Work hardening

Work hardening, or strain hardening, is an increase in mechanical strength due to plastic deformation. In metallic solids, permanent change of shape is usually carried out on a microscopic scale by defects called dislocations which are created by stress and rearrange the material by moving through it. At low temperature, these defects do not anneal out of the material (within a sufficient amount of time, since the relaxation of these defects is a very thermodynamically spontaneous albeit slow process), but build up as the material is worked, interfering with one another's motion; strength is increased thereby, and ductility decreased by considerable amount.

Any material with a reasonably high melting point can be strengthened in this fashion. It is often exploited to harden alloys that are not amenable to heat treatment, including low-carbon steel. Conversely, since the low melting point of indium makes it immune to work hardening at room temperature, it can be used as a gasket material in high-vacuum systems.

Often, work hardening is carried out by the same process that shapes the metal into its final form, including cold rolling (contrast hot rolling) and cold drawing. Techniques have also been designed to maintain the general shape of the workpiece during work hardening, including shot peening and constant channel angular pressing. A material's work hardenability can be predicted by analyzing a stress-strain curve, or studied in context by performing a hardness test before and after the proposed cold work process.

Cold forming is a type of cold work that involves forging operations, such as extrusion, drawing or coining, that are performed at low temperatures. Cold work 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 work 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. However, there is a simultaneous decrease in the ductility. The effects of cold working may be removed by annealing the material at high temperatures where recovery and recrystallization reduce the dislocation density.