andrew-theasby
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| posted on 20/6/08 at 06:24 PM |
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shielding gas for stainless steel
Is it ok to use co2 on the back of a weld when TIG'ing stainless steel, as i dont have enough t pieces or another regulator to do it off my
argon bottle, but the mig bottle is sat there ready to use. Ill be using argon on the front through the torch, or should i just not bother backing it
up? Thanks
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NS Dev
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| posted on 20/6/08 at 07:36 PM |
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its better if it is backed up, though not mandatory (I have done plenty without backgas)
never tried backing with CO2, certainly it would probably leak through the seam ahead of the weld, and that would then bugger up the tig weld
maybe worth an experiment, but I think it might cause trouble
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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andrew-theasby
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| posted on 20/6/08 at 07:46 PM |
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Ok, ill have a try, just out of interest why does it need backing up when ally doesnt. Does steel? What happens (in theory) if you dont? Thanks
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CairB
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| posted on 20/6/08 at 09:04 PM |
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I'm not sure about CO2 being suitable. I used argon.
The difference between back purging and not is significant if the weld penetrates.
I back purged when I did my exhaust recently, the weld inside was clean and smooth.
Without purging stainless reacts with the air and turns into a substance with the appearance of burnt sugar.
This isn't good from a penetration or stress raiser aspect and would expect cracks to propagate under vibration.
I managed to back purge all the welds except the silencer ends.
I did it by using discs with some fire resistant material sandwiched between, fed with argon through a drilled out bolt joining them together and a
similar blanking plug further down.
The blanking plug fitted nicely into my 'Robinson retriever" - 3 pronged finger picker that I could use to push the plug to a specific
place in the tube and more importantly pull out after.
I did think about rigging up an O2 sensor to check on the oxygen content but found that is was purged sufficiently if I turned on the back purge
betore I put my helmet on and prepped for welding.
Obviously depends on feed rate, volume and leakage path.
Have a read of some of the literature from custom exhaust system suppliers - most seem to back purge.
I reckon that it really is worth the effort. Try some test pieces to see the difference.
Colin
[Edited on 20/6/08 by CairB]
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andrew-theasby
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| posted on 20/6/08 at 09:40 PM |
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Does argon sink or float in air then?
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cryoman1965
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| posted on 20/6/08 at 10:30 PM |
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Sink
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CairB
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| posted on 21/6/08 at 07:31 AM |
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Although denser than the main constituents of air I believe that diffusion and physical mixing are the main mechanisms.
Especially if the gas is introduced with some velocity.
Under special conditions with use of diffusers I believe its possible to create a layer effect but a diffused band builds up at the argon / air
interface.
With back purging I think its more a case of the oxygen content is reduced by being displaced by the argon that is introduced, rather than creating a
totally oxygen free zone.
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