Types of Continuous Furnaces | Heat Treatment | Steel | Metallurgy

The following points highlight the two main types of continuous furnaces used for heat treatment of steel. The types are: 1. Rotary-Hearth Furnace 2. Straight-Chamber Continuous Furnaces.

Type # 1. Rotary-Hearth Furnace:

In a rotary-hearth furnace, as illustrated in Fig. 10.14, the hearth of the heating chamber rotates around a vertical axis inside a stationary roof and inner as well as outer walls with seals such as of sand between the floor and the walls. These furnaces are normally single chamber furnaces for low production with variable product mix.

The furnaces are thus useful where only one operation is used as the charging and discharging are situated next to one another, or even by a single door. Such furnaces don’t require much floor space and nor the boxes of the non-scaling steels. The speed of rotation is so adjusted that the heat treatment cycle is completed by the time the hearth completes one complete rotation.

Type # 2. Straight-Chamber Continuous Furnaces:

The following are some of the straight-chamber continuous furnaces:

1. Pusher Furnace:

In a pusher furnace, the component is pushed through the furnace chamber with the help of a pusher with hydraulic or mechanical drive. The pusher is designed to push all the components periodically through the furnace over a certain predetermined distance until the component is properly located and reaches at the discharge end for removal. Large components are loaded directly on the hearth, while the smaller components are put in trays.

Pusher type furnaces are the most widely used of continuous furnaces because of their versatility. These furnaces could be used for hardening, tempering, normalising, annealing, although these are by far the most widely used for gaseous carburising. At the discharge end, the component may be free quenched in an elevator- type quench tank or may be recirculated, to be cooled slowly. The furnaces may have more than one row of components undergoing heat treatment at the same time. Pusher furnaces may be heated by gas or with radiant tubes. Atmosphere may be controlled. The process can be automated.

2. Skid-Rail Furnaces:

In a skid-rail pusher furnace, trays containing the components are put on skid-rails (made of nickel-chrome heat resisting steels, which are being now replaced by silicon carbide refractory rails), and the trays are pushed through the furnaces to the discharge end.

3. Roller-Rail Pusher Furnaces:

Here the trays are supported on the roller rails. The pushing force needed to push the charge is very less, and thus, are used for heavy tray loads and lengthy pushes (i.e., long furnaces).

4. Buggy-Tray Pusher Furnaces:

In a Buggy-tray, wheels and axles are attached to the underside of the tray to push it through the furnaces. Heavier loads could be transported easily.

5. Walking-Beam Furnaces:

Such furnaces are provided with stationary rails and movable rails. The movable rails lift the component from the stationary rails, move it forward and then lower it back to stationary rails and the moving rail comes back to its original position to repeat the process with other component.

The first component is then lifted, moved and lowered by the second moving rails to finally move it to the discharge end. Here, normally the components are getting heated and no skidding is needed. The process can be automated. The moving rail mechanism is expensive and as sealing of the furnace is difficult, atmosphere controlled heat treatments cannot be done.

6. Conveyer-Type Furnaces:

In conveyer type furnaces, mesh or cast-link endless belts are used to move the components through the furnace. Such furnaces are preferred for smaller parts. The components are charged automatically into a mesh belt at the front of the furnace, which moves at a regulated speed through the furnace to perform the heat treatment in this time, before the component is pushed out to be collected in a box or dropped in a quenching tank Cast belts return inside the furnace. These furnaces are used for hardening, tempering, normalising. The conveyers used include woven belts and chains with projecting lugs, pans or trays connected to roller chains.

7. Tunnel Furnace:

In a tunnel furnace, the charge is loaded on a number of cars. The cars are then periodically pushed or pulled slowly through the furnace at a distance equal to the length of the car by means of a moving device. This leads to removal of a car and introduction of next charged car simultaneously. Tunnel furnaces could have a circular path. Normally, such furnaces are used for annealing of steels.  

8. Shaker-Hearth Furnace:

This furnace can be used for hardening, tempering, earbonitriding hut more commonly tor case carburising for case depth of 0.30 mm or less. In a shaker-hearth furnace, the component is moved along the hearth by means of a reciprocating motion which has to be regulated to control the heat treatment time. The hearth design incorporates a serrated cast hearth, the serrations being at right angles to the direction of flow.

As the heating is mainly confined to the components, the heating efficiency is high. The components can be charged manually, but such a furnace is limited to light weight small components. The components can be charged on to hearth by means of a metered automatic hopper or vibrating feed.

The components are then moved along the hearth by means of the reciprocating motion at controlled rate to reach the end where these are discharged through a vertical chute directly into the quench tank, which may contain oil or water-based quenchants.

After quenching, the components are conveyed automatically through the quenching bath to a washing and drying unit. Necessary atmosphere and temperature control are provided. As the hearth moves, it intermittently propels the components in the forward direction which ensures exposure and equal treatment to all surfaces of the components. Commonly the components treated are bolts, screws, springs, clips, ball-bearings, sewing needles, etc.

9. Rotary-Retort Continuous Furnaces:

This is a furnace where controlled atmosphere is used, and thus can be used for controlled hardening, carburising (more commonly) and carbonitriding of same type of small sized components such as fasteners and bearings. The retort is made of cast nickel-chrome steel. The heating may be electrical resistance or gas-fired.

The retort is mounted horizontally and rotated around the horizontal axis or an inclined axis. Commonly, a hopper is used to charge the furnace. The retort is equipped with spirals or flights. A steady flow of charge takes place due to these spirals. As the retort rotates, the parts move forward. The parts tumble inside to be uniformly exposed to the atmosphere.

The retort rotates forward and then backwards. The reversing minimises the length of the furnace. Finally the parts are discharged from discharge chute into a quenching bath, the parts still being under furnace atmosphere. The retorts could be of size between 35 to 85 cm diameters. For high rate of processing, some furnaces may have two retorts.

The loading and unloading of the retort furnace may be automated. Such furnaces if used for carburising then are suitable for case-depth between 0.4 to 2.5 mm.

The heat treatment furnaces can also be classified based on the work-medium.

The working medium of the furnaces could be:

(i) Air-in most of the heat treatment furnaces.

(ii) Controlled gas atmosphere for protecting against scaling and decarburisation.

(iii) Special gas atmospheres, such as carburising, nitriding, carbonitriding, etc.

(iv) Salt bath furnaces.

(v) Oil baths.

(vi) Metal baths like lead baths.

(vii) Solid-powered packing such as pack carburising.