Heat-treating Furnaces and Equipment

Two basic types of furnaces are used for heat treating: batch furnaces and continuous furnaces. Because they consume energy, their insulation and efficiency are important design considerations, as are their initial cost, the personnel needed for their operation and maintenance, and their safe use.

Uniform temperature and accurate control of temperature-time cycles are important. Modern furnaces are equipped with various electronic controls, including computer-controlled systems, programmed to run through a complete heattreating cycle repeatedly and with reproducible accuracy. Heating-system fuels are usually natural gas, oil, or electricity (for resistance or induction heating). The type of fuel used affects the furnace’s atmosphere: Unlike electric heating, gas or oil introduces combustion products into the furnace (a disadvantage). Electrical heating, however, has a slower start-up time, and it is more difficult to adjust and control.

Batch Furnaces. In a batch furnace, the parts to be heat treated are loaded into
and unloaded from the furnace in individual batches. Basically, the furnace consists
of an insulated chamber, a heating system, and an access door or doors. Batch furnaces are of the following basic types:

a. A box furnace is a horizontal rectangular chamber with one or two access doors through which parts are loaded. This type of furnace is used commonly and is versatile, simple to construct and use, and available in several sizes. A variation is the car-bottom furnace. The parts to be heat treated,
usually long or large, are loaded onto a flatcar, which then moves on rails
into the furnace.

b. A pit furnace is a vertical pit below ground level into which the parts are lowered. This type of furnace is particularly suitable for long parts, such as rods, shafts, and tubing, because they can be suspended by one end and, consequently,are less likely to warp during processing than if positioned horizontally within a
box furnace.

c. A bell furnace is a round or rectangular box furnace without a bottom and is lowered over stacked parts that are to be heat treated. This type of furnace is particularly suitable for coils of wire, rods, and sheet metal.
d. In an elevator furnace, the parts to be heat treated are loaded onto a car platform, rolled into position, and then raised into the furnace. This type of furnace saves space in the plant and can be especially suitable for metal alloys that have to be quenched rapidly, because a quenching tank can be placed directly under the furnace.

Continuous Furnaces. In this type of furnace, the parts to be heat treated move continuously through the furnace on conveyors of various designs that use trays,belts, chains, and other mechanisms. Continuous furnaces are suitable for high production runs and can be designed and programmed so that complete heat-treating cycles can be performed under tight control.

Salt-bath Furnaces. Because of their high heating rates and better control of uniformity of temperature, salt bat/as are commonly used in various heat-treating operations, particularly for nonferrous strip and wire. Heating rates are high because of the higher thermal conductivity of liquid salts compared with that of air or gases. Depending on the electrical conductivity of the salt, heating may be done externally (for nonconducting salts),or by immersed or submerged electrodes using low-voltage alternating current. Direct current cannot be used because it subjects the salt to electrolysis. Salt baths are available for a wide range of temperatures.Lead can also be used as the heating medium.

Fluidized Beds. Dry, fine, and loose solid particles, usually aluminum oxide, are heated and suspended in a chamber by an upward flow of hot gas at various speeds.

     

The parts to be heat treated are then placed Within the floating particles-hence the term fiuidizea' bed. Because of the constant agitation, the system is efficient, the temperature distribution is uniform, and the heat-transfer rate is high. These furnaces are used for various batch-type applications.

Induction Heating. In this method, the part is heated rapidly by the electromagnetic field generated by an induction coil carrying alternating current, which induces eddy currents in the part. The coil, which can be shaped to fit the contour of the part to be heat treated (Fig. 426), is made of copper or of a copper-base alloy. The coil, which is usually Water cooled, may be designed to quench the part as Well, after heating it. Induction heating is desirable for localized heat treating, such as that required for gear teeth, cams, and similar parts.

Furnace Atmospheres. The atmospheres in furnaces can be controlled so as to avoid (or cause) oxidation, tarnishing, and decarburization of ferrous alloys heated to elevated temperatures. Oxygen causes oxidation (corrosion, rusting, and scaling).Carbon dioxide has various effects; it may be neutral or decarburizing, depending on its concentration in the furnace atmosphere. Water vapor in the furnace causes oxidation of steels, resulting in a blue color. Nitrogen is a common neutral atmosphere, and a vacuum provides a completely neutral atmosphere. The term bluing is used to describe formation of a thin blue film of oxide on finished parts to improve their appearance and their resistance to oxidation.