muffle type furnaces are used to protect steel from oxidation during any heat-treating operation. These furnaces are so constructed that they provide a chamber, known as ‘muffle’, separate from the combustion chamber, into which the steel to be heated is placed. The gases of combustion circulate around the chamber and cannot come in contact with the steel being heated.

2. Electric Type Furnaces:

a. In electric-type furnace, the resistors are usually placed around the outside of the muffle. The muffle is made of a special heat-resisting alloy or special alloy or special refractory material.

b. The hot gases of combustion, or in case of electric furnace, electric resistors, heat the muffle and the steel within the muffle. A neutral or reducing gas may be supplied to the interior of the muffle so as to greatly increase the degree of protection afforded to the steel during the heat treating cycle. The full muffle type furnace is commonly employed with the so-called atmosphere-type furnace and is used when maximum protection is required from oxidation.

3. Batch Furnace:

A bath type of furnace with a rectangular heating chamber and one or more doors for charging and unloading, is very widely used for heat treatment purposes. Here a batch of parts is charged for heat treatment, it is taken out after heat treatment, then a second batch is charged into the same furnace, then the third batch and so on. These furnaces find applications in tool making and maintenance shops.

Batch furnaces may be classified as follows:

1. (a) Vertical (Pit type)

(b) Horizontal (Box type).

2. (a) Gas or oil fired

(b) Electric.

3. (a) Stationary Hearth:

(i) Direct fuel fired

(ii) Indirect fuel fired (muffle)

(b) Movable Hearth:

(i) Car Bottom

(ii) Rotary type

4. Semi-Continuous Furnace:

Fig. 5.31 shows a bell lift off type semi-continuous furnace in which the furnace itself is mobile and can be lifted or placed over a base containing workpieces. This furnace is heated either by electric resistors or radiant tubes. The steel hood helps maintaining a protective atmosphere around the workpieces to prevent decarburization or oxidation of workpieces. Bell furnace is used especially for annealing coils of wires or strip in steel or non-ferrous metals, often in controlled atmospheres.

Fig. 5.32 shows another type of semi-continuous furnace known as car bottom furnace in which hearth is loaded with the workpieces and moved into furnace. When the soaking period is complete, the flat car is withdrawn from the furnace and replaced by another. A car bottom furnace is used for large and heavy parts, e.g., high castings and forgings.

5. Continuous Furnace:

A continuous furnace has a horizontal work chamber and mechanical means of conveying the workpieces front one end to another. Fig. 5.33 shows a continuous heating and quenching (hardening) system. The furnace atmosphere is generally controlled.

Work pieces are carried through the heating chamber on a conveyor chains made up of heat resisting alloy steel. Besides conveyor chains, other methods of transporting workplaces through a continuous furnace are driven rollers or skids. A continuous furnace finds many advantages when used in a mass production industry.

6. Salt Bath Furnaces:

Both molten lead and molten salt baths are used for heating of steel in operations involving hardening annealing, tempering etc.

7. Molten Lead Baths:

Molten lead is fast-heating medium and gives complete protection to the surfaces of the steel with a melting point of 327°C, lead may be used successfully from 370°C to about 870°C. Lead oxidises readily with the formation of a dross, making a dirty bath, and gives trouble because of the sticking of lead and dross to the surfaces of the steel being heated.

This can be overcome by suitable lead coverings such as wood charcoal, coke, carburizing compounds, salts, etc. The steel can be further protected from lead sticking to its surfaces by the use of a coating of a thin film of salt or some other material which is applied to the steel before it is placed in the lead bath.

8. Molten Salt Baths:

These baths have been found to be satisfactory for the heating of many steel parts that are to be given heat treatments. They transmit heat quickly and uniformly and afford a protection to the steel during the heating cycle. Upon removal of the steel from such a salt bath, a thin film of salt adheres to it, giving protection from air prior to the quenching, etc. Although ideal salt bath has not been discovered, if care is used in the selection of a salt and precautions are exerted in its use and maintenance, good results may be expected.

a. Salts may be used for low-temperature tempering. They usually consist of nitrates and may be used in a range of temperatures from 150°C to 540°C.

b. Salt baths used in temperature ranges of 540°C to 870°C, consist mainly of sodium carbonate, sodium chloride, sodium cyanide, and barium or calcium chloride.

c. In a temperature range of 980°C to 1320°C, salt baths are made from mixtures of barium chloride, borax, sodium fluoride, and silicates.

d. Precautions should be observed in the use of salt baths in order to prevent violent reactions from mixing of certain salts. One should also be careful to prevent moisture from coming into contact with the fused salts. With the cyanide salts, precautions should be observed because of their poisonous nature.