Strain Harden
Strain harden is the shop-floor term for strain hardening or work hardening, where a metal becomes harder and stronger after plastic deformation because its internal dislocation structure changes when cold-worked below recrystallization temperature. In CNC machining, unintentional hardening from cutting, rubbing, or bending increases cutting forces, accelerates tool wear, and complicates subsequent operations.
On the shop floor, strain hardening appears the moment a carbide insert rubs instead of shears. Taking a too-light depth of cut on stainless or titanium lets the edge skate across a previously deformed surface, leaving a localized hardened skin. The next pass, whether roughing or finishing, hits that skin with higher cutting forces, and the tool starts squealing or chattering while flank wear accelerates. Dwell in a drill cycle and excessive peck retractions have the same effect, repeatedly compressing the same hole wall until it hardens. Good practice is to maintain a positive chip load, keep depth of cut enough to get under the altered layer, and use sharp, wear-resistant tooling. Deeper roughing passes remove stock before the hardened stratum can accumulate, and the finishing pass should then take the entire layer off in one controlled cut.
Which metals are most likely to cause trouble in strain hardening during machining?
Austenitic stainless steels, duplex stainless, titanium, and many alloys with strong work-hardening behavior are common problem materials; low-carbon steels and soft aluminums can also harden locally if cut improperly.
What are the main machining indicators that a part is strain hardening?
Rising cutting forces, squealing, shiny glazed surfaces, accelerated flank wear, and a previously easy cut becoming noticeably harder on the next pass are classic signs.
Why do finish passes sometimes fail on a hardened skin?
Because the finish tool is taking a chip thinner than the hardened layer thickness, so it is only skating over the strengthened surface instead of fully removing it.