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

 

[swall]

Where did the tooth fail and what is the orientation of the suspected quench crack relative to the tooth fracture?

 

[Tmoose]

"..... only a single tooth was found to be cracked."

Was it inspected with wet magnetic particle ?
Did the magnafluxer check it in several steps, with a new magnetization for each orientation?

 

[UCengno1]

After additional inquiry I am finding some hesistancy from the metallurgist regarding the initial failure diagnosis, e.g. not all teeth were checked just the ones near the failure point, not sure if it is a quech crack, etc... Is it possible to look for some heat treat residual material in the crack to determine if it appeared during quench?

 

[swall]

You can examine the fracture with a scanning electron microscope (SEM)and determine if there is intergranular fracture at the crack initiation. Quench cracks are intergranular; fatigue cracks are not. Overload fracture, especially in a carburized case, may exhibit some intergranular fracture in the heavy portion of the case, but near the intiation point, the fracture should be a predominantly quasi-cleavage.

 

[UCengno1]

swall,

Thank you for the very helpful guidance.

BCK