Doug68
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| posted on 19/10/09 at 02:25 AM |
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TIG Welding Closed Tubes
Hi All,
One of the things that used to happen to me TIG welding closed tube, would be the weld going pop! It took me a while to work out why as I had thought
it was due to bad technique on my part.
What I realized was that the gas trapped in the tube is being heated to the point where the pressure in there blows the weld out.
To fix this is simple, a small hole 2mm or so drilled in the tube elsewhere relieves the pressure and no more issue.
But I can't ever recall reading this as advise or in a book on the subject, so is this common knowledge?
I guess the same thing can happen with MIG, do people have this happening to them when MIGing?
Doug. 1TG
Sports Car Builders WA
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Davey D
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| posted on 19/10/09 at 05:17 AM |
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Ive capped the ends of many many tubes, and i cant say ive ever had that happen to me 
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madteg
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| posted on 19/10/09 at 05:59 AM |
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If you put a little nick in end of pipe at the top and leave it till last when welding it will not blow out, this is where the air inside the pipe
gets trapped and expands when hot and pushes weld pool out.
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907
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| posted on 19/10/09 at 07:04 AM |
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As Madteg says ^^
When doing the closing weld I tend to stop, just for a few seconds before welding the last little bit.
It gives time for the expanding gas to escape and the pressure to equalise before finishing the weld.
IMHO drilling holes will allow moisture into the tube and possibly create a corrosion problem, in say a wishbone for example.
I did however drill a 3mm hole in my chassis tees, so when welded the hole was inside the tee,
but this was to allow the argon purge to reach all the welds.
That's something I wouldn't do on a mild steel chassis but mine was stainless.
It did stop weld end blow-back though.
Cheers
Paul G
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Doug68
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| posted on 19/10/09 at 08:03 AM |
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On the holes, I've plugged those up mostly afterwards and I can do that quick enough for it not to blow out.
the other things are the holes are V small will will get plugged with paint and then theres the 100's of holes for rivets that are going to get
drilled in the thing anyway, these in theory are sealed, but nevertheless there's a lot of holes in the frame.
Also the size of the tube I've found makes a difference, bigger the volume in the tube the less likely I think its going to be to blow out.
Its interesting though to see how other people have dealt with the issue.
Doug. 1TG
Sports Car Builders WA
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tegwin
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| posted on 19/10/09 at 08:10 AM |
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The only time I TIG welded a thin wall stainless tube and needed it to be a "proper job" I drilled a 3mm hole in the pipe and fitted a
pipe connected to the argon tank to back purge the welds to stop them oxidising on the inside...
And then once I was done I welded the assembly bracket for the pipe over the hole..
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ashg
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| posted on 19/10/09 at 08:45 AM |
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weld 99% round let it cool a bit then finish it. thats how i was shown to do it when i did my welding certs at college.
Anything With Tits or Wheels Will cost you MONEY!!
Haynes Roadster (Finished)
Exocet (Finished & Sold)
New Project (Started)
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blakep82
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| posted on 19/10/09 at 09:42 AM |
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and if welding tubes together, fishmouthing etc, for a chassis for eample, drill the hole to fit under the join, that was the pressure travels
throughout the whole chassis. then at the end, just cap any open ended tubes
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