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About Heat Treating
Heat treating is a general name for various processes
that are used to harden or soften metals through a series of heating
and cooling until the desired properties are reached. Common metals that
can be used for heat treating services are aluminum, titanium, magnesium
and more, but by far the most common metal that undergoes this process
is steel. Metal heat treating is a $20 billion a year industry with high
operating costs, due to the fact that it is a very energy intensive process.
In order to change the hardness of a metal, the internal atomic structure
is actually changed during the heating and cooling. Metals with high
carbon content have the best success rate with heat treating, because
the more carbon there is, the harder the final product will be.
The most common method of heat treating steel begins by normalizing the
metal. Normalizing is an important heating and cooling step, because
it erases any effects from previous heat treatments. After this, the
steel is heated to its critical temperature at which point it becomes
red hot and nonmagnetic, an iron and carbon mixture called austenite.
The next step depends on the state of the desired final product. For
a soft steel called pearlite, the austenite must be cooled slowly through
annealing. If, on the other hand, hardness is needed, the steel is cooled
very quickly through a process called quenching, which involves rapid
cooling in a substance such as oil, water or a salt bath. The resulting
product is called martensite, and it is very hard but brittle. To ease
stress, a final process, tempering, can be used. The metal is reheated
to become less brittle, and in some cases, more flexible.
The annealing process used in heat treating services is opposite for
nonferrous metals such as aluminum, copper, silver and brass. The metal
starts out by being heated, but rather than being cooled slowly, a quenchant
is used. Metals without carbon can only be hardened through mechanical
working, such as hammering, bending and rolling or by the addition of
more durable alloying metals. There are several other methods of heat
treating. One of these is flame hardening, which is used in cases when
only a small portion of a metal’s surface needs to be hardened.
This is done by heating the area with a very hot acetylene torch and
then quenching with water. Another is case hardening, which hardens the
surface of a metal by absorbing carbon to create a hard “case.” In
this process, the metal is placed in a container heated with charcoal,
left for a while and then quenched in a salt bath.
There are some basic precautions to keep in mind when heat treating any
metal. Because of intense heat, be sure to wear protective clothing,
not only during the heat treating of the metal, but during quenching
as well, because of dangers from scalding steam and splattered hot water.
Also, quenching with oil can be a potential fire hazard, so it is important
to remove any flammable materials from the vicinity and to keep an extinguisher
nearby. Heat treating services are used to treat metals for more industries
than it is possible to name. Cutlery blades for knives and even swords
are hardened through heat treating, as are railroad components and musical
instruments. Molds for die
casting can also be heat treated to ensure
hardness. Some of the many affected industries include: automotive, hardware,
heavy equipment, construction, fire arms, military, tool and die, stamping,
trucks and trailers, aerospace, machine tools, fasteners and chains.
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Featured
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Types of Heat Treating
- is used to soften or harden aluminum materials to a desired level of hardness.
- is
the process of softening carbon and alloy steels by slowly cooling the
metals after they have been heated.
-
is a method of hardening steel by quenching into a constant temperature
medium, such as a salt bath. The resulting product is bainite.
-
is a heat treatment that involves changing the surface composition of
an iron-based alloy for additional hardness.
- is
a hardening method that involves carbon and nitrogen absorption on the
surface of steel by heating it in contact with cyanide salt and then
quenching.
-
is used when only a small part of a metal surface needs to be hardened.
This method involves heating the surface with an acetylene torch and
then quenching.
- is used to harden or soften metal materials by heating or cooling them until the desired level of hardness is reached.
- is used to soften or harden stainless steel materials to a desired level of hardness.
- uses induction heaters to harden or soften metal materials.
-
is used to harden steel by quenching the hot metal in a heat extracting,
constant temperature substance, such as salt, and waiting until the
temperature is uniform. The metal is then air cooled to form martensite,
and tempered as needed.
- is used to soften or harden steel materials to a desired level of hardness.
- involves
the reheating of ferrous metals to increase hardness and reduce brittleness.
-
involves heat treating within a vacuum furnace for metallurgical reactions
and then quenching, usually with gas.
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