When gears are case–hardened it is known that some growth and redistribution of stresses will occur which
result in geometric distortion. Aerospace gears require post case–hardening grinding of the gear teeth to
achieve necessary accuracy. Tempering of the case hardened surface, commonly known as grinding burn,
occurs in themanufacturing process when control oftheheatgeneration atthesurfaceis lost. Excessiveheat
generated atthe surface can induce surface tempering and /orre–austenitize the surface in a localized area.
The localized area will have reduced or altered mechanical properties in addition to an unfavorable residual
stress state [1 ].
A gearbox with minimal service time was removed from an aircraft, disassembled, and visually inspected.
Linearcracks along the dedendum ofthe working geartooth face were found during visual inspection in three
adjacent teeth. No teeth had been liberated. A detailed inspection of the incident gearbox found no other
components with distress.
Metallurgical evaluation determined that the cracks initiated in service at the boundary ofa localized grinding
burn, which had re–astenitized. The cracks propagated inward from the tooth surface in fatigue to a depth
greater than the depth of case.