BASIC HEAT TREATMENT
As Steelworkers, we are interested in the heat treat-
ment of metals, because we have to know what effects
the heat produced by welding or cutting has on metal.
We also need to know the methods used to restore metal
to its original condition. The process of heat treating is
the method by which metals are heated and cooled in a
series of specific operations that never allow the metal
to reach the molten state. The purpose of heat treating is
to make a metal more useful by changing or restoring
its mechanical properties. Through heat treating, we can
make a metal harder, stronger, and more resistant to
impact. Also, heat treating can make a metal softer and
more ductile. The one disadvantage is that no heat-treat-
ing procedure can produce all of these characteristics in
one operation. Some properties are improved at the
expense of others; for example, hardening a metal may
make it brittle.
The various types of heat-treating processes are
similar because they all involve the heating and cooling
of metals; they differ in the heating temperatures and the
cooling rates used and the final results. The usual meth-
ods of heat-treating ferrous metals (metals with iron) are
annealing, normalizing, hardening, and tempering.
Most nonferrous metals can be annealed, but never
tempered, normalized, or case-hardened.
Successful heat treatment requires close control
over all factors affecting the heating and cooling of a
metal. This control is possible only when the proper
equipment is available. The furnace must be of the
proper size and type and controlled, so the temperatures
are kept within the prescribed limits for each operation.
Even the furnace atmosphere affects the condition of the
metal being heat-treated.
The furnace atmosphere consists of the gases that
circulate throughout the heating chamber and surround
the metal, as it is being heated. In an electric furnace,
the atmosphere is either air or a controlled mixture of
gases. In a fuel-fired furnace, the atmosphere is the
mixture of gases that comes from the combination of the
air and the gases released by the fuel during combustion.
These gases contain various proportions of carbon mon-
oxide, carbon dioxide, hydrogen, nitrogen, oxygen,
water vapor, and other various hydrocarbons. Fuel-fired
furnaces can provide three distinct atmospheres when
you vary the proportions of air and fuel. They are called
oxidizing, reducing, and neutral.
STAGES OF HEAT TREATMENT
Heat treating is accomplished in three major stages:
Stage lHeating the metal slowly to ensure a
Stage 2Soaking (holding) the metal at a given
temperature for a given time and cooling the
metal to room temperature
Stage 3Cooling the metal to room temperature
The primary objective in the heating stage is to
maintain uniform temperatures. If uneven heating oc-
curs, one section of a part can expand faster than another
and result in distortion or cracking. Uniform tempera-
tures are attained by slow heating.
The heating rate of a part depends on several factors.
One important factor is the heat conductivity of the
metal. A metal with a high-heat conductivity heats at a
faster rate than one with a low conductivity. Also, the
condition of the metal determines the rate at which it
may be heated. The heating rate for hardened tools and
parts should be slower than unstressed or untreated
metals. Finally, size and cross section figure into the
heating rate. Parts with a large cross section require
slower heating rates to allow the interior temperature to
remain close to the surface temperature that prevents
warping or cracking. Parts with uneven cross sections
experience uneven heating; however, such parts are less
apt to be cracked or excessively warped when the heat-
ing rate is kept slow.
After the metal is heated to the proper temperature,
it is held at that temperature until the desired internal
structural changes take place. This process is called
SOAKING. The length of time held at the proper