or returning the metal to room temperature, sometimes
PREHEATING. Following the preheating, the steel is
rapidly, sometimes slowly.
quickly heated to the final temperature. Preheating aids
in obtaining uniform temperature throughout the part
Heating
being heated, and, in this way, reduces distortion and
cracking. When a part is of intricate design, it may have
Uniform temperature is of primary importance in
to be preheated at more than one temperature to prevent
the heating cycle. If one section of a part is heated more
cracking and excessive warping. As an example,
rapidly than another, the resulting uneven expansion
assume that an intricate part is to be heated to 1,500F
often causes distortion or cracking of the part. Uniform
(815C) for hardening. This part might be slowly
heating is obtained by slow heating.
heated to 600F (315C), be soaked at this temperature,
then be heated slowly to 1,200F (649C), and then be
The rate at which a part may be heated depends on
several factors. One important factor is the heat
soaked at that temperature. Following the second
conductivity of the metal. A metal that conducts heat
preheat, the part would be heated quickly to the
readily may be heated at a faster rate than one in which
hardening temperature. Nonferrous metals are seldom
heat is not absorbed throughout the part as rapidly. The
preheated because they usually do not require it.
condition of the metal also affects the rate at which it
Furthermore, preheating tends to increase the grain size
may be heated. For example, the heating rate for
in these metals.
hardened tools and parts should be slower than for
metals that are not in a stressed condition. Finally, size
Cooling
and cross section have an important influence on the
After being heated to the proper temperature, the
rate of heating. Parts large in cross section require a
metal must be returned to room temperature to
slower heating rate than thin sections. This slower
complete the heat-treating process. The metal is cooled
heating rate is necessary so that the interior will be
by placing it in direct contact with a gas, liquid, or solid,
heated to the same temperature as the surface. It is
or some combination of these. The solid, liquid, or gas
difficult to uniformly heat parts that are uneven in cross
used to cool the metal is called a "cooling medium."
section, even though the heating rate is slow. However,
The rate at which the metal should be cooled depends
such parts are less apt to be cracked or excessively
on both the metal and the properties desired. The rate of
warped when the heating rate is slow.
cooling also depends on the medium; therefore, the
Soaking
choice of a cooling medium has an important influence
on the properties obtained.
The object of heat-treating is to bring about
Cooling metals rapidly is called "quenching," and
changes in the properties of metal. To accomplish this,
the oil, water, brine, or other mediums used for rapid
the metal must be heated to the temperature at which
cooling is called a "quenching medium." Since most
structural changes take place within the metal. These
metals must be cooled rapidly during the hardening
changes occur when the constituents of the metal go
process, quenching is generally associated with
into the solution. Once the metal is heated to the proper
hardening. However, quenching does not always result
temperature, it must be held at that temperature until
in an increase in hardness. For example, copper is
the metal is heated throughout and the changes have
usually quenched in water during annealing. Other
time to take place. This holding of the metal at the
metals, air-hardened steels for example, may be cooled
proper temperature is called SOAKING. The length of
at a relatively slow rate for hardening.
time at that temperature is called the SOAKING
Some metals are easily cracked or warped during
PERIOD. The soaking period depends on the chemical
quenching. Other metals may be cooled at a rapid rate
analysis of the metal and the mass of the part. When
with no ill effects. Therefore, the quenching medium
steel parts are uneven in cross section, the soaking
must be chosen to fit the metal. Brine and water cool
period is determined by the heaviest section.
metals quickly, and should be used only for metals that
In heating steels, the metal is seldom raised from
require a rapid rate of cooling. Oil cools at a slower rate
room temperature to the final temperature in one
and is more suitable for metals that are easily damaged
operation. Instead, the steel is slowly heated to a
by rapid cooling. Generally, carbon steels are con-
temperature below the point at which the solid solution
sidered water hardened and alloy steels oil hardened.
begins, and it is then held at that temperature until heat
Nonferrous metals are usually quenched in water.
is absorbed throughout the metal. This process is called
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