MIG welding stainless steel exhaust tubing

I did a favor in welding a crack at the front muffler flang on an E36 M3 race car. It's been a while since I've welded stainless, so I bought a new spool of wire and used the 75/25 shield gas I use for carbon steel. That's what my older Hobart Handler recommends, and the repair turned out OK, but the welds are supposedly susceptible to rusting. After Googling a bit, it seems tri-mix (Helium-Argon-CO2) is now the recommended gas. Someone is also claiming a mix of 98% argon and 2% CO2 works well. Either way, I'm going to want something better when welding the custom exhaust on my TR6.

As an alternative to buying yet another bottle and paying way too much for tri-mix, I was thinking about teeing a 75/25 bottle with a pure argon bottle (that I already have) and mixing the two with regulators to lower the CO2 percentage to an acceptable level (< 5%). Has anyone tried or heard of this working?

Just use the straight Argon.

I've never had a problem with using regular old ER70S6 and 75/25. Yes it rusts but it doesn't seem to rust any faster than the stainless is going to rust anyway.

The only issue I ever have with stainless is getting the current right. Stainless is a real biznotch to weld, it does not transfer heat well so it is very easy to put too much heat into a given spot and either burn through or just make it droop like putty. So big current to get heat to transfer INTO the metal and then jump around a lot. Keeping the gaps as absolutely tight as possible helps tremendously.

Knurled wrote: I've never had a problem with using regular old ER70S6 and 75/25. Yes it rusts but it doesn't seem to rust any faster than the stainless is going to rust anyway. The only issue I ever have with stainless is getting the current right. Stainless is a real biznotch to weld, it does not transfer heat well so it is very easy to put too much heat into a given spot and either burn through or just make it droop like putty. So big current to get heat to transfer INTO the metal and then jump around a lot. Keeping the gaps as absolutely tight as possible helps tremendously.

That's what I found with the repair I just did. At first I was burning through and couldn't keep the arc going. It didn't help that I was welding up. So I turned up the voltage/wire speed and moved the gun more. Worked pretty well after that.

I'll dig up some old/scrap pipe to experiment with. My thought was that straight argon would be too hot, but it might be just a matter of altering my technique.

I feel no shame in taking some scrap stainless tube (usually chunks I'd just trimmed off of whatever project I'm welding) and practice a bit, get the welder settings right, before it's time for the welds that count. That five minutes spent is super cheap in the long run.

My technique is a little different, I'll weld a short bit then stop, watch the color change from yellow to dark red, then weld another short bit, let the metal cool off to dark red again, etc. Not putting too much heat in but also not letting it cool more than dark red. That way it is easier to start the weld bead up vs. trying to weld to "cold" metal, but it stays cool enough to not burn through.