Page 144 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 4.12 Heat-treating Furnaces and Equipment

service over a period of time. It also reduces the tendency
toward stress-corrosion cracking (Sections 2.10 and 3.8). 1800

Tempering. If steels are hardened by heat treatment, then 1600 Tensile Strength
tempering or drawing is used in order to reduce brittleness, §
increase ductility and toughness, and reduce residual stresses. E 1400 _
The term “tempering” is also used for glasses (Section 18.4). Q Yield Stress
In tempering, the steel is heated to a specific temperature, de- 5, 1200 -
pending on its composition, and then cooled at a prescribed §
rate. The results of tempering for an oil-quenched AISI 4340 1000 _
steel are shown in Fig. 4.25. Alloy steels may undergo temper |qeduCtiOn
embrittlement, which is caused by the segregation of impuri- 300 ° of area
ties along the grain boundaries at temperatures between 480°
and 590°C 200 300 400 500 600
_
Tempering temperature (°C)
Austempering. In austempering, the heated steel is

temperature until isothermal transformation from austen-
ite to bainite is complete. It is then cooled to room temperature, usually in still air
and at a moderate rate in order to avoid thermal gradients within the part. The
quenching medium most commonly used is molten salt, at temperatures ranging
from 160° to 750°C.
Austempering is often substituted for conventional quenching and tempering,
either to reduce the tendency toward cracking and distortion during quenching or to
improve ductility and toughness while maintaining hardness. Because of the shorter
cycle time involved, this process also is economical for many applications. In
modified austempering, a mixed structure of pearlite and bainite is obtained. The
best example of this practice is patenting, which provides high ductility and moder-
ately high strength, such as in patented wire (Section 15.8).

Martempering (Marquenching). In martempering, the steel or cast iron is first
quenched from the austenitizing temperature in a hot-fluid medium, such as hot oil
or molten salt. Next, it is held at that temperature until the temperature is uniform
throughout the part; then it is cooled at a moderate rate, such as in air in order to
avoid excessive temperature gradients within the part. Usually, the part is then tem-
pered, because the structure obtained is otherwise primarily untempered martensite
and thus is not suitable for most applications. Martempered steels have less tendency
to crack, distort, or develop residual stresses during heat treatment. In modified
martempering, the quenching temperature is lower, and thus the cooling rate is higher.
The process is suitable for steels with lower hardenability.

Ausforming. In ausforming, also called thermomechanical processing, the steel is
formed into desired shapes within controlled ranges of temperature and time, to
avoid formation of nonmartensitic transformation products; the part is then cooled
at various rates to obtain the desired microstructures. Ausformed parts have superior
mechanical properties.

4.l2 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.

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