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Old 06-02-2014, 04:38 PM   #1
wdevauld
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Default Stress Relief for Stainless HERMS coil

I've been working on a stainless steel heat exchanger for my brewing adventures. I managed to wrangle together a DIY 1/2 tubing coiler akin to the one that has been around HBT for some time. and and quite pleased at the diameter of the coil I achieved.

Now I want to compress the coil vertically, so that it is as low as possible. To this end I attempted to strap the whole thing as tight as possible using safety wire and then attempted to get it to stress relief temperature for stainless. According to this PDF the stress relief temperature for stainless (316) is 850F-1100F. Now the 'self-clean' mode of the average home oven reaches 900-1000F so I thought I was golden. Pop the strapped coil in the oven, and turn on the self cleaning cycle. At the end, unfortunately the coil just popped open like a spring, and now has a slightly brownish tinge to it.

Can anyone with insight into metallurgy tell me where I went wrong, and what I can do to get this coil to be as flat as possible in its relaxed state?

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Old 06-02-2014, 05:13 PM   #2
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I would try using stainless steel wire. They sell it in spools. Use the wire on multiple wraps and just keep it on there. No need to attempt to naturally compress the coil.

Good luck!

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Old 06-02-2014, 06:29 PM   #3
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LandoLincoln, I have stainless wire (Safety Wire). It's 0.02 diameter, it is what I used to hold the coil when I put it into the oven.

I was hoping to have the coil without any wires, which is why I asked. Holding it together with the safety wire is my fallback if I can't get the heat treatment to work. Someone with experience with metals must be able to point me in the right direction.

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Old 06-02-2014, 07:06 PM   #4
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Quote:
Originally Posted by wdevauld View Post
LandoLincoln, I have stainless wire (Safety Wire). It's 0.02 diameter, it is what I used to hold the coil when I put it into the oven.

I was hoping to have the coil without any wires, which is why I asked. Holding it together with the safety wire is my fallback if I can't get the heat treatment to work. Someone with experience with metals must be able to point me in the right direction.
Yeah, I don't think it's gonna happen. Your choices are 1) just make a new coil, but with a roller that doesn't put so much space in between the coils in the first place, 2) buy a new coil from stainlessbrewing, because they DO have a pro machine that coils it nice and tight, 3) invest in some welding gear and heat the coils up until they're hot enough to warp but you'll have one hell of a time keeping the coils from going oval or flat on you, 4) give up and go with the stainless wire.

But I hope I'm wrong and someone comes up with an option 5 for you.
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Old 06-03-2014, 02:38 AM   #5
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This is what I did to support my coil, but it may also work to hold yours together.

http://www.homebrewtalk.com/f170/keg-ekeggle-conversion-pics-259848/index4.html#post5560686

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Old 06-03-2014, 03:20 AM   #6
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I agree with LandoLincoln.

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Old 06-04-2014, 08:21 PM   #7
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Compress it to the height you want, and Tig weld some narrow stainless straps 120 degrees apart.
Put the straps inside the coil, so you can look from outside the coil to see if it's hiding any gunk.

We have a 1200 degree oven here at work for heat treating, of which we have used for a few "G-Jobs", and I don't think you'll be able to remove as much of the "springback" from your coil as you wish to, without securing it.

No Metallurgist here, just a lowly Aircraft Sheet Metal guy.

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Old 06-12-2014, 04:52 AM   #8
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Default Stress Relief for Stainless HERMS coil

If going with wire, what gauge is suggested?

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Old 06-12-2014, 10:13 PM   #9
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.032 Stainless safety wire should work for ya'.
.041 a little stouter, but more of a pain in the butt to work with.

Do ya' have any?

Here's but one source:

http://www.aircraftspruce.com/catalog/appages/safetywire.php?clickkey=4386

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