work-harden
To harden (a material) by work hardening.
work-harden: strengthen metal by deformation, not heat
Work-hardening is the process of increasing the hardness and strength of a metal through plastic deformation, without adding heat or changing its chemical composition. When you bend, roll, draw, or hammer a metal, you force its crystalline structure to rearrange; dislocations accumulate and tangle, locking the material into a denser, more rigid state. The metal becomes harder and stronger but also more brittle and less ductile.
This happens automatically during cold-working operations. A wire drawing die reduces rod diameter by pulling it through progressively smaller orifices; the metal work-hardens with each pass, resisting further deformation. Rolling mills flatten and compress sheet stock; after several passes, the material stiffens. Even hand forging work-hardens the piece as the hammer blows compress and align the structure. Mild steel becomes noticeably harder after just a few cycles of bending and straightening.
Annealing breaks the cycle
The hardening plateau has limits. A work-hardened metal eventually resists further deformation so strongly that further cold-working risks cracking or fracture. To continue shaping, the metal must be annealed: heated to a specific temperature (usually 400 to 900 degrees Celsius depending on alloy), held briefly, and cooled. Heat allows the dislocations to migrate and recombine, restoring ductility. The metal softens again and the work-hardening cycle can begin anew.
Aluminum, copper, and their alloys work-harden rapidly and are routinely cycled between cold-work and anneal steps during manufacturing. Steel work-hardens more slowly but still requires intermediate annealing on severe drawing or rolling schedules. Stainless steel, especially austenitic grades, work-hardens aggressively and can become dangerously difficult to machine or form after limited deformation.
The effect is sometimes wanted: a designer might specify a work-hardened temper (H or T designation on aluminum alloys) to gain strength without a separate hardening step. More often it is a constraint: production schedules must allow anneal cycles, tooling must be robust enough to handle stiffening material, and scrap rates climb if deformation exceeds the material's capacity before the next anneal.