Identification of quench cracks at tooth root

[UCengno1]

I have a small gear, 26 DP / 46 teeth, fabricated from 8620. A recent gear tooth failure problem was potentially linked to the existance of quench cracks. I question the observation only on the basis that only a single tooth was found to be cracked. Seeing as the entire gear would be submitted to the same heat treatment wouldn't I expect other teeth to exhibit the quench cracks? Comments?

BCK

 

[TVP]

Not necessarily. Remember that manufacturing and heat treating processes are dynamic in nature, with local variables in addition to global variables. Retained austenite, residual stress, surface defects, etc. are not constants, so it is possible for a gear to have only one quench crack. Quench cracks are usually easy to diagnose, due to the light oxide scale that develops during tempering.