Laser Hardening |
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The advantages of laser hardening are less touch-up work and the option to process irregular, three-dimensional workpieces |
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Thanks to the low heat input,
distortion is kept to a minimum. The cost of reworking is reduced
or eliminated entirely. |
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How laser hardening works: the laser beam heats the outer layer (case) of the metal. Rapid cooling causes the layer to harden |
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Laser hardening is a surface hardening process.
It is used exclusively on ferrous materials suitable for
hardening. These include steels and cast iron with a carbon
content of more than 0.2 percent.
To harden the workpiece, the laser beam usually warms
the outer layer to just under the melting temperature (about
900 to 1400 degrees Celsius). Once the desired temperature
is reached, the laser beam starts moving. As the laser beam
moves, it continuously warms the surface in the direction of
advance. The high temperature causes the iron atoms to
change their position within the metal lattice (austenization).
As soon as the laser beam moves away, the hot layer is
cooled very rapidly by the surrounding material in a process
known as self-quenching. Rapid cooling prevents the metal
lattice from returning to its original structure, producing martensite. Martensite is a very hard metal structure. The transformation
into martensite yields greater hardness. |
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This turbocharger shaft is laser-hardened in the sections where the bearings sit |
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The laser beam hardens the outer layer, or case, of the
workpiece. The hardening depth of the outer layer is typically
from 0.1 to 1.5 millimeters. On some materials, it may be 2.5
millimeters or more. Greater hardening depth requires a larger
volume of surrounding material to ensure that heat is dissipated
quickly and the hardening zone cools rapidly enough.
Relatively low power densities are needed for hardening.
At the same time, the hardening process involves treating extensive
areas of the workpiece surface. That’s why the laser
beam is shaped so that it irradiates an area that is as large as
possible. The irradiated area is usually rectangular. Scanning
optics are also used in hardening. They are used to move a
laser beam with a round focus back and forth very rapidly,
creating a line on the workpiece with a power density that is
virtually uniform. This method makes it possible to produce
hardened tracks up to 60 millimeters wide. |
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