Nitrogen as replacement for CO2 in shielding gas mix for MIG

CO2 is a reactive shielding gas which affects puddle characteristics. If you were to substitute nitrogen you may have to blend in some O2. I can't comment on metallurgical effects. Nitrogen is used as a purge or backup gas in some cases.

Going by the suppliers for welding gases, pure Argon is the recommendation for TIG, 75%Ar/25%CO2 for MIG, but especially for thick/deep welds. Alternatives are mixing in either Hydrogen, Helium or Oxygen, depending on need. Also suggested that pure Argon for MIG is also used.

I don't see any reason to add another inert gas that has a middling atomic weight.

I would consult with a welding expert for your exact weld process.

I've heard that some use some nitrogen in Europe, but it isn't standard practice here in the United States.

Nitrogen isn't inert. It can be used as an interstitial alloying constituent in certain austenitic stainless alloys. I don't recommend it as a purge gas or backing gas when welding austenitic stainless.

There would have to be a strong justification for using nitrogen as a shielding gas and the WPS would have to be qualified by testing to demonstrate the mechanical properties ware not degraded.

The solubility of nitrogen in liquid metal can be fairly high, but the solubility is greatly reduced as the metal changes state from a liquid to a solid. The reduction in solubility typically leads to porosity. Just think of air, it contains mostly nitrogen and roughly 20% oxygen. What happens when the shield gas has insufficient flow or if it is blown away by a breeze? Exactly, porosity. Deoxidizers can be used to counter the effect of oxygen, but there is little that can be done to counter the effects of nitrogen.

Just my thoughts on the subject.

Best regards - Al

Hey Al - hope all is well mate.
Interested in your comment " I don't recommend it as a purge gas or backing gas when welding austenitic stainless."
We used nitrogen as a purge gas for many years on many projects without any significant issues ?
Cheers,
Shane

For nitrogen modified SS grades (any LN, most 200, nearly all 6%Mo, and nearly all duplex) some nitrogen is very helpful.
It is usually less than 5% though and sometimes only 2%.
Though in nearly any SS once you have made the root pass you could back purge with N without issue since it will not contact molten metal. I have seen cases where the roots were all done first and then the fill passes later. They used pure Ar for the backing on the roots and pure N for backing on fill.
You cannot use it with martensitic or ferritic SS as it decreases ductility too much.
N is a powerful austenite stabilizer.

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P.E. Metallurgy, consulting work welcomed

EdStainless,
Very interested in learning more about this (sorry if off topic from the original post).
We used 100% nitrogen for purge on literally thousands of joints, root and fill - 316 / 304.
Visually no difference from 100 % Argon purge.
Is it a metallurgical issue ?
Obviously all WPS's have to be accepted by the Clients Welding Engineer and we never had a rejection based on purge gas ?
Cheers,
Shane

It usually doesn't hurt, but you will end up with welds that are much stronger than the base metal or filler.
Ironically the welds will have superior pitting resistance also.
I have seen cases though where weld porosity resulted.
And this was usually internal to the weld so unless you UT or RT you don't know that it is there.

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P.E. Metallurgy, consulting work welcomed

The OP would like to use N2 as a shielding gas for welding carbon steel (and not as a back purge). Nitrogen would lead to lot of interstitiality in the weld. As an extrme case, nitrides could form as a thin layer around the grain boundary leading to high hardness and brittleness of the weld.

DHURJATI SEN
Kolkata, India

It would be a bad idea to use nitrogen as a substitute for CO2 in the shielding gas for welding carbon steels for the above stated reasons. But to suit your curiosity, run some trials with it on waste carbon steels and see for yourself.