Why swelling during straightening?

[MagBen]

When straightening and cut from coil to bar, a noticable diameter increase is observed (confirmed this is not due to the measurement/material variability). Anyone knows the physical/metallurgical reason behind?

Also a strength level decrease is observed after straightening, whcih is not straightforward either, but understandable.

 

[MagBen]

More info:
Machine: Lewis straightener. Method: rotary arbor bending. Arbor die displacement, line speed, helical pitch are adjustable.
coil-to-bar process, material diameter: .25-.5''.
Temperature increase at straightening can be up to 200C. Size measurement are done when bars are cooled down to RT.

Swelling occurs at both annealed and cold worked conditions, on almost all metal materials.

Anyone, any comment please? wild guess even is welcome and appreciated!

 

[MagBen]

diameter increase varies from .001 to .004''

 

[IRstuff]

Is that even possible? What's the material? Your numbers make the density go down by 16x, like water turning into styrofoam.

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[MagBen]

This is real! At the beginning I suspected, then started to observe processing and collect data. Materials include all kinds of Austenitic SS, Duplex SS, Co-based high temp etc. I did not collect other materials, but that does not mean they are not swelling.
I requested to shave to the bottom of tolerance prior to straighten+cut to compensate the swelling, but turned out very difficult to control. material expands like crazy, range is often bigger than the tolerance. I have to add a grinding at finish to control size which adds cost.

 

[Compositepro]

Straightnening wire will take the material past its yield point and allow stress relaxation. Drawn wire has a lot of stored stress. Are you stating that the straightening process cause 200C or that up to 200C may be used during straightening?

I assume you are doing something like this

https://www.youtube.com/watch?v=FlXz3f0pP-s

 

[MagBen]

@IRstuff: so you think density decreased? but does not that imply there is some structure change?

@Compositepro: yes, the process is very similar to the youtube video you offer, except that my machine is more powerful.
200C was caused by straightening process
Stress relaxation is the cause for strength decrease if the wire is relatively heavily cold drawn. If material is annealed, strength will be increased a bit. Diameter increase happens on both drawn and annealed wire, stress relaxation is unlikely the cause for size increase. On the contrary, could stress relaxation decrease size (more stable structure)?

 

[SwinnyGG]

You're heating up 1/4" round wire 200 degrees centigrade just by straightening it?

Do you have a jammed feet roller that is 1000 degrees?

 

[EdStainless]

So how much shorter does it get? it should be clearly measurable.
One thing is are you measuring with laser mics?
How round is it before? after? And I mean on a very fine scale.
The density isn't changing.

We used to 'kill' wire if the strength was too high by pulling ti around a tight bend.
The UTS would come down a little, the Yield by a lot.
With annealed material the Yield will not drop.

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[IRstuff]

If the diameter quadruples, where does the extra mass come from? Since there can't be extra mass, then the density must go down, which is unlikely to be possible, unless it's unexpanded Styrofoam that expands. Of course, it's possible the length shrunk by a factor of 16, which also seems unlikely.

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[EdStainless]

We are talking about the OD going from 0.500" to 0.503", of something like that.
This will also happen when way too much squeeze is used in straightening.
Of course the other issue is that you get some size change (and shape change) just uncoiling material.

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We're overthinking this folks.
In general, plastic deformation implies that at least one dimension will be permanently altered.
By straightening the wire you are plastically compressing at least half of the wire. Conservation of mass means it will expand in the other directions.


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

 

[MagBen]

Thanks folks!

@jgKRI: indeed, the temperature may not as high as 200C. I did not actually measure temperature, but saw smoking, burning things sometimes. Guessed up to 100C pretty sure. The rolls were not heat up to 1000C, although I ever warmed the wire up to 800F for a better ductility.

@Ed/Ironic met: so you think length will be shortened to compensate the enlarged in diameter?! I kinda thought, but never tested it, I will try it.

@IRstuff: density decrease is a bold thought. "unexpanded Styrofoam that expands" is a genius thinking!


By the way, recommended "ironic metallurgist" change name to "non-ironic metallurgist". your posts make good/great sense, "ironic" is unfair

Have a good weekend All!

 

[IRstuff]

Oh, I misread the 3rd post, so not as dramatic as I thought. Nevertheless, a density change could still be there, but it would be a much, much smaller change.

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[berkshire]

This may be hard to measure, but do you notice that where you get a diameter increase after straightening a drawn wire, you also get a corresponding decrease in its length ?
B.E.

You are judged not by what you know, but by what you can do.

 

[EdStainless]

This kind of mistreatment is easy to do on thin wall tubing. The tube OD will grow by a few thou, and the length will shorten by inches.
Of course with solid the length change will be less pronounced but with long bars it should be clearly measurable.

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P.E. Metallurgy, Plymouth Tube

 

MagBen,
I chose it myself. Get to know me and you would understand.
It was either that or 'sarcastic metallurgist'.

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