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416 SS cracked nuts

416 SS cracked nuts

416 SS cracked nuts

(OP)
Hi,

I have this weird issue and can't figure out what's going on. Chemistry is OK, Mechanical properties are ok...but this nut sometime fails during assembly. (assembly is with automatic electric guns with preset torque. So overtorquing ain't an issue.

The nuts crack right at the corner of the base and the hex. The crack propagates until the mid of the hex. See attached pictures.

Any similar experiences??
Oh the nuts are annealed btw.

Thanks,

Herc
http://files.engineering.com/getfile.aspx?folder=090d2f55-0856-492f-9560-09



http://files.engineering.com/getfile.aspx?folder=7d832448-8f4f-4fb6-be0d-80

RE: 416 SS cracked nuts

Quote:

So overtorquing ain't an issue.

Is friction what you think it is?

RE: 416 SS cracked nuts

No file in link.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
 

RE: 416 SS cracked nuts

Your picture is not showing.
I would think that overtorquing would strip the threads or at least elongate them.
I got a buch of squirrels in my backyard that have cracking nuts all winter, may be that's one them.

RE: 416 SS cracked nuts

(OP)
And I don't think it is overtorquing. Threads were fine..

RE: 416 SS cracked nuts

Uneven force from misshapen socket? Can you mark one of the nuts, tighten it, and look for contact area?

RE: 416 SS cracked nuts

One of the pictures shows galling of the contact surface of the nut.  Look at those surfaces for stress raisers whereby surfaces may be uneven or rough. Is a lubricant used to minimize galling?  About the hardness of the nut and parent material are they the same?  What I am amazed is that the crack is not starting from the inside threaded area of the nut

RE: 416 SS cracked nuts

What is the environment?  Multiple cracks, some branching, looks intergranular, I would lean towards SCC.

RE: 416 SS cracked nuts

That' propagation of a crack from an imperfection, such as the root of a thread, through the piece.

I see this with stainless steel fasteners from time to time.  In some cases it can be magnified by thermal variations from the surroundings.

That first picture looked like intergranular separation in the substructure of the parent material.  Follow the crack to the surface, OD or ID, you will find a notch which is the stress riser for the phenonema.

Hope this helps.

Regards,
Cockroach

RE: 416 SS cracked nuts

Offshore hardware, perchance?

Not all fasteners are equal.  

RE: 416 SS cracked nuts

CHinese nuts are easy to crack

RE: 416 SS cracked nuts

Quote:

but this nut sometime fails during assembly

Are the nuts subjected to any chemical treatments that could account for intergranular corrosion prior to assembly else how could SCC be a factor in the above instances?

300 series SS's are susceptible to sensitization if held in or slow cooled through the temperature range 800-1500F.  I don't know if sensitization applies to 416 but because these nuts are annealed there might be a connection.

 

Design for RELIABILITY, manufacturability, and maintainability

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