Gas trapped in weldment

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At work I had to weld a 1/2"  schedule 160 pipe into a flange. The pipe is .840 diameter and 13" long. The flange is 7" diameter and 1 1/2" thick. The pipe goes all the way thru the flange and gets welded on both sides. One side is a 7/16" fillet weld and the opposite side is 9/16" fillet weld. Material is 316SS.  The tube gets tack welded on both sides of flange and then a 1/8" or so fillet weld around the pipe on one side and then weld the other side. When we went to weld over the root pass weld on the second weld , after about a minute of welding, the root pass got hot enough to where it blew a hole out right down to the tube.   Obviously it happened because of the trapped gas in the air space between the tube and the flange. Because physics never sleeps, the gas that was trapped early on in the procedure increasded in pressure due to the weld heat. As the weld on the second weld got hot enough the metal gave way and blew out. Has anyone experienced this before ? I'm thinking we should not have welded all the way around on the root pass on one side so that as we welded the first final weld it would have allowed the gas pressure to stay at ambient until we went to do side two's weld. Is that a good idea ?We have to, at some point , trap the gas on the part , so not welding it closed is not an option. Is there some other method we should employ ?
Reply:If you could bevel the flange and get a full penetration weld then there won't be any gas to get trapped in the joint. That would be one way to fix it. When I have to do something that I know this will happen with I get to the end of the weld and slowly add filler to the edge of the puddle. Just let it wash down the side into the bottom of the joint untill it fills and covers the "gap" where the gas is trapped. Its hard to explain this so I hope you can understand what I'm saying.Airco Ac/Dc 300 HeliwelderMillerMatic 200 (stolen)Miller Maxstar 150STLMiller AEAD200LE (welding and generating power) Hobart MIG
Reply:Pug,  I understand perfectly. I'll probably try that.The chamfer idea is of course valid but we cannot modify the part.Thanks.
Reply:Originally Posted by rcs9250Pug,  I understand perfectly. I'll probably try that.The chamfer idea is of course valid but we cannot modify the part.Thanks.
Reply:I have inspectors as second skin and then the ultimate tattletale, the XRAYmachine. Tells all.
Reply:When I do these, I leave a little bit of the "root" open for gasses to expand out of as they heat up.  By the time I get the second pass up to where I left the root open, the gas internally has equalized with the temperature of the pipe and can be sealed over without further expansion.Sent from my SCH-I535 using Tapatalk 2
Reply:Thanks Pat,That's what I will be doing on the next go around.