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Work hardening(cold working )
Published:2013-05-02 04:39:45    Text Size:【BIG】【MEDIUM】【SMALL
Summary:Work hardening, strain hardening, or cold work is the strengthening of a material by, macroscopically speaking, plastic deformation (which has the nano-scopic effect of increasing the material's dislocation density). As the material becomes increasingly saturated with new dislocations, more dislocations are prevented from nucleating (a resistance to dislocation-formation develops). This resistance to dislocation-formation manifests itself as a resistance to plastic deformation; hence, the observed strengthening.

In metallic crystals, irreversible deformation is usually carried out on a microscopic scale by defects called dislocations, which are created by fluctuations in local stress fields within the material culminating in a lattice rearrangement as the dislocations propagate through the lattice. At normal temperatures the dislocations are not annihilated by annealing. Instead, the dislocations accumulate, interact with one another, and serve aspinning points or obstacles that significantly impede their motion. This leads to an increase in the yield strength of the material and a subsequent decrease in ductility.

Any material with a reasonably high melting point such as metals and alloys can be strengthened in this fashion. Alloys not amenable to heat treatment, including low-carbonsteel, are often work-hardened. Some materials cannot be work-hardened at normal ambient temperatures; for example indium, which has a low melting point. This makes indium suitable for manufacturing gaskets, which deform to fill gaps, for high-vacuum use.

Work hardening is often produced by the same process that shapes the metal into its final form, including cold rolling (contrast hot rolling) and cold drawing. Techniques have been designed to maintain the general shape of the workpiece during work hardening, includingshot peening and equal channel angular extrusion. A material's work hardenability can be predicted by analyzing a stress-strain curve, or studied in context by performing hardnesstests before and after a process.

Cold forming is a type of cold working that involves forging operations, such as extrusion,drawing or coining, performed at low temperatures. Cold working may also refer to the process through which a material is given this quality. Such deformation increases the concentration of dislocations which may subsequently form low-angle grain boundaries surrounding sub-grains. Cold working generally results in a higher yield strength as a result of the increased number of dislocations and the Hall-Petch effect of the sub-grains, and a decrease in ductility. The effects of cold working may be reversed by annealing the material at high temperatures where recovery and recrystallization reduce the dislocation density.

History

Copper was first metal in common use for tools and containers since it is one of the few metals available in non-oxidized form, not requiring the smelting of an ore. Copper is easily softened by heating and cooling (it does not harden by quenching, as in cool water). In this annealed state it may then be hammered, stretched and otherwise formed, progressing toward the desired final shape, but becoming harder and less ductile as work progresses. If work continues beyond a certain hardness the metal will tend to fracture when worked and so it may be re-annealed periodically as the shape progresses. Annealing is stopped when the workpiece is near its final desired shape, and so the final product will have a desired stiffness and hardness. The technique of Repoussé exploits these properties of copper, enabling the construction of durable jewelry articles and sculptures (including the Statue of Liberty).

For metal objects designed to flex, such as springs, specialized alloys are usually employed in order to avoid work hardening (a result of plastic deformation) and metal fatigue, with specific heat treatments required to obtain the necessary characteristics.

Devices made from aluminum and its alloys, such as aircraft, must be carefully designed to minimize or accommodate flexure, which can lead to work hardening and in turn stress cracking, possibly causing catastrophic failure. For this reason modern aluminum aircraft will have an imposed working lifetime (dependent upon the type of loads encountered), after which the aircraft must be retired.

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