Figure 15-18.--Heat treatment temperature ranges.
twist drills, and other tools must be hardened so they
Available information indicates that the Mf of
can retain their cutting edges. surfaces of roller
high-carbon steels is actually below room temperature.
bearings, parallel blocks, and armor plate must be
Slight amounts of austenite are frequently retained in
hardened to prevent wear and penetration. Hardening is
quenched steels, especially in the higher carbon grades,
used to increase tensile strength as well as to increase
even when they are cooled to room temperature.
the material, most alloys are tempered after being
HEAT-TREATING FERROUS METALS
hardened.
The heat-treating processes most commonly used
Steels are hardened by being heated to a
for ferrous metals are hardening, tempering, annealing,
temperature just above the upper transformation
normalizing, spheroidizing, and case hardening.
temperature, soaked long enough to ensure a
Annealing is the only one of these processes that is also
completely austenitic structure, and then cooled
used for nonferrous metals. Some nonferrous metals
rapidly. This result is either fine-grained pearlite,
(including some aluminum alloys and some magnesium
bainite, or martensite. If maximum hardness is to be
alloys) can be hardened by a process generally referred
developed, the austenite must be transformed to
martensite. Martensite is extremely hard and strong,
hardening is discussed in the section of this chapter on
has great resistance, and has practically no machin-
ability.
treatment temperature ranges. You should refer to this
Carbon steels are usually quenched in brine or water
figure as you study the following information on
hardening, normalizing, and spheroidizing temper-
to ensure rapid cooling. The production of martensite
throughout a plain-carbon steel is extremely difficult,
atures.
requiring that the steel be cooled to below 1,000F in
less than 1 second and that the remainder of the cooling
HARDENING
be quite rapid. If more than 1 second is taken for the first
part of the cooling (to below 1,000F), some of the
Many tools and pieces of equipment must be
austenite will transform into fine pearlite. Pearlite varies
hardened before they are used. Cutting tools, chisels,
15-20