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Strength
/ Mechanics
of Materials Table of Content
Heat treatment of
large carbon steel components is done to take advantage of
crystalline defects nd
their effects and thus aobtain 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
500F
and 1000F.
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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