Thread: "Back purging" tig welding stainless sheet

Originally Posted by johnjohnmulley

Hi



My question is, what is the downside of this oxidisation.

I know it forms very small cracks and weakens the weld by up to 30% apparently, and looks like crap, but, for a non structural joint, does it really matter?

Can it promote "rusting" of the stainless steel, or "corrosion" would be the better term?
Can it completely rust through, even to the other side of the weld which was actually protected by the argon?

Hi ,
If non structural no I would not bother.
no it will not rust through.

If it is food grade and that the surface of the weld in question is exposed to food than that a different kettle of fish.
Cracks ,voids or granulation( sugar like lumps) can give bacteria a place to start breeding.Thats why pipes and the like are back purged -to keep welds smooth-as its imposible to smooth the internals of pipe and tube after welding .

If it's sheet, top surface exposed,well thats ground and polished in any case.
Does this help?
Grahame

grahame
Thanks,
Was exactly what I was searching for.

What if, the piping, or sheet metal object, was carrying water or something else (and not food grade) that would usually rust normal mild steel - and the stainless was NOT back purged and had the sugar lumps on it... Would it then promote any form of rusting etc

And, (another different example) what if the object was undergoing lots of cyclic temperature rises/falls like for example a truck/car exhaust. Combined with the cyclical stresses from the bumps if it ever bumps the ground, or engine movement. There's a specific example for ya.

Thanks again
John.

Originally Posted by johnjohnmulley

What if, the piping, or sheet metal object, was carrying water or something else (and not food grade) that would usually rust normal mild steel - and the stainless was NOT back purged and had the sugar lumps on it... Would it then promote any form of rusting etc

Its more a QUESTION OF WHAT GRADE OF STAINLESS YOUR PLANNING TO USE AND THE ENVIRONMENT IT WILL WORK IN.

And, (another different example) what if the object was undergoing lots of cyclic temperature rises/falls like for example a truck/car exhaust. Combined with the cyclical stresses from the bumps if it ever bumps the ground, or engine movement. There's a specific example for ya.
AGAIN THE GRADE OF STAINLESS NEEDS TO BE SPECIFIC TO THE APPLICATION IT IS INTENDED FOR

John,
Material specs and intended application mean a great deal.

It sounds like you are at pains not to give too much away.

It does sound very automotive, based on exhaust and cooling systems and given the elctrolytic problems that canarise, these are not something I would necessarily wish to advise on.

If its a business venture I would be talking to a metallurgist.

Cheers
Grahame

all 304 grade, with 309 filler
Or 316 grade too, those are my two options basically, or the two most common I would buy

No business venture. nothing to make money off.
I am making an exhaust out of stainless for myself, hence the questions about the cyclic heating and maybe bumping road or movement from engine causing stresses.
The thin stainless sheet questions about rusting was just something that came to my head for another topic thought I would ask about it all here as it's all to do with back purging.

Thanks for your help so far, not giving much away because there isn't really anything to give away, just getting some information and general ideas. Hopefully i've given all the info I can give now (the grades etc).

If its an exhaust surely it needs to be as smooth as possible inside for the fluid dynamics of the flow.
H

yeah thats true, however, the minor losses of the fluid flow against the walls of the pipe would be negligible compared to the flow through the cat converter and or the muffler.

Going to back purge all the exhaust anyway but interesting to know the effect it has on strength etc

If I was making more tuned length SS extractor systems, and I had to make joints in them, I would like to plug the joint from the backside with a split solid plug.

Then the only internal purging of the pipe would be the void between the loosely fitting plug and the pipe.

But does one really need X-ray quality welding on a BBQ flue?

Originally Posted by HammaHed

But does one really need X-ray quality welding on a BBQ flue?

Only if its a tuned length high performance barbecue header flue.

Grahame