Welding Tips and Tricks

I need to weld stainless fittings to the end of 5/16" stainless tubing with 0.020" wall thickness. The tubing is formed into a heat exchanger and the length of tubing is around 25 feet. I need a fitting on each end. I know I need to shield the back side of the weld, but don't want to fill 25 feet of the tubing with argon. I need to do about 100 joints per week. If I drill holes in a 1/4" diameter copper tube and crimp the end shut, would I get enough shielding on the back side of the joint if I slip that inside the stainless tube? I would connect the other end of the copper tubing to an argon supply. I've found a lot of "back purge" fittings for larger tubing, but nothing for tubing as small as 5/16" Please let me know your thoughts.

I think you would, let the copper tube be long enough to go at least an inch past the weld joint and then plug the end of the 5/16" tube (that the copper tube goes into) so it doesn't draw air in. You may be able to use aluminum foil as a plug or...if you have access to a lathe turn a small plug, that would slip onto the copper tube.

If you have extra 5/16" tube you could setup a couple test joints, but I really don't see any issues here.

I would like to see a photo of what your doing.

Since argon is heavier than air-if you can make a quick fixture to hold the tubing, and slightly elevate the open end-it will allow the argon to displace the air quicker with less argon used. Hope this helps.

5/16 tubing can’t hold that much Argon per foot. I don’t really see the issue or gas consumption being a high cost here.

If you held the tube upright, filled from the bottom, set the flow rate to 2cfh, time how long it takes to feel the gas at the other end (wet your finger and wait for the cool air). I’d bet it’s a few seconds only.

25 feet of 5/16" OD tube with 0.020" wall has ~17 in³ of internal volume. It would take 1 CFH about 30 seconds to fully purge it. At 2 CFH would be about 15 seconds to fully purge.

LtBadd wrote:I think you would, let the copper tube be long enough to go at least an inch past the weld joint and then plug the end of the 5/16" tube (that the copper tube goes into) so it doesn't draw air in. You may be able to use aluminum foil as a plug or...if you have access to a lathe turn a small plug, that would slip onto the copper tube.

If you have extra 5/16" tube you could setup a couple test joints, but I really don't see any issues here.

I would like to see a photo of what your doing.

Thank you for the reply Richard! I think you're right. I'll take a few pictures of the setup and post them as soon as I get this set up.

Oscar wrote:25 feet of 5/16" OD tube with 0.020" wall has ~17 in³ of internal volume. It would take 1 CFH about 30 seconds to fully purge it. At 2 CFH would be about 15 seconds to fully purge.

I should have done a little math. You're right Oscar, this shouldn't be a big deal.