This week, we weld stainless tubing.
After talking about scratch start tig welding a few weeks ago, I thought I’d talk about how to weld stainless tubing…a 6g stanless tube test using the same crude scratch start tig setup. It’s a Lincoln 225 AC buzz box with a diy rectifier hooked up to a tig torch. But with a twist.
I made a crude foot switch so that I can stop and maintain shielding gas. It might seem rinky dink, but it really makes a big difference on a scratch start tig rig.
Stainless tubing used for food and beverage applications is mostly done with no filler metal.
Why is that?
One reason is that the main quality issue on food service tubing is the surface of the root pass. Not strength.
The ability of the surface of the inside of the tube to resist bacteria growth is the main goal…
And that means a root pass free from pits, oxide islands, crevices, etc.
A sugared root from no purge is about the worst thing that can happen.
The goal is a smooth and silver root pass with no crevices that can grow critters. Periodic bacteria swab tests months and years after the welds are made will tell the story. If tests are bad, it is very expensive and your reputation is at stake… it’s a big deal.
Islands of oxide added to the puddle from the end of a filler wire can also be a problem. That is one reason why filler metal is often not used. If filler metal is used, make sure to wipe it down and keep the hot end of the rod shielded with the torch argon.
In order to get a good purge, vents must be placed in the right places, to allow the argon to displace the air in the pipe. That’s why I put the vent in the very top of the 6g tube.
To get a good purge, it is helpful to think about argon like it was water.
Argon is heavier than air. It fills a container much like water does. So best practice is to insert the argon hose low, and position whatever is being welded so that the vent is the high point.
Also remember that argon does not support life. Tig welding in a small confined space can kill you because argon displaces air in small spaces like tanks, sumps, etc. Make sure to have ventilation anywhere you are tig welding.
Another thing to remember is what water does to an argon purge.
One drop of water in a stainless tube will prevent a really good purge.
Water evaporates and adds oxygen and hydrogen to the argon and that prevents a silver root.