Work hardening, also called strain hardening or cold working, is the increase in strength and hardness that a metal undergoes as it is plastically deformed at room temperature. As the material is bent, drawn, stretched, or ironed, the dislocations that carry plastic flow through the crystal lattice multiply and tangle against each other and against grain boundaries. Each new dislocation makes it harder for the next one to move, so the metal becomes progressively stronger and harder — but also less ductile, because it is using up its capacity to deform.
The effect shows up directly on the stress–strain curve: past the yield point the stress required to keep deforming the material rises rather than staying flat, and that rising portion is the work-hardening region. Different alloys harden at different rates, described by a strain-hardening exponent (the "n value"). A high n value means the material strengthens quickly as it stretches, which actually helps in stretch forming because it spreads deformation out and resists localized necking; a low n value localizes strain and splits sooner.
Work hardening is central to multi-stage drawing. The first draw imposes the largest strain and hardens the metal the most, so each subsequent redraw can take a smaller reduction than the one before — the material simply has less ductility left. For example, a first draw might safely reduce a cup diameter by 40–50%, while later redraws are limited to perhaps 20–30% because the wall has work-hardened. Push past that limit and the work-hardened wall tears. The same mechanism explains why a piece of sheet bent back and forth repeatedly grows stiff and finally cracks at the bend.
Annealing is the direct remedy: heating the metal to recrystallize its grain structure relieves the accumulated work hardening and restores ductility, allowing forming to continue. So in a severe forming sequence, work hardening and annealing alternate — the metal hardens as it is formed, is softened by an intermediate anneal, and is formed again. Work hardening is not always unwanted, though; it is deliberately exploited to strengthen finished parts, such as the work-hardened walls of ironed beverage cans.
It is important not to confuse work hardening with heat-treat hardening; the two raise hardness by entirely different mechanisms. Work hardening comes purely from mechanical deformation and the dislocation tangles it creates, so it can be undone by annealing. Heat-treat hardening (such as quenching and tempering steel, or precipitation hardening aluminum) changes the metal's phase or precipitate structure and is reversed only by a different thermal cycle. Because work hardening also reduces ductility, designers track how much cold work a part accumulates — measured roughly by the total true strain it has undergone — so they can schedule an anneal before the remaining ductility runs out. Forming-limit analysis builds on the same idea, mapping how much strain a sheet can absorb before a work-hardened, thinned region necks and splits.