welding a stainless exhaust....need some help

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What's up guys, first post here.  My welding leaves something to be desired, but it's fun to learn.    I've informally taught myself some welding basics, but obviously need help.  Here's my current problem:This is a 45 degree 3" pipe slip-fitted to a 3" stainless Burns muffler, MIG welded with stainless 0.03" wire and tri-mix gas.  I had trouble adjusting between burn-through and cold welds, and i had several apparent gaps.  I ground down the welds, rewelded, and ground again.  What I can do to make sure the joint is solid, without leaks.  Reweld it with a colder setting to fill in the gaps, or is there a "patch" I can apply to seal leaks and improve strength?    This is a stainless exhaust for a BMW racecar, and the 45 deg fitting will attach to the headers, so it's fairly close to the engine (ie, heat may be an issue in cracking the welds)Last edited by jmitro; 08-08-2008 at 08:41 PM.
Reply:Welcome and thanks for the post.I would grind out any of the bad weld and reweld it. I have never had much luck with the tri mix gases for stainless nor for that matter have I ever liked hard wire GMAW stainless. If you brought it to me I would do some grinding on it and weld it with GTAW (tig) Not any patch that I know of. You are correct to have a concern about cracking. make sure it is not hanging off the header nor off the welded joint. Get a hanger near the front end to support the weight.Luck TJ
Reply:Looking at those pictures I'd say to cut the section out that you welded and have a good welder show you how to splice in a new section...we all can learn from watching an experienced hand. I also wouldn't have a problem doing it with solid wire but it would be easier with tig.Anything worth doing is worth doing RIGHT
Reply:are you by chance back gassing?Millermatic 211weldpack 3200 squirtgunsquare wave 200victory journeyman kitHypertherm 30xptoo much other crap to listtinkerer extraordinaire
Reply:Cut it out and try again would be my best bet. I would use TIG if you have it available. If not you can stick weld SS. Honestly it looks like you have some cold lapping and a general lack of fusion over the majority of the weld area. Try again or bring it to someone and ask if they will let you watch how they fix it.'Mike
Reply:Fat bastard had some great points. Unless you can cut out the section, grind it down, and get a solid run on the seam. I would think a second pass will go better, because you have already filled the gaps. I would try and practice a little on some scrap. I find that SS mig tends to hump up a bit even if you are doing a good job, it is the nature of the beast. Filling a gap will give a flatter weld bead, but it is hard to mig thin SS with a gap. Usually I have better success with lapping over the seam, and I make the joints as tight as I can on thinner SS. After it gets to .090 thick, it welds very dependably. (Mig. Tig works down to .030 pretty well.)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:It is a little hard to tell whats going on,  but I would say leave it alone at this point.You did not tell us how thick the stainless tubjng is.You finished well, but it is better not to sand or grind welds if you want maximum strength.If it really matters to you I would start all over and TIG it with #309 filler.Tack in four places before welding.When you weld an exhaust with mig there may be some metal globs protruding into the gas exhaust flow inside the tube.AWS certified welding inspectorAWS certified welder
Reply:ok thanks.  Forgot to mention, this was 0.06" wall thickness tube.I thought about cutting out the bad section and trying again.....but thought I might grind it down and redo it first.  If I can't find someone to TIG stainless around here, I will either make a second pass or cut it out and take it somewhere for some TIG weldingI'm using a Lincoln 110V SP-135T welder.  I used both B and C voltage settings for the above welds, and wire speed of 2.5 - 4.0 on the machine.  any suggestions on what to use if i make a second pass?
Reply:No point in making a second pass over the existing weld, it's not going to help any.  If there is porosity and lack of fusion in there (which there appears to be from the picture), even if you lay a perfect bead over the top of it, cracks will still propogate from the defects that are already in there.  You'd be light years ahead just cutting the section out.  I wouldn't be so much concerned about the heat inducing cracks as I would the vibration.
Reply:If you're don't have OCD with your welding, it will probably be fine just like it is.  I see a few spots that are low, but I'd bet you have plenty of material thickness to hold what you're doing...unless that muffler will be supported only by the pipe.