Workhardening

Work hardening, also known as strain hardening or cold working, is the increase in strength and hardness of a metal that results from plastic deformation at temperatures below its recrystallization temperature. The phenomenon occurs as dislocations are generated and interact, making further dislocation motion harder and raising the stress required for continued deformation.

Deformation introduces dislocations, which multiply and interact, creating tangled networks and subgrain structures. These defects impede subsequent dislocation motion, raising yield and tensile strength. Recovery processes such as rearrangement, annihilation, and, at higher temperatures, recrystallization can reduce dislocation density and reverse some hardening.

Most metals exhibit work hardening to some degree, especially copper, aluminum, nickel, steel, and other ductile alloys. Common cold-working methods include rolling, drawing, extrusion, bending, and stamping. Some alloys also harden by precipitation or solid-solution strengthening; in those cases, hardening mechanisms may combine with work hardening to set final properties. Recrystallization annealing can restore ductility by forming new grains.

Work hardening increases hardness and strength but reduces ductility and toughness. It can introduce residual stresses and anisotropy, affect surface appearance, and raise the risk of cracking if deformation is excessive. Process parameters, such as reduction per pass and total strain, influence the balance between strength and formability.

Hardness testing (for example Rockwell or Vickers) and tensile testing quantify the degree of hardening. Work-hardened metals find use in wires, springs, tubes, and formed parts where high strength is required. Designers must balance strength gains against reduced ductility and potential residual stresses when planning forming operations.