1.
INTRODUCTION
Heat
treatment is any one of a number of controlled heating and cooling operations
used to bring about a desired change in the physical properties of a metal. Its
purpose is to improve the structural and physical properties for some
particular use or for future work of the metal. There are five basic heat
treating processes: hardening, case hardening, annealing, normalizing, and
tempering. Although each of these processes brings about different results in
metal, all of them involve three basic steps: heating, soaking, and cooling.
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.
2. 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 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
2.1 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.
2.2 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. 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 pre heat, 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.
Nonferrous metals are seldom preheated, because they usually do not require it,
and pre heating can cause an increase in the grain size in these metals.
2.3 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. Brine or water is used for metals that require a rapid cooling
rate, and oil mixtures are more suitable for metals that need a slower rate of
cooling. Generally, carbon steels are water-hardened and alloy steels are
oil-hardened. Non-ferrous metals are normally quenched in water.
HEAT COLORS FOR STEEL
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.
Table 2.1—Heat Colors for Steel
Table 2.2—Approximate Soaking
Periods for Hardening, Annealing, and Normalizing Steel
3. 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.
3.1 ANNEALING
In general, annealing is the
opposite of hardening, anneal metals to relieve internal stresses, soften them,
make them more ductile, and refine their grain structures. Metals are annealed
to relieve internal stresses, soften them, make them more ductile, and refine
their grain structures. Metal is annealed by heating it to a prescribed
temperature, holding it at that temperature for the required time, and then cooling
it back to room temperature. The rate at which metal is cooled from the
annealing temperature varies greatly. Steel must be cooled very slowly to
produce maximum softness, This can be done by burying the hot part in sand,
ashes, or some other substance that does not conduct heat readily (packing), or
by shutting off the furnace and allowing the furnace and part to cool together
(furnace cooling).
3.2 NORMALIZING
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, forging, forming, or machining.
Stress, if not controlled, leads to metal failure; therefore, before hardening
steel, we should normalize it first to ensure the maximum desired results.
Usually, low-carbon steels do not re-quire normalizing; however, if these
steels are normalized, no harmful effects result. Castings are usually annealed,
rather than normalized; however, some castings 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 the metal. Normalized
steels are harder and stronger than annealed 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 structure. Thin pieces
cool faster and are harder after normalizing than thick ones. In annealing
(furnace cooling), the hardness of the two are about the same. 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.
3.3 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.
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 warping.
3.4 TEMPERING
After the hardening treatment is
applied, steel is often harder than needed and is too brittle for most
practical uses. Also, severe internal stresses are set up during the rapid cooling
from the hardening temperature. To relieve the internal stresses and reduce
brittleness, you should temper the steel after it is hardened. Tempering
consists of heating the steel to a specific temperature (below its hardening
temperature), holding it at that temperature for the required length of time,
and then cooling it, usually instill air. The resultant strength, hardness, and
ductility depend on the temperature to which the steel is heated during the
tempering process. The purpose of tempering is to reduce the brittleness
imparted by hardening and to produce definite physical properties within the
steel. Tempering always follows, never precedes, the hardening operation.
Besides reducing brittleness, tempering softens the steel. That is
un-avoidable, and the amount of hardness that is lost depends on the
temperature that the steel is heated to during the tempering process. That is
true of all steels except high-speed steel.
Tempering increases the hardness of high-speed steel.
