Damaged threads

[GonzaloMartinez]

Hello everyone, I want to know if someone can help me identify this mechanism of damage so I can look it up in the bibliography by its proper name.

Thank you.

(Reference in millimeters)

 

[yaschief]

Looks like Galling to me.

 

[RafaelCosta]

I agree with yaschief.
Is this thread stainless steel? If it is, the probability of galling increases a lot. Maybe you should consider apply anti-galling coatings.

best regards

 

[GonzaloMartinez]

Thank you, I appreciate your replies. RafaelCosta the material is medium carbon steel SAE 4140, quenched and tempered. The material was checked by chemical composition measurement and metallography. I would have expected higher hardness, the measurement gave 27 HRC.

Regards.

 

[racookpe1978]

It does look like galling, BUT! The stripped metal is coming from the other piece: The gals are pulling metal out of the nut (unlikely) or tapped hole (more likely).

If it is from a tapped hole, and because the tapped hole no longer has any metal in its threads to be cleaned and re-tapped, you must use a threaded insert or choose a larger diameter stud on re-assembly. Cleaning these bolts (studs ?) may allow re-use, but it will likely be cheaper to get new bolts/studs.

Edited remark: Looked at the photo again: The bolt itself looks like it galled. Perhaps the tapped holes are in better shape than I expected.

 

[tbuelna]

The damage on those external threads is mostly material displaced from the thread tips. There also seems to be progressively greater material displacement from left to right in the photo. It almost seems like the mating internal thread was tapered, like an NPT pipe thread.

 

[RolMec]

Could you pls. check both threads for a (even small) difference in pitch?
What is the material of the counterpart, could it be that it is of a significantly different grade / strength?
To wind in a M16 (pitch 2) that deep should have used quite a bit more than the usual force to be applied. Any input from the mechanic? What did you lubricate with?
27 HRC seems way too low, I'd expected sthg like 35. But this in itself would not have an effect on the mating of threads..

 

[GonzaloMartinez]

Here is a picture of the mating threaded hole. I looks in good shape to me. Could this damage corresponds to an overtorque of the joint?? The material is torn in the direction of friction force on tightening procedure (left handed thread). Something I haven't told you yet is that the joint fractured not on the threaded portion but in the base of the shank.

Sorry RolMec I don´t have data on the material of the counterpart but I can get it. I'll try to get more information about the tightening procedure with the mechanic.

Regards.

 

[Tmoose]

Is this a one time event?
Is it a production part, or a unique assembly ?
What happened to the severed bolt body off to the left?
What size is the thread? How is it supposed to be tightened?

As suggested by others, confirming the thread geometry of both components is an important first step.
Also asking the folks that were there what happened would have some clues.
And, observing the assembly of a few qualified correctly manufactured components.

I'd expect to find some measurable geometrical discrepancies.

At least one inch-sized fastener has a pitch close enough to a similar size (but slightly larger diameter) metric fastener that an inch sized screw
will assemble easily into the metric nut. The screw will wobble alarmingly if tested that way, as the pitch diameters are way off.
For instance, a 3/4"-16 screw installed in a M20-1.5 tapped hole
I picture if such a combo was tightened or otherwise subjected to significant axial load the threads might fail br ripping off the thread crests similar to those in your picture.

 

[tbuelna]

Although it's hard to be 100% certain just looking at the picture, but the internal threads shown in the second photo sure look like tapered pipe threads to me. While the external threads in the OP photo look like straight threads. The external thread damage in the OP photo appears to be primarily material displaced from the thread tips, and not so much from the thread flanks. There also appears to be progressively greater displacement of tip material on the external threads from left to right in the OP photo. It might be possible that the external threads were straight, the internal threads were tapered, but both threads were of similar nominal diameter and pitch.

 

[desertfox]

Hi GonzaloMartinez

So the failure occurred at the bottom of the tapped hole in the region of the last thread?

In addition to finding out the material of the counter part see if you can get information on bolt tightening procedure, size, lubrication, torque and did this failure occur during tightening or in service

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[chicopee]

Rolmec seems to have the right inclination and that is a mismatch of thread pitch as it is evident from the reference ruler.