As Steelworkers, we are interested in the heat treatment
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-treating
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.
HEAT-TREATING THEORY
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 methods
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 monoxide,
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 l-Heating the metal slowly to ensure a
uniform temperature
Stage 2-Soaking (holding) the metal at a given
temperature for a given time and cooling the
metal to room temperature
Stage 3-Cooling the metal to room temperature
HEATING STAGE
The primary objective in the heating stage is to
maintain uniform temperatures. If uneven heating occurs,
one section of a part can expand faster than another
and result in distortion or cracking. Uniform temperatures
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 heating
rate is kept slow.
SOAKING STAGE
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
temperature is called the SOAKING PERIOD.
The is used for metals that require a rapid cooling rate, and
soaking period depends on the chemical analysis of the oil
mixtures are more suitable for metals that need a
metal and the mass of the part. When steel parts are slower
rate of cooling. Generally, carbon steels are
uneven in cross section, the soaking period is deter- water-
hardened and alloy steels are oil-hardened. Nonmined
by the largest section.
Ferrous metals are normally quenched in water.
During the soaking stage, the temperature of the
metal is rarely brought from room temperature to the
final temperature in one operation; instead, the steel is
slowly heated to a temperature just below the point at
which the change takes place and then it is held at that
temperature until the heat is equalized throughout the
metal. We call this process PREHEATING. Following
preheat, the metal is quickly heated to the final required
temperature.
When apart has an intricate design, it may have to
be preheated at more than one temperature to prevent
cracking and excessive warping. For example, assume
an intricate part needs to be heated to 1500°F for hardening.
This part could be slowly heated to 600°F, soaked
at this temperature, then heated slowly to 1200°F, and
then soaked at that temperature. Following the final
preheat, the part should then be heated quickly to the
hardening temperature of 1500°F.
NOTE: Nonferrous metals are seldom preheated,
because they usually do not require it, and preheating
can cause an increase in the grain size in these metals.
COOLING STAGE
After a metal has been soaked, it must be returned
to room temperature to complete the heat-treating process.
To cool the metal, you can place it in direct contact
with a COOLING MEDIUM composed of a gas, liquid,
solid, or combination of these. The rate at which the
metal is cooled depends on the metal and the properties
desired. The rate of cooling depends on the medium;
therefore, the choice of a cooling medium has an important
influence on the properties desired.
Quenching is the procedure used for cooling metal
rapidly in oil, water, brine, or some other medium.
Because most metals are cooled rapidly during the hardening
process, quenching is usually associated with
hardening; however, quenching does not always result
in an increase in hardness; for example, to anneal copper,
you usually quench it in water. Other metals, such
as air-hardened steels, are cooled at a relatively slow rate
for hardening.
Some metals crack easily or warp during quenching,
and others suffer no ill effects; therefore, the quenching
medium must be chosen to fit the metal.
HEAT COLORS FOR STEEL
You are probably familiar with the term red-hot as
applied to steel. Actually, steel takes on several colors
and shades from the time it turns a dull red until it
reaches a white heat. These colors and the corresponding
temperatures are listed in table 2-1.
During hardening, normalizing, and annealing,
steel is heated to various temperatures that produce
color changes. By observing these changes, you can
determine the temperature of the steel. As an example,
assume that you must harden a steel part at 1500°F. Heat
the part slowly and evenly while watching it closely for
any change in color. Once the steel begins to turn red,
carefully note each change in shade. Continue the even
heating until the steel is bright red; then quench the part.
The success of a heat-treating operation depends
largely on your judgment and the accuracy with which
you identify each color with its corresponding temperature.
From a study of table 2-1, you can see that close
observation is necessary. You must be able to tell the
difference between faint red and blood red and between
dark cherry and medium cherry. To add to the difficulty,
your conception of medium cherry may differ from that
of the person who prepared the table. For an actual
heat-treating operation, you should get a chart showing
the actual colors of steel at various temperatures.
TYPES OF HEAT TREATMENT
Four basic types of heat treatment are used today.
They are annealing, normalizing, hardening, and tempering.
The techniques used in each process and how
they relate to Steelworkers are given in the following
paragraphs.
ANNEALING
In general, annealing is the opposite of hardening,
You anneal metals to relieve internal stresses, soften
them, make them more ductile, and refine their grain
structures. Annealing consists of heating a metal to a
specific temperature, holding it at that temperature for
a set length of time, and then cooling the metal to room
temperature. The cooling method depends on the
metal and the properties desired. Some metals are Ferrous Metal
furnace-cooled, and others are cooled by burying them
To produce the maximum softness in steel, you heat
in ashes, lime, or other insulating materials.
the metal to its proper temperature, soak it, and then let
Welding produces areas that have molten metal next it cool
very slowly. The cooling is done by burying the
to other areas that are at room temperature.
As the weld hot part in an insulating material or by shutting
off the cools, internal stresses occur along with hard spots
and furnace and allowing the furnace and the part to cool
brittleness. Welding can actually weaken the metal.
