In the previous pages on the subjects of alloying and the binary
phase diagram, the microstructures of alloys that were allowed
to solidify by slow cooling were considered. It should also be
known, however, that it is possible to modify the microstructure
of an alloy by subjecting it to various thermal treatments. Heat-treating
is a term used to describe all of the controlled heating and cooling
operations performed on a material in the solid state for the
purpose of altering its microstructure and/or properties. The
focus of this discussion will be on metals but is should be noted
that heat-treatment is also used on ceramics and composites to
modify their properties.
The major objectives of the different kinds of thermal treatments
- Soften the material for improved workability.
- Increase the strength or hardness of the material.
- Increase the toughness or resistance to fracture of the material.
- Stabilize mechanical or physical properties against changes
that might occur during exposure to service environments.
- Insure part dimensional stability.
- Relieve undesirable residual stresses induced during part
Different metals respond to treatment at different temperatures.
Each metal has a specific chemical composition, so changes in
physical and structural properties take place at different, critical
temperatures. Even small percentages of elements in the metal
composition, such as carbon, will greatly determine the temperature,
time, method and rate of cooling that needs to be used in the
heat treating process. Depending on the thermal treatment used,
the atomic structure and/or microstructure of a material may change
due to movement of dislocations, an increase or decrease in solubility
of atoms, an increase in grain size, the formation of new grains
of the same or different phase, a change in the crystal structure,
and others mechanisms.
Since there are so many ways in which metals are heat treated,
it is not practical to discuss them all. But, as an example, let’s
look at how heat treatment is used to strengthen a copper aluminum
In designing alloys for strength, an approach often taken is to
develop an alloy with a structure that consists of particles (which
impede dislocation movement) dispersed in a ductile matrix. Such
a dispersion can be obtained by choosing an alloy that is a single
phase at elevated temperature but on cooling will precipitate
another phase in the matrix. A thermal process is then developed
to produce the desired distribution of precipitate in the matrix.
When the alloy is strengthened by this thermal treatment, it is
called precipitation strengthening or hardening.
Precipitation hardening consists of three main steps: solution
treatment, quenching, and aging. Solution treatment involves heating
the alloy to a temperature that allows the alloying atoms (called
the solute) to dissolve into the solution. This results in a homogeneous
solid solution of one phase. Quenching rapidly cools the solution
and freezes the atoms in solution. In more technical terms, the
quenching cools the material so fast that the atoms of the alloying
elements do not have time to diffuse out of the solution. In the
as-quenched condition, the solute is supersaturated meaning that
the lattice is overly stressed by the alloying atoms. Aging is
the process where the solute particles diffuse out of solution
and into clusters that distort and strengthen the material.
The precipitation hardening process for a copper-aluminum alloy
is shown graphically in the image below. On the right is phase
diagram, which is a very useful tool for understanding and controlling
polyphase structures. The phase diagram is simply a map showing
the structure of phases present as the temperature and overall
composition of the alloy are varied. The images on the right in
the image show the resulting microstructure at each step in the
Common Heat Treating Processes
A few of the more common terms used in heat treating are introduced
below. It should be noted that not all of the term are applicable
to all alloys.
Age Hardening is a relatively low-temperature
heat treatment process that strengthens a material by causing
the precipitation of components or phases of alloy from a super-saturated
solid solution condition.
Annealing is a softening process in which metals
are heated and then allowed to cool slowly. The purpose of annealing
is to soften the material for improve machinability, formability,
and sometimes to control magnetic properties.
Normallizing is much like annealing, but the
cooling process is much faster. This results in increased strength
but less ductility in the metal. Its purpose is to refine grain
structure, produce more uniform mechanical properties, and sometimes
to relieve internal and surface stresses.
Precipitation Heat Treatment is the three step
process of solution treating, quenching, and age hardening to
increase the strength or hardness of an alloy.
Solution Heat Treatment involves heating the
material to a temperature that puts all the elements in solid
solution and then cooling very rapidly to freeze the atoms in
Stress Relieving is a low temperature heat treat
process that is used to reduce the level of residual stresses
in a material.
Tempering involves gently heating a hardened
metal and allowing it to cool slowly will produce a metal that
is still hard but also less brittle. This process is known as
Quenching is the rapid cooling of a hot material.
The medium used to quench the material can vary from forced air,
oil, water and others. Many steels are hardened by heating and
quenching. Quenching results in a metal that is very hard but
More information on heat treatment can be found in the material
(ie aluminum, steel, titanium, etc.) sections