Not exactly porosity, but..

--Sometimes, just at the end of a bead I'm doing in tig, after I
shut off the power, the bead ends with this thing shaped like a volcano;
i.e. one burst bubble or some such I guess. Going back and trying to fill it
can sometimes be difficult; i.e. I think I've got it, then I turn off the
power and the damned flaw is back! Can someone describe what's going on?
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EXACTLY the same problem I have with some 3/16 x 1" strap Ive been using for this and that.
Once you have moved on up the weld look back to see this bubble form, pop and leave a crater.
Hope someone comes up with an answer.
"I think this is because of your belief in biological Marxism. As a genetic communist you feel that noticing behavioural patterns relating to race would cause a conflict with your belief in biological Marxism." Big Pete, famous Usenet Racist
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I've had the same thing happen to me. Don't know for sure, but am wondering if there is too much heat deing applied at the end of the bead, allowing components of the steel to 'boil' out.
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John Miller
I've had this too a couple of times but I could get rid of it by adjusting downslope to a bit longer.
Not an expert though, just someone who's teaching himself to TIG; with the help of this group.
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I have had problems with that in the past. I think that a combination of: getting stuff really clean. throttling back on the heat as the end approaches holding the torch over the weld puddle until it stops glowing or the postflow switches off solved it.
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I am a LONG way from being an expert at this, but when I asked how to solve this problem of the local folks who are really good at welding, they suggested that at the end of the bead I throttle down the voltage and run the torch backward over the bead I've completed for a "dab" or so. This seems to have helped the end of my beads look a little cleaner, and minimized the "cratering".
Ernie will probably chime in and tell me this is the wrong thing to do (and then I will adopt his suggestion, of course), but FWIW this has helped me.
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Peter Grey
Sounds like Hydrogen to me. Hydrogen is fully soluble in the liquid steel in the weld pool, but as it freezes the hydrogen is expelled, hence your 'volcano'. Hydrogen is not present in your material or filler rod, but may be being generated by decomposition of grease, oil, or water on the job. (or any organic material for that matter!) Try thoroughly cleaning the metal to be joine and mak sure there's no condensation on it by warming it.
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Porosity. Your gas shield is going away before the metal is solidified and the molten metal is oxidizing to form a crater.
When this happens resist the urge to just weld over it. It is better to grind out the crater with a die grinder, then weld it up. Otherwise you are just stirring the oxides right back into the weld puddle.
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Ernie Leimkuhler
I'm really really sure this isn't the case for steel. I studied hydrogen in steel welds for my Doctoral research. Is so for Al apparently - hydrogen in melt causes blowholes. But steel - really strong convincing evidence says no, hydrogen does not get rejected out on solidification. So immune is steel to this fault that you can use cellulosics (xx10's), which deliberately give sky-high hydrogen levels to make the arc fierce - narrow, stiff and penetrative (and get shield without much slag). There is no way that TIG even running across slatherings of grease like for a pure joke could come within a tenth (???) of the hydrogen level cellulosics give.
[reasoning -- cellulose resists breakdown 'til the last second, then shoots down between the steel core wire and the flux covering into the arc - it is very difficult to jack up the hydrogen level in the arc so high without this "injection mechanism"]
I'd ask our keen student of welding aobut slope-down, with view to explanation beng a "shrinkage pipe" to start with.
Richard Smith
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Richard Smith

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