Strain Hardening
Strain hardening, also called work hardening or cold working, is the increase in a metal’s hardness and strength after plastic deformation beyond its yield point. As deformation accumulates, the material becomes harder to deform further and ductility drops. In CNC machining, this creates a hardened surface skin that makes subsequent cuts more difficult and accelerates tool wear.
In a CNC cell, strain hardening typically appears when the cutting edge rubs instead of shears. This happens with a light depth of cut, a dull insert, poor chip load, dwell, or peck cycles that repeatedly compress the same zone. The hardened layer may be shallow but it is consequential: the next pass encounters higher cutting forces, more chatter, poorer finish, and shortened tool life because the tool is cutting a tougher skin than the base metal. Stainless steels, titanium, and other alloys prone to work hardening demand a strategy that keeps the cut under control with sharp tooling, stable feeds, and enough depth of cut to get below any previously hardened layer. In millwork and edgebanding, excessive pressure or dwell on metal hardware and formed components can locally harden or embrittle the work zone, affecting fit-up and secondary machining.
What microstructural mechanism causes strain hardening?
Plastic deformation increases dislocation density and dislocation interactions, which raises resistance to further slip and therefore increases flow stress.
Why does strain hardening matter in machining more than in simple forming?
Because the hardened zone changes the effective machinability of the next tool engagement, so even a small hardened surface layer can force the cutting edge to work harder than expected and degrade surface integrity.
What is the main process control rule to avoid strain hardening in CNC cutting?
Do not let the tool rub; maintain a feed and depth of cut that keeps the edge cutting below any previously hardened layer, using sharp tools, stable chip load, and appropriate coolant strategy.