Backing Gas in SS Vessels
Backing Gas in SS Vessels
(OP)
Why back purging gas is used in SS welding?
Nasir
Welding Engineer
DESCON ENGINEERING LIMITED
PAKISTAN
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Backing Gas in SS Vessels
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RE: Backing Gas in SS Vessels
RE: Backing Gas in SS Vessels
RE: Backing Gas in SS Vessels
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
RE: Backing Gas in SS Vessels
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.
RE: Backing Gas in SS Vessels
RE: Backing Gas in SS Vessels
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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RE: Backing Gas in SS Vessels
RE: Backing Gas in SS Vessels
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.
RE: Backing Gas in SS Vessels
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.