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.

 

[3DDave]

I have seen hydrogen embrittlement - snapping Belleville washers as easily as a corn chip with finger pressure, for example.

Maybe? Embrittlement results in brittle failure along the grain boundaries rather than plastic deformation. Did the bolts get baked at a high temp at some time? A requirement for some hydrogen exposure is to oven bake to drive the hydrogen back out; could someone have done that?

 

[mfgenggear]

Tug
If the parts are under 40 HRc then
It will not effect the parts in this case bolts. As a rule of thumb parts can be post bake as soon as processed
In chemical process.
Generly -50 degrees from the tempering temperature.
See this link

https://www.sae.org/standards/content/ams2759/9e/

Hydrogen embrittlement to my knowledge
Causes instan brittle fracture.
Plastic deformation means it was not heat treated correctly or parts were over torque at previous install.

 

[MintJulep]

Well, phosphoric acid certainly has plenty of H to give as it reacts with various things.

A grade 10.9 bolt is susceptible both material and hardness. (Generally 32 HRC and above is considered susceptible).

An hour at room temperature is enough time.

But, hydrogen embrittlement causes brittle failure. So, your report of a yield failure is at odds with embirttlement.

Any chance that the mechanic set a Nm torque spec number on the lb.ft scale of the wrench?

 

[mfgenggear]

That is incorrect 32Hrc will not obtain
Hydrogen embrottlement

 

[TugboatEng]

These bolts have never been exposed to anything but ambient temperature. This is their 2nd torquing cycle.

 

[jessieleo99]

Although the bolts did not fracture, the reduction in torque and plastic deformation may indicate that the material's microstructure has been affected.
It is recommended to perform a dehydrogenation treatment after cleaning to reduce the risk of hydrogen embrittlement.

https://powder.samaterials.com/

 

[TugboatEng]

I am purchasing new bolts. It's only $140 for bolts in a $3.5 million project. We have a small team so purchasing contributes a significant labor burden on us so we prefer to not purchase when we don't have to. I have always reused these bolts in the past without issue but now that my coworker exposed them to phosphoric acid two failures have occurred. This has piqued my curiosity.

 

[LittleInch]

And so it should. You need to treat this like the failure it is. Only luck has meant it didn't fail in service.

I would pull whatever records and certificates you have for those bolts and look at them closely.

Also check the vendor carefully.

A lot of items like this are are counterfeit or they forge the certs. And supply from Far east is often suspect.

Yielding at 50% tension is not good, but agree this doesn't sound like brittle failure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MFAMET]

phosphoric acid solution could cause hydrogen embrittlement depending on the concentration, temperature and duration
A non-residual stress release part could cause cracking while submerged in the acid solution. This is called stress corrosion cracking (SCC). A simple test could determine whether the solution will generate hydrogen embrittlement on the par.

To bend a wire (.2"-.3" dia.) by 120-150 degree. This will generate a tensile residual stress at inner surface, and then submerge it to your acid solution for 30-90 minute. check and look the inner surface to see any cracking.

 

[mfgenggear]

Mfamet
Very well explain, and agreed how ever again it would be brittle fracture.
The bolts yielded. It would be interesting to see if an electron microscope would detect hydrogen embrittle.
I have many years processing parts and have not seen corrosion cracking on
Parts with less than 40 HRc,
Which anything is possible. And weird crazy issues have happened.

 

[Compositepro]

It's certainly possible that defective bolts got treated with phosphoric acid. It was not brittle failure, so it was not hydrogen embrittlement. Hypothesis made and rejected. Next hypothesis and test.

 

[RolMec]

Hello OP,
do I get it correct that these bolts have already been torqued into place once, then disassembled, and are now re-used?
Pls. check, to what level / standard the original torqueing has been done: to yield (70 or 90 %) or beyond yield. If the second, theses bolts are imo as a rule not reusable. But even in case of torqueing below the yield limit, there's a probability that some bolts got over the edge anyway. ASME PCC-1-2013 might be a source.
Over here (Europe / Germany) and in my field (heavy outdoor machinery), 10.9 grade bolts are not to be re-used, regardless of torque history.
Regards

Roland Heilmann

 

[weldstan]

There have been enough forged bolt documentation to mount finding sourcing of bolts and checking chemistry as possible suspects. Also check failed bolt profile to determine any obvious flaws.

 

[EdStainless]

1. These didn't fail from HE
2. The Phos cleaning could cause HE

I worked in an environment where we were using custom made Monel K500 bolts.
The rule was never reuse. Period.
The field guys would bring big bags of them back to the plant and our test area would use them for test assemblies once and then throw then in the recycle bin.
Even though there were 12 @ $15 x 6 per unit, the risk was too great.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[mrfailure]

RolMec is correct: You should not be reusing the bolts in the first place. Always install new ones instead as they will not have experienced any degradation of mechanical properties from previous tightening or service. This is your likely root cause of failure no matter the mechanism.

Regarding the failure mechanism: Can you measure Rockwell hardness to see what it is? That will then help focus the failure analysis discussion. I have found a minimum threshold hardness of 35 HRC (others have suggested differently) is required before hydrogen embrittlement is considered a possibility.

A longitudinal cross-section through the deformed area might be worth looking at to see if there is any actual subsurface damage to further help understand.

I also put out the possibility that during previous service the fastener could have been insufficiently tightened, resulting in it accumulating fatigue cycles that could have reduced tightening strength even though no crack had yet formed. Another reason to replace the bolts in this situation.

 

[WKTaylor]

Tug... EXACTLY WHAT 'SPEC' [made-per] BOLTS are you using? The spec will tell use exactly what You have... ASME, SAE, proprietary, etc???

Also... exactly what grade of Phos Acid were they cleaned in???... and was the mechanic using any existing tech/maintenance data/procedures?

Almost always SCC results in very low-stress brittle fracture... little/no deformation.

Also... what is the operating environment for these bolts??
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Last minute edit...

'What finish' is applied to 'what spec bolts'...

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[Tmoose]

"forged bolt documentation"

documentation for forged (not machined from bar stock) bolts
or
forged documentation for bolts?

 

[TugboatEng]

This is going to be a hard one to confess to but I gave the mechanic the wrong torque spec. 35% too high.

 

[3DDave]

It's the right torque, just for the wrong fastener. Be optimistic!