WORKING OF METALS
Properties of Metals
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.
to soften the steel and improve ductility
to relieve internal stresses caused by previous processes such as heat treatment,
welding, or machining
to refine the grain structure
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.