work hardening
The repeated plastic deformation of a material, causing a permanent distortion of its crystal structure, and an increase in its strength and hardness; the resultant hardness increase itself. (It can be either desirable or undesirable depending on the aims and circumstances in a metalworking task.)
work hardening: metal gets stronger when you bend it
When you repeatedly deform a metal through rolling, drawing, bending, or hammering, its crystal structure undergoes permanent rearrangement. Dislocations multiply and tangle within the lattice, making further movement of atoms harder. The result is a measurable increase in tensile strength and hardness, alongside a decrease in ductility. This phenomenon, also called strain hardening or cold working, is unavoidable whenever metal undergoes plastic deformation below its recrystallization temperature.
The practical consequence depends entirely on your goal. A wire drawer exploits work hardening intentionally: successive passes through progressively smaller dies work-harden the wire, building strength without adding material. A spring manufacturer relies on it to achieve the required elastic properties. Conversely, a metalworker forming a complex shape may find the material becoming too brittle to work further, requiring an annealing cycle to restore ductility by recrystallizing the structure.
The mechanics and measurement
The degree of hardening depends on the amount of plastic strain imparted. A small bend produces modest hardening; multiple passes through dies or repeated impacts produce severe hardening. Quantitatively, tensile strength can increase by 50 percent or more in heavily work-hardened material compared to its annealed state. However, elongation to fracture drops correspondingly, sometimes to just 2 or 3 percent. Hardness typically increases 20 to 30 Brinell points depending on the metal and deformation rate.
Different metals respond differently. Copper and aluminum harden significantly and quickly; iron and steel show less dramatic increases. The rate of deformation matters: slow, steady cold working produces more uniform hardening than rapid impacts. Temperature also matters: any heating during work, whether from friction or ambient conditions, accelerates recovery and reduces net hardening.
The name reflects the mechanism: work, in the physics sense, is energy applied to the material; that energy is locked into the deformed crystal structure as increased hardness. The effect is permanent unless the material is heated above its recrystallization point, which typically lies between 40 and 60 percent of the absolute melting temperature. For copper, that threshold is around 200°C; for steel, much higher. Understanding when hardening becomes a liability rather than an asset is central to planning any metalworking sequence.