Newbie with a question about MIG and stainless

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Hello,I'm rather inexperienced at welding and have a question. I just put a new stainless steel Magnaflow exhaust system on my '68 Mustang, and need weld some hangers and a few of the joints. I've got a Lincoln MIG that I've used on steel before and want to know what I need to do the work on the exhaust. Using the wire that came with the welder, I laid down a bead on a piece of the scrap tubing and it seemed to take it just like my practice steel. Should I get some special wire? How bout the MIG gas? anything special? If I can't do it with this setup, I'll probably take it to a muffler shop. For the small amount of welding I need, might not be worth buying a lot of supplies. ThanksJohn
Reply:Welcome to Weldingweb!In a bind, you can use the regular ER70S-6 wire but it will lead to corrosion and eventually cracking or breaking. The GMAW SS wire, read: MIG Stainless wire, is rather expensive but it is the proper way to do it. It is recommended to use straight Argon with it as well. By the time you buy or lease the bottle plus buy a 2lb. spool of 308L wire, you'll end up spending near or over $100. If $100 isn't bad, then do it yourself, otherwise take it to a shop that will do the Stainless and keep your car looking rust free.John -  fabricator extraordinaire, car nut!-  bleeding Miller blue! http://www.weldfabzone.com
Reply:The recommended gas mix for welding stainless steel with the small Lincoln 120v MIG machines is 98-2 Ar-O2, not 100% argon.  That's with 0.030 inch 308L wire. (example machine picked was Weld-Pak 3200HD, late model operator manual IM786 page 28 Table 1).Or a 90-7.5-2.5 helium-argon-CO2 tri-mix gas (SP-175+, operator manual IM610, page B-6 Table B.1).Pure argon is a recommended gas to use for MIG on aluminum, but it is not a recommended gas for any steel (mild, alloy, stainless, etc) MIG welding.Some stainless wire may also list the use of Ar-CO2 mixes with -low- CO2 amounts (typically less than 5%, more often 2-2.5% max) for single-pass stainless steel welding.  Higher CO2 percentages and/or multi-pass welding increases the carbon pick-up from the gas into the weld pool, and that kind of ruins the "stainless" aspect of the stainless steel.Yes you can weld stainless together with the usual ER-70S-x wire and your usual C25 gas, but the weld will not be stainless and will rust.  Sometimes pretty quickly!Supplies-wise, you would need a spool of stainless wire and a cylinder of the appropriate shielding gas (tri-mix, 98-2 Ar-O2, or possibly 98-2 Ar-CO2).  The big cost will be the cylinder of gas, unless you have a cylinder and swap it for  a cylinder of 'stainless' gas.
Reply:Don't mean to be contradictory, but 100% argon will work just fine on SS thinwall exhaust pipe welded to some (hopefully SS) hangars and to itself.You're right that the penetration profile will be very narrow using 100% argon, but this ain't a battleship.  Know what I mean. Favorite right now is a Miller Syncro 200.Tons of tools and I blame at least one of them when things don't go right.
Reply:Not being contradictory, but have you found any wire supplier who recommends the use of pure argon for MIG on steel?  Miller, ESAB, Lincoln all list pure argon for MIG on aluminum or reactive metals, but not for MIG on steel.I tried, I asked point blank about the use of pure argon for MIG on steel, the answer was "Nope, plain argon gas is not recommended for MIG welding steel."You could weld some stainless steel pieces together using ER70S-x wire and pure argon, but that doesn't mean it was done right.The wire maker isn't going to back the weld done with your own idea of procedures and gases.  I figure they've tested and experimented and measured more gas and wire and procedure and environmental conditions and combinations than I'll be able to.  I'll take their recommendations.
Reply:Unless the entire system is SS, from hearers back to tailpipe, then some of thew junctions will be SS to mild or aluminized steel. If that is the case, I would weld it with 70-s6 rather than 308 filler wire. You will get more cracking with the SS filler to mild than a mild filler to SS. Do it all the time, and yeah, eventually it will rust. Like in 10 years or so. By then, you'll probably sell the ride.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:For that situation, since this is a one-shot deal and you dont appear to have a need for buying another gas cylinder for other purposes, I would weld it with regular steel wire (ER70S-6) and steel-mix gas (argon/c15 or argon/c25) and paint the welded area with high-temp header paint or maybe some of that paint-on zinc oxide coating to slow down the rusting.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:I welded a stainless turbo downpipe with mild steel Flux-Core before, and it held together just fine and didn't discolor any more than the stainless itself after a few thousand miles. This was even when I was first learning to weld so the welds were of far lower quality than they would be now.