Batch Furnaces for Heat Treatment of Steel: 7 Types | Metallurgy

The following points highlight the seven main types of batch furnaces used for heat treatment of steel. The types are: 1. Box-Type Batch Furnace of Steels 2. Bogie-Hearth Furnace of Steels 3. Muffle Furnace of Steels 4. Pit Furnace (Vertical Furnace) of Steels 5. Bell Furnaces of Steels 6. Salt Bath Furnaces of Steels 7. Fluidized-Bed Furnaces of Steels.

Type # 1. Box-Type Batch Furnace:

The simplest of the box-type has an opening (door) at one face just as in a box, and that is why it is named so. The furnace chamber is commonly rectangular in section. It is used for small and medium sized parts. Generally, loading and unloading (after the heat treatment) is done manually through this door. For heavy components, a zig-crane, or an overhead crane may be used.

Box-type furnaces are quite flexible, and can be used for annealing, pack-carburising, and hardening of low alloy steels. The furnace should be used to its full capacity to drive the maximum advantages.

Though such furnaces can be heated by a fuel, but commonly electric resistance heating is done. The resistance wire in the form of coil is placed inside the refractory (high alumina) grooves. A thermocouple fitted through the rear wall of the chamber is connected to a pyrometer to automatically control the temperature.

Now a days, to facilitate the loading and unloading, a detachable and movable bottom, which is rolled into position underneath the furnace and raised into the furnace by motor-driven mechanisms, after putting the charge on the hearth, is used. Such a furnace is called ‘Elevator-type’ furnace. Large and heavy loads can be handled, and can be cooled rapidly by high velocity gas system internally or externally such as for solutionising treatment of precipitation hardenable type non-ferrous alloys.

Type # 2. Bogie-Hearth Furnace:

It is a modified version of box-type batch furnace having a movable hearth mounted on wheels. This car-type hearth is moved out to load and unload (after the heat treatment) the charge. The car hearth with the loaded charge is put inside the furnace and then sealed with granular sand sealing troughs, or solid seals. Such furnaces are normally non-atmospheric controlled. The heating may be done by a fuel, or by electric resistance heating elements. It is also possible to use a programmed cycle here.

Normally bogie-hearth furnaces are used in temperature range of 540°C to 1100°C such as for stress- relieving, annealing, hardening of components. It is commonly used for heat treating bulky and heavy components although it can also be used for small components.

Type # 3. Muffle Furnace:

A muffle is a hollow cuboid or cylindrical retort made of special refractory material, or non-scaling steel. A furnace, in which the heat source does not directly make contact with the material being heat-treated, is described as a muffle furnace. The components are charged in a muffle, and gas firing, or electrical energy can be used to heat the muffle from outside.

The gas is burnt outside the muffle, and the heating is effected by the hot gases which are made to circulate through the ring like space between the interior-wall and the exterior-muffle wall.

The products of combustion of the gas do not enter the heating chamber (the muffle), and thus, the atmosphere of the furnace can be controlled, and thus, scaling of the components can be prevented. Also, such a furnace gives reasonable uniformity of temperature distribution. Fig. 10.2 illustrates such a furnace schematically. Gas carburising of small parts such as parts of cycle-chain is carried out in a non-scaling steel retort (muffle) revolving around the horizontal axis.

Commonly, electrically heated muffle furnaces are extensively used for the heat treatment of small sized components. Fig. 10.3 illustrates heating element like nichrome or kanthal wire wound around the muffle, or are placed in the ring-like space to heat the muffle with its contents. For high temperatures, electric muffle is heated by glow bars, or radiant elements, where the steel gets heated by direct radiations. Muffle furnaces are used for bright annealing, nitriding, carburising, bright hardening.

There are two main types of muffle batch-type furnaces, depending on the design. A horizontal type is illustrated in figures 10.2 and 10.3. The second type is a vertical muffle pit furnace. (Fig. 10.4)

Type # 4. Pit Furnace (Vertical Furnace):

A pit furnace consists of the furnace placed in a pit. The furnace extends to the shop-floor level or slightly above it. It has a cover or lid put on top of the furnace. The long and slender parts such as tubes, spindles, shafts, rods, etc. are suspended from suitable fixtures from top, or may be supported from the lower end to remain in vertical position. Heating in such a manner reduces distortion and warpage.

