[TimB]

New guy to TIG here:

Reading posts that mention back purging when welding stainless. From the context I take that to mean bathing the back of the weld with argon? Is that right?

Is that just to better mitigate contamination of the weld, or are there also other benefits?

Thanks,
Tim

[LtBadd]

Yes and yes.
It will make the weld easier to do, without the purge you'll be fighting the puddle especially if it's a butt joint.

[Oscar]

Properly done it completely eliminates the contamination of the rear of the weld bead, thereby reducing/eliminating the chance of weld defects.

[tweake]

sugaring can lower the strength of the weld.
but i've seen a lot of products where its not back purged. out of sight out of mind.
don't forget it only matters if you do pull penetration welds.
also you can use things like backing plates which takes the heat out of the back of it.

the problem of course is you use a ton of argon back purging and that gets costly.

[Arno]

sugaring can lower the strength of the weld.

As well as ruining the corrosion resistance of stainless as it boils out all the chromium and nickel on the back of the weld.

Very bad on engine exhausts where the rust/corrosion problem always starts from the inside with the acids in the gasses and the moisture. Gets worse the further back from the engine you go as temperatures drop and water condenses.

If purging is not really feasible or gets too costly because of large sizes then somethings like solar-flux B can help to stop the back side from surgaring. Such a flux is not suitable for food-grade applications though as it will leave a glassy inert residue on the back of the weld that's not allowed on those applications.

Bye, Arno.

[snoeproe]

Not back purging a stainless pipe/tube tig weld will always end up with sugaring on the inside. Sugaring is a weld defect and is not something you want.
I’m a solarB weld flux fan myself.

[Coldman]

You can also purge with industrial dry nitrogen to keep the purge costs down. I've purged hundreds of welds with nitrogen that takes 150psi every day. Gives a slightly gold shiney but clean and hygienic finish. I wouldn't recommend it for nuclear or aerospace but for non-critical work it's fine if you have it on hand. For small jobs or critical, it's easier to just put a dual output regulator on your argon cylinder and purge that way.

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[LtBadd]

Good example Coldman, and something most don't realize.