Strain Hardening

Strain Hardening or Work hardening is the process of increasing the strength of a material using plastic deformation.  The metal to be strain-hardened is stretched beyond its yield to a point just before it will fracture.  At this point, the metal becomes stronger and will resist deformation.  More stress will be required to deform the material.

For instance, low carbon steel is stretched beyond the yield point and aged (left for a few days).  The material will have a higher yield stress.  Steel can be hardened by rolling it between a pair of rollers.  The dimension of the steel sheet is reduced and the metal is hardened. 

Strain hardening occurs as the dislocations cause the atoms to move to other locations and are anchored there.  Strain hardening is a cold working process.  It is done below the recrystallization temperature of the material. 

A simple example would be to bend a piece of wire or a paper clip in opposite directions for a few times.  Fatigue sets in the material.  The point of the bending becomes hardened.  Further bending will occur at a point above or below the earlier point.