Steel with an extremely low-carbon content re-
quires the highest annealing temperature. As the carbon
content increases, the annealing temperatures decrease.
Copper becomes hard and brittle when mechani-
cally worked; however, it can be made soft again by
annealing. The annealing temperature for copper is be-
tween 700°F and 900°F. Copper maybe cooled rapidly
or slowly since the cooling rate has no effect on the heat
treatment. The one drawback experienced in annealing
copper is the phenomenon called hot shortness. At
about 900°F, copper loses its tensile strength, and if not
properly supported, it could fracture.
Aluminum reacts similar to copper when heat treat-
ing. It also has the characteristic of hot shortness. A
number of aluminum alloys exist and each requires
special heat treatment to produce their best properties.
Normalizing is a type of heat treatment applicable
to ferrous metals only. It differs from annealing in that
the metal is heated to a higher temperature and then
removed from the furnace for air cooling.
The purpose of normalizing is to remove the internal
stresses induced by heat treating, welding, casting, forg-
ing, forming, or machining. Stress, if not controlled,
leads to metal failure; therefore, before hardening steel,
you should normalize it first to ensure the maximum
desired results. Usually, low-carbon steels do not re-
quire normalizing; however, if these steels are normal-
ized, no harmful effects result. Castings are usually
annealed, rather than normalized; however, some cast-
ings require the normalizing treatment. Table 2-2 shows
the approximate soaking periods for normalizing steel.
Note that the soaking time varies with the thickness of
Normalized steels are harder and stronger than an-
nealed steels. In the normalized condition, steel is much
tougher than in any other structural condition. Parts
subjected to impact and those that require maximum
toughness with resistance to external stress are usually
normalized. In normalizing, the mass of metal has an
influence on the cooling rate and on the resulting struc-
ture. Thin pieces cool faster and are harder after normal-
izing than thick ones. In annealing (furnace cooling), the
hardness of the two are about the same.
The hardening treatment for most steels consists of
heating the steel to a set temperature and then cooling it
rapidly by plunging it into oil, water, or brine. Most
steels require rapid cooling (quenching) for hardening
but a few can be air-cooled with the same results.
Hardening increases the hardness and strength of the
steel, but makes it less ductile. Generally, the harder the
steel, the more brittle it becomes. To remove some of
the brittleness, you should temper the steel after hard-
Many nonferrous metals can be hardened and their
strength increased by controlled heating and rapid cool-
ing. In this case, the process is called heat treatment,
rather than hardening.
To harden steel, you cool the metal rapidly after
thoroughly soaking it at a temperature slightly above its
upper critical point. The approximate soaking periods
for hardening steel are listed in table 2-2. The addition
of alloys to steel decreases the cooling rate required to
produce hardness. A decrease in the cooling rate is an
advantage, since it lessens the danger of cracking and
Pure iron, wrought iron, and extremely low-carbon
steels have very little hardening properties and are dif-
ficult to harden by heat treatment. Cast iron has limited
capabilities for hardening. When you cool cast iron
rapidly, it forms white iron, which is hard and brittle.
And when you cool it slowly, it forms gray iron, which
is soft but brittle under impact.
In plain carbon steel, the maximum hardness ob-
tained by heat treatment depends almost entirely on the
carbon content of the steel. As the carbon content in-
creases, the hardening ability of the steel increases;
however, this capability of hardening with an increase
in carbon content continues only to a certain point. In
practice, 0.80 percent carbon is required for maximum
hardness. When you increase the carbon content beyond
0.80 percent, there is no increase in hardness, but there
is an increase in wear resistance. This increase in wear
resistance is due to the formation of a substance called
When you alloy steel to increase its hardness, the
alloys make the carbon more effective in increasing
hardness and strength. Because of this, the carbon con-
tent required to produce maximum hardness is lower
than it is for plain carbon steels. Usually, alloy steels are
superior to carbon steels.