13-08-2012, 01:29 PM
BASIC HEAT TREATMENT
heat treat ment2.pdf (Size: 206.86 KB / Downloads: 276)
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.
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.
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.
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.
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.
Nonferrous Metal
Copper becomes hard and brittle when mechanically
worked; however, it can be made soft again by
annealing. The annealing temperature for copper is between
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.
HARDENING
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 hardening.
Many nonferrous metals can be hardened and their
strength increased by controlled heating and rapid cooling.
In this case, the process is called heat treatment,
rather than hardening.
heat treat ment2.pdf (Size: 206.86 KB / Downloads: 276)
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.
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.
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.
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.
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.
Nonferrous Metal
Copper becomes hard and brittle when mechanically
worked; however, it can be made soft again by
annealing. The annealing temperature for copper is between
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.
HARDENING
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 hardening.
Many nonferrous metals can be hardened and their
strength increased by controlled heating and rapid cooling.
In this case, the process is called heat treatment,
rather than hardening.