always follows, rather than precedes, the hardening
in hardness, but it is much softer than martensite. If the
process. Tempering is occasionally done after materials
purpose of the hardening is to develop something less
than maximum hardness, the first part of the cooling (to
have been normalized, but its major use is after
below 1,000F) must still be accomplished within 1
hardening.
second; then the remainder of the cooling must be done
In some alloy steels tempering may increase
at a somewhat slower rate.
hardness when tempered to certain temperature
The rate of quenching alloy steels depends upon the
ranges. In most other materials, however, tempering
composition of the material In general, a slower quench
causes an unavoidable loss of some hardness. The
is used for hardening alloy steels than is used for
amount of hardness removed by tempering depends
hardening plain-carbon steels.
upon the tempering temperature; the higher the
temperature, the softer the material will be.
Various quenching mediums are used to produce
the desired end results. The more common quenching
Tempering is always done at temperatures below
mediums are oil, water, and brine. When water or brine
the lower transformation point. In this respect,
is used, the temperature of the bath must not rise above
tempering differs from hardening, annealing, and
80F. Generally, 70F is the best temperature for these
normalizing, which all involve heating the material to
quenching mediums. Above 80F, water loses thermal
temperatures above the upper transformation point.
The temperatures used for tempering are selected
The plain-carbon steels are generally quenched in
on the basis of the properties required in the final
water or brine; brine is preferred because it has less
product. For example, permanent magnets are tempered
tendency to cause steam pockets to form. Steam pockets
at 121F because they must retain considerable strength
may cause soft spots or uneven stresses. These steels
and hardness. Case-hardened objects are also tempered
require a very active agitation or shaking action when
at relatively low temperatures (212 to 400F) because
quenched. The agitation removes the steam bubbles
the surface of such objects must remain hard. Many
from the surface of the metal. When the quenched
cutting tools are tempered at 430F or below so they
material reaches about 200 to 250F, or a temperature
will retain hardness. Battering tools must have great
low enough to dry the material thoroughly without too
impact resistance and must be able to cut or penetrate
much sizzle, it can be removed from the water and
metal; therefore, battering tools are tempered
tempered immediately.
between 450 and 600F even though the higher
The alloy steels, with the exception of the
temperatures mean some sacrifice of hardness to
high-alloy, air-hardening steels, are generally oil
quenched. The oil bath should be maintained at a
between 600 and 900F because the property of
temperature of 120 to 150F. (This is just below the M f
line of most low-alloy steels.) At this temperature the
property of hardness. Tools made of high-speed steels
heat-treating oils have a lower viscosity, and therefore,
are tempered at 1,050 to 1,100F. Note, however,
a better and more uniform cooling effect. The warm oil
that with high-speed tools the high tempering
also results in a more moderate cooing rate, which
temperature increases, rather than decreases,
lessens the chance of residual stresses and cracking.
hardness. This increase in hardness occurs because
Although steels must be heated and soaked so the
high-speed steels retain austenite during quenching;
structure becomes completely austenitic, you should
when the hardened steel is tempered, the austenite
take care to see that they are not overheated and that
changes to martensite.
they are not held too long at temperature. Using too high
Since tempering uses temperatures below the
a temperature or too long a soaking period allows a
lower transformation point, the rate of cooling
coarse grain structure to develop.
generally has no effect upon the structure of the
material. However, some nickel-chromium steels and
a few other special steels become brittle if they are
TEMPERING
heated to the tempering temperature and then allowed
to cool slowly. These steels, which are often called
After hardening, most alloys are tempered to reduce
temper brittle or blue brittle steels, must be quenched
rapidly from the tempering temperature to prevent
stresses developed during hardening. Tempering
15-21