Capping Box Tubing Cautions?????

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I have been working with box tubing lately and have been capping the ends for a professional finish look.  I know somewhere on the net about cautions welding tanks and building up heated gases inside the tank during welding. Does this apply also to other enclosed tubing? DanMy instructor said someone will catch on fire during class...He wasn't joking!!
Reply:You may get blow out at the end of the weld as the pressure inside can punch out. It is more of a problem with tig than mig or stick. A little practice at the end of the weld, and you'll get past it.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:I have welded tubular chromoly aircraft engine mounts and this is a common concern. The way it is dealt with is a small hole is drilled in the tubing about 1/16 or so in a low stress area. The tube is capped off by welding.After the tube has cooled then the small hole is sealed to prevent corrosion.Enough pressure can develope that It will become impossible to close that last hole in the cap you are trying to finish unless you drill a hole somewhere.Some race car frames mount a pressure guage in the frame tubing, so if a crack developes in the frame, it can be detected by a pressure drop.
Reply:what size tube are you using? i have been capping the ends of 1x1x16 ga sqaure tube with no problems. i just cut a scrap piece of 16 ga stock i have laying around, tack it at the corners to resist warping, then lay a contiunous bead along opposing edges. so far, i have not had any blow-out issues. the guys at work are constantly capping 2x2x7 ga 304/304L SS square tube the same way. as far as i am aware, they don't have much of an issue, but either way, the tubes are fairly long, which helps dissipate the heat.Later,Andy
Reply:for almost all carbon steel tubing this is not a problem
Reply:I do end caps on 3" X 8" X 1/4" all the time. Never had an issue with that.  I have had a problem in the past working with old or rusty tubing that has moisture trapped in the crud. The heat from welding creates steam. This will blow out if you don't leave 1/2 " or so till tube is cool
Reply:it all depends on how much welding is done on the tube after its sealed, and how long of a piece it is.I had a problem making a SS cart a few months ago, but I drilled a few 1/8 holes and fixed it.Have we all gone mad?
Reply:The tubing size is generally 2 x 2 and at least 3/16" wall. Some can range from 1-1/2" to over 6 feet. I have not seen any blowout like you describe. I lay some beads let it cool a bit, grind a bit and lay some more beads. I think I will drill a 1/16" holeDanMy instructor said someone will catch on fire during class...He wasn't joking!!
Reply:No.weld all but the last 1/16" then set aside let it cool go back spotit in and finish as you like No mater how much heat you add to during welding you can't build up dangerous pressures.Jimmy
Reply:Originally Posted by worldsgreatestwelderNo.weld all but the last 1/16" then set aside let it cool go back spotit in and finish as you like No mater how much heat you add to during welding you can't build up dangerous pressures.Jimmy
Reply:Large pieces rarely have problems with high internal pressure when sealed, smaller pieces can build up explosive pressure during welding process, just be careful.