results for hardening cylindrical parts of small or me-
dium diameters. The part is mounted between lathe
centers and turned at a high rate of speed pasta station-
ary torch. Enough torches are placed side by side to heat
the entire part. The part can be quenched by water
flowing from the torch tips or in a separate operation.
When you perform heating and quenching as sepa-
rate operations, the tips are water-cooled internally, but
no water sprays onto the surface of the part.
In flame hardening, you should follow the same
safety precautions that apply to welding (see chapter 3).
In particular, guard against holding the flame too close
to the surface and overheating the metal. In judging the
temperature of the metal, remember that the flame
makes the metal appear colder than it actually is.
After the hardening treatment is applied, steel is
often harder than needed and is too brittle for most
practical uses. Also, severe internal stresses are set up
during the rapid cooling from the hardening tempera-
ture. To relieve the internal stresses and reduce brittle-
ness, you should temper the steel after it is hardened.
Tempering consists of heating the steel to a specific
temperature (below its hardening temperature), holding
it at that temperature for the required length of time, and
then cooling it, usually instill air. The resultant strength,
hardness, and ductility depend on the temperature to
which the steel is heated during the tempering process.
The purpose of tempering is to reduce the brittleness
imparted by hardening and to produce definite physical
properties within the steel. Tempering always follows,
never precedes, the hardening operation. Besides reduc-
ing brittleness, tempering softens the steel. That is un-
avoidable, and the amount of hardness that is lost
depends on the temperature that the steel is heated to
during the tempering process. That is true of all steels
except high-speed steel. Tempering increases the hard-
ness of high-speed steel.
Tempering is always conducted at temperatures be-
low the low-critical point of the steel. In this respect,
tempering differs from annealing, normalizing, and
hardening in which the temperatures are above the upper
critical point. When hardened steel is reheated, temper-
ing begins at 212°F and continues as the temperature
increases toward the low-critical point. By selecting a
definite tempering temperature, you can predetermine
the resulting hardness and strength. The minimum tem-
perature time for tempering should be 1 hour. If the part
is more than 1 inch thick, increase the time by 1 hour for
each additional inch of thickness.
Normally, the rate of cooling from the tempering
temperature has no effect on the steel. Steel parts are
usually cooled in still air after being removed from the
tempering furnace; however, there are a few types of
steel that must be quenched from the tempering tem-
perature to prevent brittleness. These blue brittle steels
can become brittle if heated in certain temperature
ranges and allowed to cool slowly. Some of the nickel
chromium steels are subject to this temper brittleness.
Steel may be tempered after being normalized, pro-
viding there is any hardness to temper. Annealed steel is
impossible to temper. Tempering relieves quenching
stresses and reduces hardness and brittleness. Actually,
the tensile strength of a hardened steel may increase as
the steel is tempered up to a temperature of about 450°F.
Above this temperature it starts to decrease. Tempering
increases softness, ductility, malleability, and impact
resistance. Again, high-speed steel is an exception to the
rule. High-speed steel increases in hardness on temper-
ing, provided it is tempered at a high temperature (about
1550°F). Remember, all steel should be removed from
the quenching bath and tempered before it is complete] y
cold. Failure to temper correctly results in a quick failure
of the hardened part.
Permanent steel magnets are made of special alloys
and are heat-treated by hardening and tempering. Hard-
ness and stability are the most important properties in
permanent magnets. Magnets are tempered at the mini-
mum tempering temperature of 212°F by placing them
in boiling water for 2 to 4 hours. Because of this low-
tempering temperature, magnets are very hard.
Case-hardened parts should not be tempered at too
high a temperature or they may loose some of their
hardness. Usually, a temperature range from 212°F to
400°F is high enough to relieve quenching stresses.
Some metals require no tempering. The design of the
part helps determine the tempering temperature.
Color tempering is based on the oxide colors that
appear on the surface of steel, as it is heated. When you
slowly heat a piece of polished hardened steel, you can
see the surface turn various colors as the temperature
changes. These colors indicate structural changes are
taking place within the metal. Once the proper color
appears, the part is rapidly quenched to prevent further
structural change. In color tempering, the surface of the
steel must be smooth and free of oil. The part may be
heated by a torch, in a furnace, over a hot plate, or by