ASTM A563 apparent incongruity

[Tmoose]

I have the "opportunity" to evaluate some 1-5/8" -5.5 heavy hex nuts with a odd behaviour.

One obvious difference between the good ones and the bad ones is hardness.
This led to checking out ASTM A563 Grade A, the spec for the "heavy hex nuts" that would be provided by Fastenal.

Attached is an image showing what I believe is a minimum specified nut hardness that would be unable to begin to meet the 100 ksi proof load stress that is also a requirement for Grade A nuts.

Am I missing something?

thanks,

Dan T

 

What is hardness of the good ones and of the bad ones? And why are the bad ones bad?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Tmoose]

When re-used, a few of the "bad" ones loosened in service. Repeatedly. Brand New commercially available "heavy hex" nuts have been working fine.
The one "bad" one available is HRB65, just a couple of points below the ASTM A 563 minimum of HRB 68. The grain structure is pretty coarse. The threads don't appear "pulled" or deformed in any way, other than wear/erosion on the first couple of threads. The lone "good" used nut is HRC 30, with a much finer grain structure, as if quenched and tempered. Despite those differences between the two nuts, the cause of The self loosening is kind of a mystery. There are lots of things about the very mature design that I don't like, but apparently it normally works just fine.

I posted because What really hangs me up, looking strictly at the ASTM spec in the previously attached image, is how a Grade A nut with the required minimum proof stress load could possibly also have the low allowable minimum hardness.

 

[EdStainless]

Isn't the proof load test on an actual article? In that case it is a shear test.
I suspect that a nut that size with UTS=60ksi would allow you to exert 100ksi on the test bolt just fine.

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P.E. Metallurgy, Plymouth Tube

 

If I'm not mistaken you normally want to have nut hardness somewhat less than that of the stud/bolt, but 68 HRB minimum seems extremely low. Also, the range between B68 and R32 is very wide. It seems like ASTM set some arbitrary lower limit not quite reflective of reality.
For critical fastener applications I advise clients to set up a QC regimen where they sample fasteners for hardness testing, magnetic particle examination, thread profile inspection, and metallographic sectioning to look for structure, decarburization, and forming defects such as laps and cracks. Then segregate the lots these tests represent. Class of fit is another criterion to pay attention to.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

TM, you might try reaching out to the ASTM committee that handles that spec.
Often there is some background information on these limits.
I am sure that they have some test history to base this on.

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P.E. Metallurgy, Plymouth Tube