An increase in hardness and strength caused by plastic deformation of the material.
Most metals get harder when deformed, but the effect varies enormously. Austenitic stainless steels, titanium and nickel alloys harden severely — often doubling their surface hardness after a single pass — while low-carbon steel and free-machining grades harden very little.
For machining, the consequence is a single rule: do not dwell. A tool that rubs rather than cuts hardens a layer beneath itself, so the next pass cuts a harder material than the drawing specifies, and the pass after that harder still. It is why light finishing passes on stainless and titanium are counter-productive.
Taking a very light finishing pass on stainless or titanium to improve the finish. It hardens the surface instead. Take a full-depth cut with a sharp edge.
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