Backing Gas in SS Vessels

[Fizza453]

Why back purging gas is used in SS welding?

Nasir
Welding Engineer
DESCON ENGINEERING LIMITED
PAKISTAN

 

[vesselfab]

so the back side of weld will not be contaminated and "sugar" up

 

[stanweld]

Or in other words, to prevent severe oxidation of the backside of the weld in single side welded, full penetration joints. Backpurging is not required when the root is backgouged to sound metal and reverse side welded.

 

[Fizza453]

Stanweld,

I like to add a little, when two welders at the same time weld a single side full penetration butt joint from opposite sides with an overlap of approx 25-50 mm, in this case back purging is also not required.

Vesselfab,

What is sugar up? is it a specific welding term?



Nasir
Welding Engineer
DESCON ENGINEERING LIMITED
PAKISTAN

 

[racookpe1978]

Imagine the blackened, coarse-crystaled appearance of sugar after it has been melted and burnt in an open flame.

That is what the un-purged, un-protected "back-side" of a stainless steel weld looks like.

So, you have two choices: Grind off the black oxide to pure metal (by getting access to the back of the weld and grionding off what you have so expensively just welded, then re-weld (from the back side) the weld back up to the thickness you want ...

Or back purge and prevent any oxygen from touching the molten hot back side of the weld. The front side is (obviously) already purged with the regular He or Ar purge gas from the welding torch.

 

[stanweld]

When back purge cannot be done, use specially designed GTAW flux coated or flux cored filler metals. Solar Flux B can also be used.

 

[vpl]

This is a curiosity question on my part:

Is it possible to have two people weld a single joint at the same time, one on the front and one on the back?

If you have two people, one on each side, would a back purging gas be needed, or would having the two welders prevent the oxidation?

Would both welders have to be in the same location on the weld at the same time?

(As you can tell, I don't do any welding!)

Patricia Lougheed

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[Metaljon]

VPL I think there may a safety issue with using two welders at the same time. But if it was common practice I would want to be on the outside LOL. I think standard practice is to back purge so the root is sound.

 

[racookpe1978]

I have done it (a double weld) that way - though it is difficult to keep both moving at the same rate, and even harder to have both welders stop and start at "exactly" the same times as they used up weld filler rod and have to shift body positions, etc. Possible - but in practice, very difficult to execute.

I have also done it (back-purging, that is) with a hand-held back-purge hose directed over the (very visible) glowing-red hot spot. That worked adequately, but was better was a welder outside with a purge "can" (a fitted steelmetal "box" folded so one side of the six sides was open. The inert gas filled the box from the hose into a little fitting in the sidewall, but the box could slid along the surface as needed. The gas was held contianed against the wall where the other welder was working inside the exhaust, but the outside welder didn't have to match the position of the "hot spot" exactly. A little leeway in positioning allowed the better back-purge.

 

[racookpe1978]

Answering your implied question about using two welders: Technically, the only thing that is required is that the entire part of the stainless steel that is above the oxidation temperature has to be kept covered by an inert gas - the oxygen in the air has to be kept away from the hot (melted) steel pool until it has cooled down and is no longer reactive.

So, if you have two welders, each with a GTAW torch and each covering his/her side of the glowing red-hot weld pool with is/her inert gas purge, then the weld would be safe and would cool without the black oxidation. Easier to say than to execute.

Now, if you have a weld configuration that doesn't allow the back of he weld to be exposed to oxidizing air, then you're just fine: so a stainless steel fillet weld doesn't need a back purge. A "backed" weld (one with a backing strip) where there is steel behind the weld won't need a backing purge either. Many pipe welds use an a consumable (melted) backing strip such as an EB Ring in an open root pipe butt weld, and those are typically back purged by filling the whole pipe with inert gas.

Safety? You can kill people with a inert gas by pushing the oxygen away from the welder into a low spot in an enclosed space. True, there will be no flame possible, but there will be no breathing either. Air analysis gear needs to read and alarm on both combustible gasses, explosive gasses, poisonous gasses (CO and H2S for example) and low levels of oxygen.