Page 145 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment
Uniform temperature and accurate control of temperature-time cycles are
important. Modern furnaces are equipped with various electronic controls, includ-
ing computer-controlled systems, programmed to run through a complete heat-
treating 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 fur-
naces 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 com-
monly 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 low-
ered. 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 plat-
form, 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 pro-
duction 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 be-
cause 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.