cold working
Cold working means deforming metal below its recrystallization temperature, usually at room temperature, so the damage you do stays done. Bend a paperclip back and forth: the bent spot gets stiffer and harder to move, and eventually it snaps. That stiffening is cold work in action. Because the metal cannot grow fresh grains to heal itself, every bit of deformation piles up dislocations that tangle and jam each other, so the metal gets stronger and harder but less able to stretch before it breaks.
The amount is measured as percent cold work, the fractional reduction in cross-section: %CW = (A0 minus Af) divided by A0, times 100. Drawing a rod down from 10 mm to about 7 mm diameter roughly halves its area, which is about 50 percent cold work, and it comes out markedly stronger and harder but much less ductile. Cold rolling, cold drawing of wire, and stamping all use this. Because there is no scale and no thermal shrinkage, cold working also gives a smooth surface and tight dimensions.
The honest trade-off is the classic strength-ductility exchange: you cannot keep cold working forever, because the metal runs out of ductility and cracks. So heavy cold work is broken into steps with an intermediate anneal (heating to recrystallize and soften) to restore ductility before continuing. Cold work also leaves residual stresses locked in, which can warp a part later or, in the wrong environment, drive stress-corrosion cracking. Strengthening always has a price.
Copper wire is cold drawn: pulling it through successively smaller dies at room temperature raises its strength and hardness while dropping its ductility, so it must be annealed between heavy draws or it will crack.
Cold work strengthens by tangling dislocations, and always spends ductility to do it.
Cold working strengthens but spends ductility and locks in residual stress; there is a limit before cracking, which is why heavy cold work needs intermediate annealing.