COLD WORKING

Cold working refers to the hardening of a ductile metal under the effect of plastic deformation performed cold, that is to say below its recrystallization Temperature. When a metal is hammered, rolled, drawn or stretched, its crystalline structure changes at the microscopic scale: dislocations, linear defects in the lattice, multiply and become entangled, which limits their own mobility. This loss of mobility raises the yield strength and the Hardness of the Material, but correspondingly reduces its Ductility and can, if the process is taken too far, cause the onset of cracks. Cold working can be reversed by annealing, which restores the crystalline structure through recovery and then recrystallization. In watchmaking, this phenomenon is deliberately and carefully exploited. Hairspring (balance spring) blades are hardened by Rolling cold to achieve the required elastic characteristics before shaping the terminal curve; barrel spring blanks are cold worked by wire drawing then profiling to obtain the stiffness and fatigue resistance necessary for a stable driving torque; the rolling of the pivots, finally, superficially cold works the steel and improves its wear resistance without resorting to additional heat treatment.