Machinery Repairman

slowly from the tempering temperature by cooling in
still air.
Tempering is usually done before the material has
completely cooled from the hardening process. The
holding time at temperature varies according to the
thicknessof the material, but the minimum time is about
1 hour. If the part is more than an inch thick, the holding
time should be increased by about 1 hour for each
additional inch of thickness.
ANNEALING
ANNEALING is a term used to describe any heattreatment
process that is used for the primary purpose
of softening the metal. Two types of annealing
processes are commonly used. FULL ANNEALING is
done to soften the metal and make it more ductile, and
to relieve internal stresses caused by previous treatment
such as casting, cold working, or hot working. The
operation known as PROCESS ANNEALING or
STRESS RELIEF ANNEALING is done to soften the
metal somewhat, although not as much as by full
annealing, and to relieve internal stresses.
In general, full annealing requires higher
temperatures, longer soaking time, and slower cooling
than process annealing. In the full annealing of steels,
the steel is heated to a temperature that is 25° to 50°F
above the upper transformation point. In the process
annealing of steels, lower temperatures are generally
used. The rate of cooling used for annealing varies
greatly, depending upon the metal being annealed and
the degree of softening required.
NORMALIZING
The form of heat treatment known as
NORMALIZING is used only for ferrous metals.
Normalizing is sometimes used as a preliminary step
before full annealing. The chief purposes of
normalizing are (1) to relieve internal stresses caused
by forging, bending, machining, or other working, or by
uneven cooling; and (2) to give a uniform predictable
grain structure. Steel that has been normalized is soft
and ductile enough for many purposes, but it is harder
than steel that has been fully annealed. Normalizing is
sometimes followed by tempering, particularly in the
case of certain steels that tend to become brittle when
normalized.
15-22
Figure 15-19.—Microscopic structure called spherodite magnified
1,000 times.
In normalizing, the steel is heated to a temperature
above the upper transformation point and is cooled in
still air.
SPHEROIDIZING
SPHEROIDIZING is a term used to describe any
heating and cooling process that produces a rounded or
globular form of iron carbide in the steel. This globular
or spheroidal form of iron carbide is developed when
fine pearlite is heated to a temperature just below the
lower transformation point of the steel, held at this
temperature for a long time, and then cooled very
slowly. Temperatures, holding time, cooling rates, and
other details of the process vary, depending upon the
carbon content of the steel and the extent of
spheroidizing that is required. In general, the object of
spheroidizing is to improve machinability.
Spheroidizing is sometimes regarded as being a special
annealing process. The typical globular form of
spheroidized steel is illustrated in cross section in figure
15-19.
AUSTEMPERING
AUSTEMPERING is a hardening treatment of
metals that involves quenching the metal in a bath of
molten salt maintained above the start of the martensite
temperature and holding it until transformation is
complete. The product formed is called bainite; a tough,
hard structure.
There are two distinct advantages of austempering—(1)
the high degree of freedom it provides