The components can be put on the bottom hearth of the furnaces for heating or held in a basket inside the furnace (Fig. 10.4). The non-scaling steel retort can help high degree of control of atmosphere. Fans promote both uniformity of temperature as well as gas composition. Fig. 10.4 furnace is used for gas carburising or other case-hardening treatments.

Fig. 10.4 Muffle pit furnace

A Pit furnace may not have a muffle. Pit furnaces are particularly suited for parts to be cooled in the furnace. Direct quenching particularly with large charge and in a large furnace is not feasible. Not only that the temperature may fall during opening of lid, the brief exposure to atmosphere results in the formation of adhering black scale.

To avoid this formation, horizontal type batch furnace with provisions of quenching under a cover of protective atmosphere is used. Pit furnace is cheaper and gives a better pay load-cost ratio, particularly suiting long slender components.

Type # 5. Bell Furnaces:

Bell furnaces have removable covers, called ‘bells’. The charge is put on a hearth, and on top of it is put a retort (Fig. 10.5 c), which is sealed at the bottom with sand, etc. The supply of the protective gas is constantly given to inside of this scaled retort for continuous protection.

An outer ‘bell’-shaped container having the heating elements fitted on inner wall is lowered to cover such an assembly as being done in Fig. 10.5 (b) and finally looks like as in Fig. 10.5 (a). The heating of the charge is done in controlled protective gas atmosphere for the required length of the time.

After heating the first charge, the bell is taken off and is put on another assembly and the heating is then done of the second assembly for the required time. Mean-while, the first assembly and its charge cools under the protective atmosphere as the gas is constantly being fed in and out of the sealed retort.

The hearth could be rectangular, or circular. One bell can take care of several retort assemblies. Fans may be provided inside the hearth assembly for rapid heating and uniformity of temperature.

Temperature could be controlled by having thermocouples, automatic temperature controllers. As the heating bell shifts after completion of heating of one assembly to another, it leads to economic heating procedure as the heating bell is not continuously cooled and reheated. This procedure needs more floor space and overhead cranes, etc.

Bell furnace is used for bright annealing, nitriding, bright normalising, ion-nitriding stress relieving, etc.

Type # 6. Salt Bath Furnaces:

Molten salt bath furnace essentially consists of an oval or rectangular container made of steel, cast iron, or a refractory pot which holds the molten salts. The pol is heated to and maintained at the required heat treating temperature either by the combustion of a fuel (fuel oil, or gas), or by electrical resistors.

Electrically heated salt bath furnaces are much more common in use, now-a-days. Refractory pots are preferred for use with the neutral salts (free from cyanides or carbonates). Steel or cast ire ns are suitable for other salts used for cyaniding, or liquid carburising, etc.

The mode of heat transfer to the charge is mainly by convection through the liquid bath. As the molten salt bath comes in best intimate contact with the charge, the heat transfer to the charge is very quick. Moreover, the molten salts possess high heat capacity resulting further in very fast heating up of the charge as compared to air furnaces (around five times). Thus, the heat treatment time is drastically reduced resulting in good economy.

Salt bath furnaces can be used in a very wide range of temperatures from 150°C to 1300°C depending on the salt mixture used and the heat treatment requirements such as from tempering to hardening of high speed steels, including cyaniding, liquid carburising, liquid nitriding, austempering, martempering. The temperatures of the bath can be controlled within plus or minus 5°C. These furnaces are most commonly used for small and medium sized components particularly for mass heat treatment of parts.

Advantages of Salt Bath Furnaces:

Some of the advantages of salt bath furnaces are:

1. Because of better temperature control, all the components at a time are healed to the same uniform heat treatment temperature, resulting in highly reproducible properties.

2. There is no danger of oxidation and decarburization.

3. As the heating rate is high, heat treatment time is reduced.

4. Selective heat treatment can be done by immersing only the desired section of the components in the molten salt bath.

5. Different shapes, sizes of variable section thicknesses of light and heavy parts can be given heat treatment simultaneously with different heal treatment times at the same heat treatment temperature.

