Welding Stainless Steel with Mild Steel Wire


I need to weld a threaded boss into a car exhaust system for an O2 sensor. The boss is stainless steel and the exhaust is stainless, however the wire in my MIG machine is for mild steel.

Question is will it weld? I'm not too concerned about corrosion as it will only be in for a few months whilst the engine is set up but would like some assurance as to whether it will even work?

If it will what gas should I be using? I currently have a CO2 and Argon mix.



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The general rule is that you want to use welding wire that has more nickel and chrome than the metals being welded because some of the nickel and chrome will go away because carbides are formed. So I think using mild steel wire may lead to welds that crack. The CO2 will add to the problems of carbides.

But for a few months you are likely to get by with the gas you have and the mild steel wire. What is the worst that can happen? You have to grind off the welds and redo it with stainless wire and argon.

=20 Dan

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I welded a stainless piece to mild steel with 6013 (it was a custom made "christmas tree" to hold cables, torches and so on on top of my welder).

It is holding great and is not experiencing any stresses.

Seems that your exhaust is not safety critical and does not get very stressed out, so the risk is low and so is the strain.

I would do it, worst case is you will have to redo it and get a new part.

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I would either switch out my gas mix for pure argon and my wire for whatever grade stainless you have (304, 308, 309, 316,...) and weld it using the correct wire, or switch to TIG and use the correct wire.

I think your mild steel weld will hold for a little while, but if you will have more than a few heating / cooling cycles, you will definitely have welds cracking out. The coefficient of expansion is quite different between mild steels and many grades of stainless, meaning that the stainless will expand and shrink more than the mild steel weld will. Cracks will appear at the toes of the weld on the stainless exhaust pipes.

Good luck!

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For what it is worth? I had a stainless aircraft exhaust system temporarily repaired using mild steel wire and CO2 MIG and the weld cracked almost immediately after one or two heating cycles. I repaired the cracked weld without grinding out the mild steel using TIG with stainless rod and argon gas and the weld lasted as long as I flew the airplane which was for several more years.

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"R" wrote: (clip) I repaired the cracked weld without grinding out the mild steel using

^^^^^^^^^^^^^^^^^^ I take it that a crack in the exhaust system would not jeopardize the safety of the airplane.

BTW, Al. Have you considered brazing? That works well on dissimilar materials.

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Leo Lichtman

If it had been a concern the repairs would have been done differently. Thanks,


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