Induction Gear Cracking

[sodeen]

Analyzing a very large round gear (~15 feet across) that has teeth case hardened to ~.100-.150 case depth.

The teeth are on the inner circumference of the round gear. Induction hardening does an "every other gap" process when case hardening the gear teeth. On the second pass, the areas not hardened previously are hardened.

After a bit of grinding, mag particle inspection revealed cracks in the center of some of these gaps. All of the cracks followed an every-other-gap pattern when they were present.

This is the biggest gear tooth that we've attempted to induction harden in this fashion.

It seems to me like hardening one area on either side of a gap and then heating/cooling it rapidly on the next pass could cause enough stresses to see those vertical cracks, and the every-other pattern seems to argue that the issue is likely related to the heat treatment method, but this is a pretty important issue considering the size and cost of making one of these so I thought I'd see if anyone had other ideas.

 

[arunmrao]

Can you use a slower quench after heating the profile,perhaps this may reduce thermal stress. I assume,you have stress relieved or normalised the gear once again after cutting the profile and that no cracks were created during the process.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[swall]

It is not clear where the cracks are. Are they in the teeth or in the roots? What is the material? What is the quenchant?

 

[sodeen]

Sorry, was in a rush when I wrote the original.

The cracks are in the "root" between the teeth. Material is regular old AISI 4140 H. Quenchant is polymer-based.

More info has come to light since this post that has all but defined the problem. I appreciate the help.