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Author: Subject: Work hardening of annealed stainless tubing
Panache
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[*] posted on 25-10-2009 at 14:44
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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[*] posted on 25-10-2009 at 14:50


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? :P

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bbartlog
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[*] posted on 25-10-2009 at 20:38


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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[*] posted on 26-10-2009 at 05:17


Quote: Originally posted by Panache  
The tubing regularly cycles between ambient and -90C in a 24hr period.
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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[*] posted on 26-10-2009 at 14:34


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? :P

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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[*] posted on 26-10-2009 at 18:35


Quote: Originally posted by Panache  
Its 316L, i assume the L denotes its heat treatment.
316L is a lower-carbon version of 316.
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merrlin
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[*] posted on 26-10-2009 at 22:48


Quote: Originally posted by watson.fawkes  
Quote: Originally posted by Panache  
Its 316L, i assume the L denotes its heat treatment.
316L is a lower-carbon version of 316.


A lower carbon content is desirable when welding or heat treating might produce carbide precipitation that can reduce corrosion resistance.

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