Panache
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Work hardening of annealed stainless tubing
Annealled stainless tubing (seamless) must be one of the most useful things for a home laboratory or (like me) a small commercial laboratory with
limited funds.
It has one good bend in it before work hardening makes it stiff as a Siberian corpse.
Just this morning i observed however what appeared to be contrary behaviour with the material and wondered if forum members with more complete
metallurgy knowledge could speculate as to the reasons for this behaviour.
Due to lack of foresight i was forced to muscle a section of annealled stainless tubing (5mm od, 3mm id), that previously had been bent into shape,
and had therefore been work hardened. This section of tubing was ordinarily very stiff and almost impossible to bend by hand (i had tried a few times
previously). This time however the tube was at -90C and it bent as if it had never been work hardened previously. I was expecting to use all my
significant strapping muscles in the task, however all that was needed was a firm wrist to affect the bend?
I would have thought that very cold tubing would have been more difficult to bend again?
Does the cold temperature re-anneal? The tubing regularly cycles between ambient and -90C in a 24hr period.
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12AX7
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What stainless? I wouldn't expect that from 304 or 316. Maybe 17-4 PH. Even so, I think the intent of cryotreatment is to improve hardened
characteristics, which should have little effect...
Sure you aren't simply stronger than you think? 
Tim
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bbartlog
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I can't speak specifically to the case of stainless steel, but there certainly are other examples of metals that become more rather than less workable
at some lower temperature. Zinc for example supposedly is more malleable at slightly elevated temperatures (>100C) but then becomes brittle again
above 200C until it melts at 400C. So it's quite believable that your stainless just happens to be more bendable at -90C...
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watson.fawkes
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I would have to guess that the
cycling caused it. There are two ways that heat treatment works. First, high heat increases atomic mobility enough that solid-state diffusion is
significant. Second, temperature changes create internal strains that act at the microscale, driving lattice rearrangement.
In the present case, each cycle might not do very much in the way of annealing, but over time the cumulative effect could be significant. The time and
energy it takes to do this cycling would make it an exorbitantly expensive commercial treatment, so you'll be unlikely to find a direct comparison,
although possibly in a research paper.
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Panache
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Quote: Originally posted by 12AX7  | What stainless? I wouldn't expect that from 304 or 316. Maybe 17-4 PH. Even so, I think the intent of cryotreatment is to improve hardened
characteristics, which should have little effect...
Sure you aren't simply stronger than you think?
Tim |
Its 316L, i assume the L denotes its heat treatment. I have left the item out of the freeze overnight to see if it bends as easily at ambient. If i
had an instron i could measure things accurately, funnily i don't have one of those lying around, oh wait no there it is next to the GC-MS. This
tardis really is huge inside.
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watson.fawkes
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316L is a lower-carbon version of 316.
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merrlin
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A lower carbon content is desirable when welding or heat treating might produce carbide precipitation that can reduce corrosion resistance.
Attachment: ASM Handbook, Vol 01 - Properties and Selection - Irons, Steels, and High Performance Alloys.pdf (50kB)
This file has been downloaded 6428 times
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