Plastic deformation which is
carried out in a temperature region and over a time interval such that the
strain hardening is not relieved is called
cold work. Considerable
knowledge on the structure of the cold-worked state has been obtained. In the
early stages of plastic deformation, slip is essentially on primary glide planes
and the dislocations form coplanar arrays. As deformation proceeds, cross slip
takes place. The cold-worked structure forms high dislocation density regions
that soon develop into networks. The grain size decreases with strain at low
deformation but soon reaches a fixed size. Cold working will decrease ductility.
Hot working
refers to the process where metals are deformed above their recrystallization
temperature and strain hardening does not occur.
Hot working is usually performed at elevated
temperatures. Lead, however, is hot-worked at room temperature because of its
low melting temperature. At the other
extreme, molybdenum is cold-worked when deformed even at red heat
because of its high recrystallization temperature.
The resistance of metals to plastic
deformation generally falls with temperature. For this reason,
larger massive sections are always worked hot by
forging, rolling, or extrusion. Metals display
distinctly viscous characteristics at sufficiently
high temperatures, and their resistance to flow
increases at high forming rates. This occurs not
only because it is a characteristic of viscous
substances, but because the rate of
recrystallization may not be fast enough.
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