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I was building some lift pontoons for an overloaded patio boat and ran into a problem maybe you can help me with. These are basically rectangular watertight boxes made of 12 ga. mild steel. I welded the inside with mig, then tried to weld the outside with tig. My tig weld is porous, contaminated. I think it is because of slag contamination from the root of the mig weld. The steel is new and as clean as it can be, and when I tig in places I haven't migged the weld is just fine. Is there any way to do this, or is it just a bad idea? I can mig the outside instead, I just wanted a more reliable watertight weld, as well as wanting more tig practice. I was using 2% thoraited (red) tungsten,DCSP, and argon. Machine for Mig was lincoln SP200, .035 wire, C25/AR75 gas. Tig was miller econotig.Bart
Reply:Assuming that you used solid MIG wire, ther would be no slag inclusions, and any silicate would have floated to the top of the bead. If teh back side ws open to atmosphere you could have porosity at the bottom of your foot tht your TIG welding is bringing to the surface. You don't mention what filler rod you are using. 70S-6 may work better thatn 70S-2. I often use 309L when I can't avoid spitting.
Reply:Yes solid mig wire, without back shielding. I'm not sure whattig filler rod I was using, I'll have to look tomorrow, It was clean copper coated 1/16" wire for mild steel, I don't think my welding store has options on mild steel, at least not on the shelf. I don't think it is the filler because I can run a perfect bead with it on any other areas that were not mig welded on the opposite side.
Reply:did you weld everything in position or out?also might have neded to back grind after the mig pass to clean up after the lack of shielding gas on the other side. that could give you a bad base to start your tig on.you say that if you tigged places without migging then it was fine.if thats true then it sounds like the mig was the problem.G
Reply:Back gouge it and then tig it upMiller Xmt 350Lincoln Ln-25Ahp 200xSmith Gas Mixer AR/HTig is my Kung FuThrowing down dimes and weaving aboutInstagram http://instagram.com/[email protected]
Reply:Are you working indoors or out doors?AWS certified welding inspectorAWS certified welder
Reply:we are talking 12 ga metal right? I don't thing back gouging will accomplish anything. I do think you should hit the metal with a flap wheel. I think the back side of the mig weld is being contaminated with atmospheric elements. I would think the metal is different from the raw unwelded edges due to being heated to near melting temps with no shielding gas. You are now trying to tig weld contaminated metal; I would think. You may also be dealing with some elements in the metal that have surfaced due to being in plastic form (slag) which you mentioned.Why Mig one side and tig the other? 12 ga right? open butt, single pass would be ok.Last edited by Scott Young; 03-21-2011 at 07:48 AM.
Reply:Just my opinion but why are you welding both sides? The material thickness is thinner than your corner weld so I can't see a weakness problem there...... Miller Dynasty 350Twenty Six HammersThree Crow BarsBig Rock
Reply:Sorry for the delay, I was using 70S-2 filler. Sanding and wire brushing the surface doesn't help, probably because it is a no gap fit up with 12 Ga. Like I stated, when I weld where I have not Migged the tig weld is fine, So I'm pretty sure it is atmospheric contamination on the backside of the mig weld like some of you thought. Yes, it was indoors, all in good position. As far as why I welded both sides, the corners on these pontoons take more abuse (rentals) so I don't want a comeback. I Tigged one side only because I was trying to let some new welders practice, and I Tig alot of Aluminum, but not much steel, and obviously I needed practice as well, because the contamination caught me by surprise. Thanks for all the replies. Bart |
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发表于 2021-8-31 23:29:00