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.

 

[EdStainless]

Those two are very close in hardness.
I would like to see a lot more difference.
How much galling are you getting?

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P.E. Metallurgy, consulting work welcomed

 

[3DDave]

Probably burned the dry-film off and also baked the surfaces into a combination that galls. This is especially the case for repeated post-bake assembly cycles. There are higher resistance coatings available, but I expect they aren't good for more than one shot and then discard. The MoS2 is likely more temp resistant than the binder; once the binder is gone the MoS2 is not going to stick.

"The temperature limitation of MoS2 at 400°C (752°F) is imposed by oxidation. MoS2 oxidizes slowly in atmospheres up to 600°F."

I stopped reading too soon.

" In a dry, oxygen-free atmosphere it can function as a lubricant up to 1300°F. The oxidation products of MoS2 are molybdenum trioxide (MoO3) and sulfur dioxide. MoS3 is hydroscopic and causes many of the friction problems in standard atmosphere."

Not only does it no longer work properly, MoS2 can make things worse when overcooked with oxygen available.

 

[EdStainless]

Using HBN or WS2 may be better alternatives.

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P.E. Metallurgy, consulting work welcomed

 

[rollingcloud]

Dave,
The required dry film lubricant is EVERLUBE® ESNALUBE™ 382 Gray PWA-36545T Spec Low VOC Thermally Cured Mos2 Solid Film Lubricant, pretty much exactly what you said. The spec does require coating the lubricant prior to any 750 F+ heat and allows the coating to be reapplied between cycles as required. But does this lubricant play such big role? I thought the lubricants are more useful for long term operation.

 

[rollingcloud]

Ed,
The picture above is all I have right now, it looks like a lot of material is stripped off. The previous supplier did not have this issue, and the new supplier have been having issues with parts lately.

 

[3DDave]

Thread lubricants are certain to work the first time the fastener is installed and are likely to work the first time the fastener is uninstalled. That's it.

Any more than that is stepping closer to the cliff edge. Maybe the deformation changed a little, maybe the binder changed of the thickness changed or the roughness changed. All I know is nearly guaranteed is one install and one removal.

 

[lucky-guesser]

Thread lubricants are certain to work the first time the fastener is installed and are likely to work the first time the fastener is uninstalled. That's it.

Agreed. If the threads have particularly loose tolerances then you may get lucky and have some residue left for a second run, but I would only do that if the coating isn't critical.

Tight threads going on a critical (in this case anti-galling) coating, I would 100% reapply each time. You may have just been getting lucky before with your previous supplier.

 

[EdStainless]

Clean and reapply each cycle.

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P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

Rolling cloud
Different perspective. Did you research the max temperature for the dry film.

Secondly crimping or peening as I know is to prevent loosing from vibration.
As I know it.
Why are you teststing by removing and reinstalling.
In critical fasteners such as this
Remove and replace. Why is this being tested in this fashion. If I remember correctly 718 and A286 are precipitation harden. Of course it will change. Verify the heat treat procedure and what is the max stress value that can be applied.

 

[mfgenggear]

Once threads are peened that's it. Done. Not normally reusable.

 

[TugboatEng]

In my experience, A286 nuts and bolts pair well together. Why not make both from the same material?

 

[CWB1]

Start with the basics - Review your BJA and assembly procedure. IME that failure generally means you're over-torquing or over-speeding the fastener.

 

[mfgenggear]

When there is very similar material, with out a coating, which the coating is destroyed. And addition baked.
Is a definite cause of galling, and threads being peened or crimped.
Being very lucky so far.

 

[Tmoose]

https://www.eng-tips.com/viewthread.cfm?qid=326577

 

[rollingcloud]

mfgenggear,
The DFL's max operating temp is 800 F. The drawing calls for a spec which requires reusability test, this spec (active since 1985) is used for other similar nuts as well, I would have to assume it can be reused under some conditions, otherwise, I don’t see the point of the testing either. The nut is a self-locking nut, and upper portion of the thread is deformed to produce the locking feature. I believe the nut should be able to become round when assembled and return to its oval shape when removed.

For reference, FAA AC43.13-1B allows the reuse of self-locking nuts as long as they are meeting the minimum prevailing torque (table 7-2).

 

[rollingcloud]

TugboatEng,
Having the same material would make the galling issue worse, no?

 

[rollingcloud]

CWB1,
Good point, I will double check with the supplier. What's BJA? Bolt joint assembly?

 

[EdStainless]

They are different materials, but nearly the same strength.
The problem is that the bolt is the stronger material, so when you remove the nut it is the nut that deforms the most.
This makes reuse very problematic.

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P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

Rolling cloud
OK
Add a procedure to verify the threads of the nut and bolt.
With a go and no go gage.
If it does not accept the go gage reject
If it accepts the no go , scrap
If it does not accept the go gage, chase the threads with tap and die.
If the threads cross thread then it
Is yielded and damaged, it should be rejected.
If upon removal of the nut damages the threads. It then becomes a safety issue.