Techniques for Heating and Cooling Metal
Before modern metal working techniques were invented,
blacksmiths used heat to make metal workable. Once the metal was formed into
the desired shape, the heated metal was quickly cooled. Quick cooling made the
metal harder and less brittle.
What Is Heat Treatment?
Heat treatment is the process of heating and cooling metals to
change their microstructure and to bring out the physical and mechanical
characteristics that make metals more desirable.
The temperature metals are heated to
and the rate of cooling after heat treatment can significantly change metal's
properties.
The most common reasons metals undergo heat treatment are to
improve their strength, hardness, toughness, ductility and corrosion
resistance. Get a better idea of the process with the examples of common heat
treatments that follow.
Understanding Annealing
Annealing is a form of heat treatment that brings a
metal closer to its equilibrium state. Annealing softens metal making it
more workable and providing for greater ductility. In this
process, the metal is heated above its upper critical temperature to change its
microstructure. Afterward, the metal is slow-cooled.
What Is Quench
Hardening?
Less expensive than annealing, quenching is a heat treatment method that quickly returns
metal to room temperature after it is heated above its upper critical
temperature. The quenching process stops the cooling process from altering
the metal's microstructure.
Quenching, which can be done with
water, oil and other media, hardens steel at the same temperature that full
annealing does.
The Precipitation
Hardening Process
Precipitation hardening is also known as age hardening. It is a heat treatment process
that creates uniformity in a metal's grain structure.
This process makes the material
stronger.
The process involves heating a solution treatment to high
temperatures after a fast cooling process. Precipitation hardening is
usually executed in an inert atmosphere at temperatures ranging from 900 to
1150°F. It can take anywhere from an hour to four hours to carry out this
process. The length of time typically depends on the thickness of the metal and
similar factors.
Why Are Metals Tempered?
Commonly used in steelmaking today, tempering is a heat treatment used to
improve hardness and toughness in steel as well as to reduce brittleness. The
tempering process creates a more ductile and stable structure. The aim of
tempering is to achieve the best combination of mechanical properties in
metals.
What Is Stress Relieving?
Stress relieving is a heat treatment process that decreases
stress in metals after they have been quenched, casted, normalized, etc. Stress
is relieved by heating metal to a temperature lower than that required for
transformation. After this process, the metal is then slow cooled.
The Benefits
of Cryogenic Treatments
When a metal part is cryogenically treated, it is slowly
cooled with liquid nitrogen. The slow cooling process helps prevent
thermal stress of the metal.
Next, the metal part is maintained at
a temperature of roughly −190 °C for about a day.
When it is later heat tempered, the metal part undergoes an
increase of temperature up to approximately +149 °C. This
heat tempering helps to lower the amount of brittleness that may be
caused when martensite forms during cryogenic treatment.
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