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Heat treatment of large carbon steel components is done to take
advantage of crystalline defects and their effects
and thus obtain certain desirable properties or conditions.
During manufacture, by varying the
rate of cooling (quenching) of the metal, grain size and grain patterns
are controlled. Grain characteristics are controlled to produce different levels
of hardness and tensile strength. Generally, the faster a metal is cooled, the
smaller the grain sizes will be. This will make the metal harder. As hardness
and tensile strength increase in heat-treated steel, toughness and ductility
decrease.
The cooling rate used in quenching
depends on the method of cooling and the size of the metal. Uniform cooling is
important to prevent distortion. Typically, steel components are quenched in oil
or water.
Welding can induce internal
stresses that will remain in the material after the welding is completed. In
stainless steels, such as type 304, the crystal lattice is face-centered cubic
(austenite). During high temperature welding, some surrounding metal may be
elevated to between 500�F and 1000�F. In this temperature region, the austenite
is transformed into a bodycentered cubic lattice structure (bainite). When the
metal has cooled, regions surrounding the weld contain some original austenite
and some newly formed bainite. A problem arises because the "packing factor" (PF
= volume of atoms/volume of unit cell) is not the same for FCC crystals as for
BCC crysta
The bainite that has been formed
occupies more space than the original austenite lattice. This elongation of the
material causes residual compressive and tensile stresses in the material.
Welding stresses can be minimized by using heat sink welding, which results in
lower metal temperatures, and by annealing.
Annealing is another common heat
treating process for carbon steel components. During annealing, the component is
heated slowly to an elevated temperature and held there for a long period of
time, then cooled. The annealing process is done to obtain the following
effects.
a. to soften the steel and improve
ductility
b. to relieve internal stresses
caused by previous processes such as heat treatment, welding, or machining
c. to refine the grain structure
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