Localized thermal damage caused when grinding heat overtempers the hardened case, leaving a soft patch, or reheats it enough to form brittle untempered martensite. Burn lowers pitting and bending resistance and can start cracks. It is invisible to the eye and is found by nital etch inspection, where tempered areas etch dark and rehardened areas white, or by Barkhausen noise testing.
Nital etch shows grind burn on three teeth, dark patches right at the pitch line, scrap them and dress the wheel.
Burn is invisible until it fails, so critical gears get an etch or Barkhausen check after every grind.
Also heard as
- grinding burn
- temper burn
Related terms
Gear grinding
Finishing hardened gear teeth with abrasive wheels to correct heat treat distortion and reach high accuracy.
Pitting
Surface fatigue of gear teeth that breaks out small craters, typically on the flank just below the pitch line.
Carburizing
A heat treatment that diffuses carbon into the surface of low-carbon steel gears, then quenches for a hard case.