Hydrogen Embrittelment 5

[TugboatEng]

I showed up at our shop today to find my mechanic cleaning bolts in a phosphoric acid solution. Later, during assembly, two of the bolts yielded at ~50-75% of the torque spec. The bolt are class 10.9 M14 socket head capscrews. Is it possible that a 1 hour exposure to phophoric acid at 75% concentrations could cause hydrogen embrittelment in this material? The bolts didn't break but there was a substantial amount of plastic deformation.

 

[TugboatEng]

True, it looks like I pulled the value off the fine thread table. I was working on other things and "Googled" the torque value.

 

[3DDave]

Everyone here managed to miss that option. Does this teach the assemblers to double check when it twists the bolt heads off?

 

[TugboatEng]

I have to commend the assembler because he came to me and said something didn't feel right. We didn't break any bolts. I put my hands on it and could feel it yielding. I always try to impress on people that if you take something to yield it's failed. Stop now and do not continue trying to apply more force because you are not.

 

[3DDave]

I like how the pitch changes. I have seen that before.

I had a project where weld studs were snapping off in the field and the question was, were they over torqued and being twisted off. I had some sample studs welded and made an impromptu test - enough to see the quality of the failure rather than to quantify it.

That result you have is what I was seeing. At the the highest torque the studs weren't twisting off but the pitch was changing. This meant that the assemblers were seeing a false torque because of interference in the threads and so the clamp load was not being developed. Being loose on a vehicle that saw moderate vibration and where the studs then became the shear path led to rapid fatigue at the weld stud base. Somehow it had something to do with dis-assembly and re-assembly. They were fine when initially installed, but failed after servicing.

I don't recall what the action after that was. It has slipped my memory after 25 or so years. But I do recall the satisfaction at seeing what appears to me to be the reason that threads strip - the incoming pitch is increased too much to fit rather than it being a simple friction or galling problem, no different than trying to put a 20 TPI bolt thread into a 28 TPI nut.

 

[Tmoose]

https://res.cloudinary.com/engineer...638982/tips/PXL_20240916_222206273_jn1xfi.jpg

Is the installed "grip length" about 20 mm?
By grip length I mean from under-head to the first/upper-most thread of the tapped hole.

 

[LittleInch]

That's still an interesting failure. How much of the bolt was "free" and how much embedded in whatever was threaded?

Now the question is how many were torqued to the "wrong value"??



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