Heat Treatment Furnaces and Its Classification | Steel | Metallurgy

In this article we will discuss about the meaning and classification of heat treatment furnaces.

Meaning of Heat Treatment Furnaces:

Heat treating furnaces are essentially heating chambers, i.e., a refractory vessel which holds the steel stock as well the heat. The furnace chamber is heated with some source of heat. The supply of heat has to be regulated depending on the requirement. More heat is needed during the heating period, but almost a constant heat is required when the furnace has attained the required temperature and is to be maintained at that temperature.

The heat has to be supplied to the whole of the properly designed furnace in a way that the temperature is constant everywhere, or at the places where the charge is being kept otherwise some parts may get under heated, or overheated. The doors, or openings are kept as small sized as possible to reduce the heat losses. Tempering and low temperature furnaces may require provisions for forced air or atmosphere circulation. The carburised parts may be quenched inside the furnace itself.

The heat treatment furnaces play a very important role in imparting reproducible useful properties to the steel components. The design of the furnace is determined by the stock which is to be treated and the particular treatment which has to be carried out at the special temperature. When parts are required to be heat treated at different temperatures, several furnaces may be required as a furnace which may be suitable for use at 1300°C, may be unsuitable for use at 300°C, although the latter temperature is within its maximum temperature range.

This is because heat transfer below 700°C and particularly at 300°C or so occurs mainly by conduction and convection. Radiation plays a relatively unimportant part. Thus, the box-type furnace is unsuitable. Forced-air-circulation furnaces are needed. Thus, a hardening furnaces cannot be used for tempering purposes. Modern furnace designs have progressed to such a state that there is a special furnace for each range of temperature. There is no furnace which will efficiently carry out all the heat treatments.

Classification of Heat Treatment Furnaces:

The heat treatment furnaces may be classified according to following criterion:

Based on Source of Heat:

Though the choice of the right fuel depends upon the availability and cost of the fuel, but technical suitability plays an important role in determining the best fuel. The initial cost of the furnace, the running cost of the fuel, supervision charges, maintenance charges, etc. factors must be taken into account and compared with the relative convenience of using a particular fuel, before a correct choice can be made.

When comparing the cost of the fuel, the relative efficiencies of utilization must also be considered. For example, the efficiency of a coal fired furnace may be 5 to 10%, while a properly designed and insulated electric furnace may have efficiency of utilization of 96 to 98%. The factors like the ease with which automatic temperature controller and time controlling devices can be arranged to bring the furnace in operation, or switch it off also influences the choice of fuel.

Although solid fuel for firing furnaces are almost no longer in use for heat treatment, these are given here just because these too were used earlier.

The choice of fuels is given in brief as:

1. Solid Fuels:

(a) Coal:

Advantages:

Cheap, easily available, luminous.

Disadvantages:

Lack of proper temperature control- Proper grade of coal is required; Labour oriented; storage problem; Smoke problem.

(b) Pulverised Coal:

Advantages:

Same as solid coal. Most of the disadvantages of solid coal firing are overcome. Temperature control is possible by the control of firing. Cheaper low grade coals can also be burnt satisfactorily. No labour required for charging the furnace. Smoke problem can be overcome through control of combustion.

Disadvantages:

It Increased cost of pulverising equipment and the attended mechanical complications. Coals of high ash contents cannot be used successfully.

(c) Coke:

It is similar to coal but coke ensures general freedom from smoke, but as the calorific valve is less than that of coal, a larger grate area is required for the same heat output.

2. Liquid Fuels:

If oils are available at economic prices, then, they have many advantages:

Easier control of combustion as no labour is needed for firing, though in winter season, fuel oils thicken and oil pipe lines have to be specially warmed up by steam. Though liquid fuels are easy to store and the furnace can be fired at any time, but fuel oil has to be stored in a separate building. The designs of the furnaces are simpler; there are no ashes remaining after the combustion of fuel oil.

Temperature control is effected with great accuracy. Oil-fired furnaces are economical only at temperatures of around 1000°C. Luminous flames of oil enable rapid heat transfer to steel components. Commonly gasoline and kerosene are used successfully.

Disadvantage:

Temperature is not uniform throughout the chamber and thus, use of heat circulation arrangement is essential.

3. Gaseous Fuels:

Of the fuel-fired furnaces, gas-fired are the best and most economical. Gas fired furnaces are simple in design and possess better control over temperature, and can be used up to 1500°C. Gas-fired furnaces are highly perfected, being inferior only to electric furnaces. The gas-fired furnaces can be designed from smallest to the largest size.

Gaseous fuel used could be:

(a) Coal-gas or Town gas

(b) Producer gas

(c) Blue-water gas

(d) Cracked oil gas

(a) Coal Gas or Town Gas:

Scarcity of high-quality gassing coal in India led to least development of gas industry. Steels plants having their own coke ovens supply this gas to be used as a fuel.

(b) Producer Gas:

As this gas has rather low calorific value, and unless enriched with other rich gas like coke oven, etc., requires specially designed large burners, ports and passages to enable the furnaces to be brought up to heat treating temperatures within reasonable time. To attain higher temperatures, preheating of the air blast is almost essential.

(c) Blue Water Gas:

This gas has higher calorific value than producer gas specially if coal is used in place of coke in the producer-bed. The calorific value and the content of methane, is much lower than coal gas.

