Strain hardening in which deforming a metal raises its hardness and strength. Austenitic stainless, nickel alloys and manganese steels harden fast, so a tool that rubs or dwells leaves a skin far harder than the base metal and the next pass dulls on contact. The defense is a sharp tool, a positive feed that cuts under the hardened layer, and never letting a drill or endmill sit spinning in the cut.
Don't let the drill dwell in the 304, it'll work-harden the bottom and the next peck won't cut at all.
If a drill starts squealing in stainless, back out and change the approach; letting it dwell only hardens the bottom more.
Also heard as
- strain hardening
Related terms
Rubbing
A tool sliding over the work without taking a proper chip, usually because the feed is too light or the tool is dull.
Chip load
The thickness of material each cutting edge takes per revolution, also called feed per tooth.
Peck
To drill a hole in short steps, backing out between them to clear chips and let coolant in.
HSS
High-speed steel: the tough, hand-grindable tool steel used for drills, taps, lathe bits and some endmills.