cold work

Cold work means shaping a metal by plastic deformation while it is cold, below the temperature where it would re-soften on its own. Rolling sheet at room temperature, drawing wire through a die, pressing a car panel, bending a bracket: all are cold work. Because the metal cannot heal as fast as you deform it, cold work is the practical way to trigger work hardening and lock strength into a part.

Engineers measure how much a metal has been cold-worked as a percentage of cross-section it has lost: percent cold work = (A_0 - A_d) / A_0 times 100, where A_0 is the original cross-sectional area and A_d the area after deformation. Roll a 10 mm-thick strip down to 6 mm and you have (10 - 6)/10 = 40 percent cold work (using thickness as the stand-in for area in flat rolling). The higher the percent cold work, the more dislocations pile up, so the higher the strength and hardness, and the lower the remaining ductility. Charts of these three quantities versus percent cold work are standard design data.

The upside is strength and precision at no alloying cost, with clean surfaces and tight dimensions because there is no heat to scale or distort the part. The downside is that ductility runs out: past a certain percent cold work the metal will crack if pushed further, so it must be annealed to reset it soft before more forming. Cold work also leaves residual stresses and directional properties (texture) that can matter in service.