What would be the most likely cause of nut thread stripping? 3

[rollingcloud]

The new supplier stated that the thread of the nut is chipping out during the first removal cycle after baking (image attached) and the nut is getting stuck after 3-4 cycles of room temperature torque test.

https://imgur.com/a/c18sqkU

.190-32 UNJF Nut material: A286, hardness 24- 37 HRC, 130 ksi tensile strength. Dry film lubricated.
Actual measured hardness: 40 - 43 HRC. I believe the crimping process would increase the hardness?

Test bolt material: Incol 718, hardness 36 HRC min, min of 180 ksi tensile strength. Actual hardness: 45-46 HRC

Nut-bolt assembly is heated to 850° F for 6 hours for testing.

I checked the reported thread dimensions, no deviation from the standard.
Did they over-crimp the nut? How do manufacturers typically perform the crimping? I feel it might be tricky to get it right. I attempted to screw the nut (we have some good parts left in house) onto the bolt by hand, but when it reached the deformed thread area, it was far too tight without a tool.

 

[Doug Hunter]

I would check to make sure that the root radius of the threads is not sharper than it was before. Note that the temperature of the test might be causing some tempering of the material.

Best regards,
Doug Hunter
Altarium Technical Consulting

 

[CWB1]

What's BJA? Bolt joint assembly?

Bolted joint analysis. For fasteners that are to be reused you need to ensure that you're only torquing them to ~70% of yield. Non-reusable are typically ~90%.

 

[mfgenggear]

Nice post CWB1, important info