heat parts that are uneven in cross section, even though
the heating rate is slow. However, such parts are less apt
to be cracked or excessively warped when the heating
rate is slow.
The object of heat treating is to bring about changes
in the properties of metal. To accomplish this, the metal
must be heated to the temperature at which structural
changes take place within the metal. These changes
occur when the constituents of the metal go into the
solution. Once the metal is heated to the proper
temperature, it must be held at that temperature until the
metal is heated throughout and the changes have time to
take place. This holding of the metal at the proper
temperature is called SOAKING. The length of time at
that temperature is called the SOAKING PERIOD. The
soaking period depends on the chemical analysis of the
metal and the mass of the part. When steel parts are
uneven in cross section, the soaking period is
determined by the heaviest section.
In heating steels, the metal is seldom raised from
room temperature to the final temperature in one
operation. Instead, the steel is slowly heated to a
temperature below the point at which the solid solution
begins, and it is then held at that temperature until heat
is absorbed throughout the metal. This process is called
PREHEATING. Following the preheating, the steel is
quickly heated to the final temperature. Preheating aids
in obtaining uniform temperature throughout the part
being heated, and, in this way, reduces distortion and
cracking. When apart is of intricate design, it may have
to be preheated at more than one temperature to prevent
cracking and excessive warping. As an example, assume
that an intricate part is to be heated to 1,500°F (815°C)
for hardening. This part might be slowly heated to 600°F
(315°C), be soaked at this temperature, then be heated
slowly to 1,200°F (649°C), and then be soaked at that
temperature. Following the second preheat, the part
would be heated quickly to the hardening temperature.
Nonferrous metals are seldom preheated because they
usually do not require it. Furthermore, preheating tends
to increase the grain size in these metals.
After being heated to the proper temperature, the
metal must be returned to room temperature to complete
the heat-treating process. The metal is cooled by placing
it in direct contact with a gas, liquid, or solid, or some
combination of these. The solid, liquid, or gas used to
cool the metal is called a cooling medium. The rate at
which the metal should be cooled depends on both the
metal and the properties desired. The rate of cooling also
depends on the medium; therefore, the choice of a
cooling medium has an important influence on the
Cooling metals rapidly is called quenching, and
the oil, water, brine, or other mediums used for rapid
cooling is called a quenching medium. Since most
metals must be cooled rapidly during the hardening
process, quenching is generally associated with
hardening. However, quenching does not always result
in an increase in hardness. For example, copper is
usually quenched in water during annealing. Other
metals, air-hardened steels for example, may be cooled
at a relatively slow rate for hardening.
Some metals are easily cracked or warped during
quenching. Other metals may be cooled at a rapid rate
with no ill effects. Therefore, the quenching medium
must be chosen to fit the metal. Brine and water cool
metals quickly, and should be used only for metals that
require a rapid rate of cooling. Oil cools at a slower rate
and is more suitable for metals that are easily damaged
by rapid cooling. Generally, carbon steels are
considered water hardened and alloy steels oil hardened.
Nonferrous metals are usually quenched in water.
FORMS OF HEAT TREATMENT
The various heat-treating processes are similar in
that they involve the heating and cooling of metals. They
differ, however, in the temperatures to which the metals
are heated, the rates at which they are cooled, and, of
course, in the final result. The most common forms of
heat treatment for ferrous metals are annealing,
normalizing, hardening, tempering, and case hardening.
Most nonferrous metals can be annealed but never
tempered, normalized, or case hardened. Successful
heat-treating requires close control over all factors
affecting the heating and cooling of metals. Such control
is possible only when the proper equipment is available,
and the equipment is selected to fit the particular job.
Annealing is used to reduce residual stresses, induce
softness, alter ductility, or refine the grain structure.
Maximum softness in metal is accomplished by heating
it to a point above the critical temperature, holding at
this temperature until the grain structure has been
refined, followed by slow cooling.