(d) Cracked Oil Gas:

This is an excellent fuel of a very high calorific value, almost double than that of coal-gas. Owing to the higher cost of oil as compared to coal, this system has only been used for small scale plants.

4. Electricity:

Electric furnaces are used most extensively now-a-days and are replacing the gas-fired furnaces. Heating furnaces with electric current is probably the most perfect means of power use. Electric furnaces offer many advantages: Much simple in design; no combustion chamber, gas ducts, nor stack flues. Uniformity of temperature in furnaces and the precise control over temperature can be easily effected by means of automation.

As heat is not lost in flue gases, the efficiency of heat utilisation is highest. The efficiency of batch type hardening and normalising electric furnaces is 65% to 75%, while that of continuous heat treating furnaces is 70 to 80%. Very high temperatures can be attained in electric furnaces. No pollution with neat and clean hygienic working conditions. Minimum requirement of accessories. It is very convenient to start and switch off the electric furnaces.

Of the electric heating methods, electric resistance furnaces are the most commonly used furnaces for heat treatment of metals and alloys. Induction heating is the most common, perfect and even cheap (on mass production of similar parts) surface hardening method. Laser and plasma heating for surface hardening have limited field. In resistance heating furnaces, the temperatures can be controlled easily and with a high degree of accuracy. The commonly used resistors with the maximum temperature to which they could be used are given in table 10.1.

Based on Use i.e. Type of Heat Treatment:

The furnaces can be classified depending on the type of heat treatment done such as:

(a) Tempering or Sub-Critical Annealing of Steel:

Range of temperature is 0 to 700°C.

Types:

(i) Dry furnace with forced air circulation.

(ii) Liquid baths- Oil, lead, or salt.

(b) General Purpose Furnaces:

Range 700 to 1050°C- For hardening, normalising, carburising of carbon and low alloy steels.

Types:

(i) Dry furnaces.

(ii) Liquid baths- Lead or salt.

(c) High Temperature Furnaces:

Range from 1000 to 1400°C- For heat treatment of high speed steels; other high temperature cycles.

Types:

(i) Dry furnaces

(ii) Liquid baths- Salt baths

(d) Carburising or Carbonitriding Furnaces:

Sealed quench furnace.

Based on Types of Operations:

1. Batch Furnaces:

The name ‘batch’ signifies that a batch of components are charged in a furnace, which is heated from room temperature at a specified rate to a predetermined maximum temperature, kept at that temperature for a specified time, and then cooled at specified rate.

Then, the next batch of components is charged into the furnace and undergoes the heat treatment. If needed, one batch of components may differ from another in weight, size, grade of steel and the type of heat-treatment required (i.e., even the temperature of heating and the soaking time etc. may also vary).

A batch furnace basically consists of a refractory lined-insulated furnace chamber enclosed in a steel shell with one or more access doors and a heating method inside the chamber. Different types of heat treatment cycles can be carried out in the same furnace, but one at a time with one batch of components, for example, carburising, hardening, annealing, nitriding, normalising, stress-relieving annealing, etc.

If there are a number of batch type of furnaces in a heat treatment shop, then the work should be specialised, i.e., one set of furnaces is used for annealing, another for normalising, etc. otherwise, the productivity of furnace is lowered and consumption of fuel or electric power is increased. These furnaces, thus, can be operated with, or without a controlled atmosphere.

A batch furnace is generally used to heat treat low volumes of parts (i.e., low weight per hour), as well as for carburising parts that need deep cases, i.e., long cycle times (such as pit furnaces used for ball bearings), or for parts difficult to be handled by a conveyer system in continuous furnaces, or parts which are large sized but only a few in number.

A batch furnace could be of horizontal or vertical type. The initial cost of a batch furnace is much less than a continuous furnace. Generally, the loading and unloading of the charge is done manually, i.e., it may require a lot of labour. Some very commonly used batch furnaces are- Box-type batch furnace, bogie-hearth furnace, salt bath furnace, muffle finance, pit furnace, sealed-quench furnace, bell furnace, tempering furnace, vacuum furnace, fluidized-bed furnace.

2. Continuous Furnaces:

In a continuous furnace, the components to be heat treated are almost continuously charged at one end of the furnace and then discharged at the other end of the furnace after the heat treatment is over. The components are moved through the furnace, as a rule, by mechanical means.

These furnaces are generally operated at permanent temperature conditions, and thus, are invariably used to heat the same components, made of the same grade of steel and subjected to the same heat treatment cycle, i.e., only to annealing, to hardening, to normalising, or carburising, etc., i.e. these furnaces are specialised furnaces.

Thus, these furnaces can be easily programmed and are typical of plants which heat treat high volume components, or for mass production. Continuous furnaces may or may not have controlled atmospheres. The initial cost of such furnaces is high, but the running costs are low due to reduced labour cost and high efficiency of flow of charge, particularly if the furnace is run 24 hours a day.

A continuous furnace may have different zones, necessitating proper control on the travel of the components through the zones. For example, for carburising, the furnace may have separate chambers for heating, for carburising and for the diffusion process.

In a continuous furnace, movement of the components has to be done from the charging door to the discharging door. Two general designs of the furnaces could be, one having the rotating hearth and the second a straight-chamber furnace.