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Old 09-19-2012, 10:34 AM
El Rudo El Rudo is offline
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Default yield strength increment after cold forming

Hi, I'm new to this forum, I've got some questions one of you may have the answer to:
I'm designing a construction from thin stainless sheet, AISI 316 grade as it's used for water sports. I have tested CNC milled parts and they seem just a bit too weak. I now need to assess how far I should depart from the current design to make it strong enough.
Now I have heard that the yield strength of sheet metal increases considerably after cold working (work hardening). Questions:
> Is there any telling how much the yield strength can increase?
> Could the yield strength as much as double?
> Does such strengthening occur thoughout the part or only around the areas with the most "3D" changes?
The parts will be pressed and stamped, the deformation is between 10-20%. Thickness around 1.0mm.

Looking forward to what you come up with!
cheers,
Rudo
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Old 09-19-2012, 12:30 PM
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Page 11 of this document shows some strength figures based in the percentage of reduction: www.atimetals.com/Documents/ati_316_tds_en.pdf

You'd have to seriously reduce the section to double the tensile strength. Cold working only affects the area worked.
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Old 09-19-2012, 01:22 PM
El Rudo El Rudo is offline
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Hi USMCPOP,
That's exactly what I'm looking for, great stuff!
I now see that double would be tough, the shape in the parts will reduce by a few % only, yet I can expect a noticeable gain in yield strength.
thanks, great forum so far!
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Old 09-19-2012, 02:13 PM
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You can get some varieties of stainless in annealed, 1/4 hard, 1/2 hard and full hard. Depending on the amount of subsequent cold working and forming you need to do, you might be able to use something partially cold worked.
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Old 09-19-2012, 10:48 PM
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Quote:
Originally Posted by El Rudo View Post
.
thanks, great forum so far!
Welcome aboard El Rudo, do a little reading, you ain't seen nothing yet!
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Old 09-20-2012, 06:53 AM
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Rudo,

Are you looking for yield strength or stiffness? The only way to make a thin sheet stiffer is by forming a lip or ridges or something like that. You have to alter the section.
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Old 09-20-2012, 02:01 PM
El Rudo El Rudo is offline
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Hi 1911 man, thanks for the tip, I'll read what I can!

@ USMCPOP, thanks for asking, stiffness is fine for the moment. I only get a little bit too much plastic deformation with the CNC'd parts - which I suspect to be annealed. The mass produced parts will certainly have some yield strength increment.
Usually I ask the supplier/manufacturer these kinds of questions, but for this project it's not yet known who's going to make it, or where in the world it will be made and where they get their steel. With the information such as yours I can get very close to the final design, hopefully we can speed through manufacturing with minimal engineering and tooling changes.

Over the past 2 decades I've been engineering and designing products professionally in lots of different materials, plastics, composites, concrete, fabric, alloys, steel, you name it. Heard and saw a lot, but I'm still more a generalist rather than a specialist
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Old 09-20-2012, 02:33 PM
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I'm more of a hack, but I do know stainless steel can be a blessing or evil. Your CNC folks aren't going to like working with anything partially work-hardened, at least I don't think so. I know 304 is evil, and 316 is supposedly not far behind.

I hear you about the specialist/generalist thing. There are sooo many materials, processes and techniques in use that it boggles the mind. Then you get different suppliers with slightly different grades and it all goes out the window. Good luck.
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