6. Desired furnace atmosphere can be obtained by properly selecting the salt mixture.

7. Initial cost of installation of such a furnace is very low.

8. The floor area and the maintenance required are minimal.

9. Worn-out electrodes can be easily replaced while the furnace is in operation, and does not need to be shut down for a long time.

10. Electrode-salt baths have higher heating efficiency, and much higher temperature can be attained. These furnaces are almost always used for heat treatment of high speed tool steels.

11. Temperature control ± 5°C can be easily obtained.

Disadvantages of Salt Bath Furnaces:

1. High cost of pots which are to be replaced periodically.

2. Replacement of pots is quite time and labour oriented problem.

3. Pollution problems about fumes, but more critical the disposal of spent salts.

4. A large number of salt mixtures have cyanide as an important gradient. All necessary precautions have to be taken while using such salts. Proper ventilation, fume hoods, separates gloves, tongs etc. are required. Even otherwise too these salts are hazardous lo labour working there.

5. A steel hardened in cyanide bath should not be tempered in a salt bath without completely removing any sticking salt otherwise violent explosion can occur.

Types of Salt Bath Furnaces:

There are three main types of salt bath furnaces:

(i) Externally heated furnaces.

(ii) Immersion heating element type furnaces.

(iii) Immersed electrode type furnaces.

The externally heated salt bath furnaces could be fuel fired or electrical resistance wound salt bath furnaces. In both these, the pot or retort is heated from outside to bring it to the required heat treatment temperature.

Fig. 10.6 illustrates schematic diagrams of these furnaces:

In the immersion heating element type salt bath furnaces, the heating element is immersed in the salt and remains there for heating, maintaining the temperature and even after shut down of the furnaces.

Immersed Electrode Type Salt Bath Furnace:

The most commonly used industrial salt bath furnace is the immersed electrode type furnace. It has molten salt bath with immersed electrodes to supply the power. The electrodes normally are of mild steels, or of steel having 28% chromium and 2% nickel. Electrodes are flats with square or rectangular cross section as the opposing surfaces of such electrodes cause better concentration of magnetic flux than a round surface.

The electrodes are connected to the secondary of the transformer to power the furnace. As the molten salts have electrical resistance, the heat is generated within the salt bath when the current is passed through the electrodes to the bath. Fig. 10.8 illustrates one immersed electrode salt bath furnace.

Starting of the Salt Bath Furnace:

Solid salts are bad conductors of electricity, but once the ionic salts become molten, then the ionization produces cations and anions, which move through the molten salts under the influence of applied current to opposite electrodes and thus, conduct electricity.

In the salt bath furnaces, the fused salt is at the same time a working medium, i.e., transfers heat to the components, as well as acts as heating element, i.e., the electrical resistance of the molten salt to the flow of current generates heat to heat the salt bath to the required heat treatment temperature and also continue maintaining it at this temperature for the required time.

As the cold solid salt does not conduct electricity, the salt has to be melted first by means of an auxiliary device, or a oxy- acetylene blow torch till there is enough molten salt between the electrodes to conduct the electricity between the electrodes and thus generate heat to melt more solid salt added later to fill the heating chamber of the furnace with molten salt.

The auxiliary device normally consists of a ‘U-shaped’ mild steel frame connected at the both ends with copper connectors (Fig. 10.7 a). For starting a salt bath furnace, a layer of salt is spread at the bottom of the empty furnace, and the U-shaped section is put in the heating chamber of the furnace. It is connected to either a separate power supply or to the provisions made on the electrodes outside the heating chamber.

Some salt is added into the chamber. Now, the power is supplied at low taps of the transformer. The electrical resistance of this mild steel U- section, generates heat and melts the salt. More salt is added and made molten till there is sufficient molten salt in the furnace to make good electric connection between the electrodes through the molten salt, i.e., it itself can act as heat producer.

The ‘U-shaped’ device is then taken out and then the power is switched on through the electrodes to the molten salt. Before closing down the furnace, and while the salt is still molten, the dry ‘U-shaped’ device is again placed inside the heating chamber. As the salt solidifies, the device remains inside it, so that starting the furnace for the next heat treatment can be done by it.

