Work hardening is an increase in hardness of a material, especially metal, as it is deformed. This happens both during deformation due to overt deflection (bending) and that which is less obvious, such as machining. The readiest example of work hardening is in the form of a paper clip - when attempting to straighten a paper clip or a similar piece of wire one will note that the bent portion is more resistant to straightening than the wire next to it is resistant to being bent, resulting in a wave around the original position of the bend.

All types of cold working of metal cause work hardening, which while it typically makes metal stronger (or at least more rigid) also makes it more brittle. The best example of a functional use of work hardening is the forging of tools or other metal items which results in a much higher level of strength than a simple cast part. It is also a factor in bent sheet metal structures, which have structures called style lines stamped into them in order to add rigidity (though the work hardening is only part of the picture, and the monocoque effect is the other.)

Work hardening is removed by the application of heat. Work hardening can be removed from steel in particular by heating it to a cherry red color. Removing work hardening is a prerequisite for repairing deformation damage in steel structures.

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