What IS porosity?

Hi all,
I'm battling a porosity problem that's either contamination, or an air leak AFAICT. I'll figure it out. Anyhow, while spending more time than I'd like
with my grinder, I started to wonder about the nature of the problem. I know that one has to grind out all the porous metal before continuing to weld. It seems that if I don't grind out all the affected metal, then the porosity can just continue to eat into metal that wasn't affected before. IOW, if I apply heat to a small area that's porous it'll expand (indefinitely?) into the surrounding area.
What is it that causes porosity to "expand" (or does it just appear this way to me)? Is there something that happens to the metal during the process of becoming porous that changes its composition? Do the porosities that comes from different problems exhibit different characteristics?
Peter
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If this is on a MIG machine, doublecheck that the gun's back end gas nipple is pushed all the way in. I took out my gun once and put it back in and then had terrible porosity until I realized I hadn't fully seated it and the O-ring wasn't sealing.
GWE
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Grant Erwin wrote:

It's TIG. It's intermitant so if I've got a leak it's small and probably being affected by position of the torch/hose.
Peter
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The first thing I do is to change GAS tanks , I have spent a day with water buckets checking for torch and hose leaks , only to find that I had a Argon tank that was contaminated , supplier told me that this isn't uncommon , some steels especially cold rolled are layered like ply wood and can release material as it's heated , plain water is very bad for TIG , would not eliminate anything . Good Luck Phil

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There are a number of things that could be causing your problems, some of the more obvious are: 1) Do you know what kind of metal your welding on? Make sure its not an alloy that is weldable. Watch out for leaded steel, bronze alloyed steel. 2) make sure all gas connections are tight and not leaking, also check torch lines for leaks as it is notorious for aspirating air into your weld zone. 3) Make sure you dont have any fans or breezes from open doors or windows blowing away your shield gas. 4) Make sure your metal is cleaned of oils or glues. 5) Make sure there is no moisture coming up from your work surface (table). Good luck, I hope this helps

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Porosity usually happens due to contaminants in either the shielding gas or base metal, or a loss of shielding gas. MES and Phil covered the basics of what to look for there.
Don't forget to check your tungsten- if it's oxidized, problem is in your shielding gas.
AWS Welding Handbook, 9th edition, states:
1) Entrapped gas impurities (hydrogen, nitrogen, air, water vapor) 2) Defective gas hose or loose hose connections 3) Oil film on base metal.
Porosity will travel into sound base metal as you heat it to its melting point in the arc. Remember, all gases expand when heated, and by melting the metal, you are giving the gas elbow room to expand into new territory. I've witnessed a test where porosity was started at the beginning of a joint (on purpose), then the gas was turned on, purged, new tungten used, and the joint was welded out from the porosity. It traveled the entire length of the joint, and you simply would have cried if you saw the x-rays! The test joint was on HTS steel, 3/8 inch thick, 12 inches long.
Oh- If you are using CO2 in your gas mix, check your regulators for icing up...
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Note: CO2 is not considered a tig gas, it is an active gas not an inert required by TIG

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TinLizziedl wrote:

So it sounds like eventually the gas that's trapped inside the weld puddle would expand enough to dissapate and stop the porosity (assuming that the rest of the joint is clean and prepped well). I'm just astounded by how far the porosity will infect the surrounding metal. It's like watching some type of disease take hold.
Peter
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Hi
My experience - simple "textbook" case of everything exactly as Peter says http://www.weldsmith.co.uk/tech/welding/weld_misc/040324_TIG_blowholes/tc_blowholes.html
Richard Smith
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Richard Smith wrote:

My pososity looked more like a sponge - smaller holes very close together, but the basic look of the holes is the same.
Peter
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