Many times, instead of the ‘U-shaped’ mild steel auxiliary device, a graphite disc is placed on the salt spread on the bottom of the furnace so that the disc presses against the ends of the auxiliary electrodes. The power is switched on with maximum voltage supplied to it. The graphite disc becomes incandescent and thus, melts the salt. More salt is added till that also melts. After a sufficient molten salt has been produced, the auxiliary electrodes and the graphite disc are removed.

Now the temperature of the salt may be raised by the direct application of the current to the molten salt and more salt may be added to the furnace. Increasing the currents helps in faster rate of heating of the salt bath to the required heat treatment temperature. The temperature of the bath is automatically controlled with the help of thermocouples, pyrometers, temperature controllers within plus or minus 5°C of the required temperature.

Type # 7. Fluidized-Bed Furnaces:

Fluidized-bed furnaces have become recently quite common in use replacing neutral salt baths, atmosphere furnaces for hardening, annealing, normalising in 750-1050°C range; cyanide salts bath, atmosphere furnaces for surface treatment; salt baths and forced-air circulation furnaces for tempering in range 100-750°C; and even continuous furnaces.

The furnace essentially consists of a porous plate above which is the bed of dry fine particles of sieve size 80 to 100 grits commonly of aluminium oxide for heat treatment of metals, though sand, or zirconium oxide, etc. are also used. The bed is made to act like a liquid (fluid) by a moving gas fed upwards, so that the bed goes into motion and the upper surface of the bed disappears.

This is called the disperse (or lean) phase fluidized bed with pneumatic transport of the solids. Although the disperse phase bed furnaces are used for long and thin parts like shafts and plates, but more commonly, dense phase fluidized bed furnaces are used in which the components to be heat treated are submerged in a bed of fine solid particles held in suspension without any particle entrainment by a flow of the gas.

The bed is fluidized by air, or a mixture of gases, depending on the requirement of the process. For example- for neutral hardening or tempering, nitrogen gas is used; for carburising a mixture of methanal, N₂ and propane, or a mixture of propane and air, is used.

Fluidized beds have high heat transfer efficiency-around 8 to 25 times of forced air circulation type of furnaces. This is because of turbulent motion and rapid circulation of the particles in the fluidized furnace, and due to the high solid-gas interfacial area. This is attained when the flow rate is two to three times the minimum fluidization velocity.

As said, it is possible to obtain various types of atmospheres in fluidized beds such as reducing or oxidising, neutral and carburising. The atmospheres in the fluidized beds could be ammonia, neutral gas, N₂ and air; mixtures of propane and air for carburising to obtain high effective case depth of 1 mm in 1.5 hour. Normally larger volume of propane is consumed in such furnaces.

Commonly used fluidized-bed furnaces are of following types:

1. External Electric-Heating Fluidized Furnace:

The retort containing the fluidized bed is heated from external electrical resistance elements or silicon carbide rods as illustrated in Fig. 10.9. Here, fluidized gas can be maintained at any desired composition. It takes normally 3-4 hours to heat to the normal heat treatment temperature of 820-870°C.

2. External Gas-Fired Fluidized Furnace:

A burner burns a fuel gas- air mixture to flow around and heat the retort containing the bed. Burners can be controlled well to even get low tempering temperatures.

3. Sub-Merged Combustion Furnace:

In such furnaces, the products of combustion of fuel are passed through the bed and parts, which results in excellent rate of heat transfer.

4. Internal Combustion Gas-Fired Furnace:

Here, the gas-air mixture, for heating and for fluidization, are ignited in the bed, generating heat by internal combustion, i.e., the bed is fluidized by burning gases to generate heat in the bed. If the furnace is controlled properly, it is possible to heat to 800°C in 1 and 1½ hours.

As there is uniform temperature is the fluidized furnaces, and as the fluidized beds have high thermal conductivity, it is possible to quench many parts in fluidized bed that are normally air cooled resulting in least distortion or cracks. Fluidized-bed furnaces are cheaper to install than other furnaces except the salt bath furnaces.