Use of Nitrogen as Backing Gas 2

N2 is not a inert gas.

Regards

In case there is still doubt after r6155's comment:
N2 is not an inert gas.

Nearly all alloys that we weld have a significant solubility for nitrogen.
As a weld gas I never go above 3-4% even for DSS or SASS (2% is more typical).
For backing gas it depends on if we are talking about the root pass or fill passes.
For fill passes I wouldn't care either way
For a root pass you are likely to end up with porosity and/or cracking issues if you use straight nitrogen.
I have done it in a pinch doing autogenous welds with zero gap in Nitronic 40.
Very much not recommended.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

I will have to get my money back for being taught that Nitrogen doesn't react with the weld pool (and shields it from oxygen)

Also I will need to edit a whole lot of websites...

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1709585464/tips/SHIELDING_GASES_FOR_STAINLESS_STEELS_ep1o9k.doc[/url]

Interesting subject.
I also have always been of the understanding Nitrogen is acceptable as a backing (purging) gas.
The attached from Sandvik seems to support that.

The Sandvik page says purge.
But do they mean for the root pass or only for fill passes?
I stick by what I said earlier.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

Ed,
Not wanting to argue - this is just one of those things that is not conclusive.
So, good to discuss.
This is from a White Paper by Dr. Michael.J.Fletcher - M.Sc Metallurgy

The majority of major users concur that additional nitrogen in the shielding and purge gases is essential to compensate for loss of nitrogen in the weld and heat affected zones in duplex stainless steels. There is however little consensus on the optimum amount of nitrogen required.

‘..too much nitrogen will increase tungsten wear, and it is not to be recommended in the majority of cases..’ (Wiktorowicz and Crouch,- Welding Research Abroad, 1996).

Sandvik recommends 100% nitrogen for purge gas when welding SAF 2507 super duplex steel.

Outokumpu suggest the use of 3% nitrogen in the purge gas, but claim that pure nitrogen can lead to excessive levels of austenite in the root pass.

A private communication from a USA source states they rarely use more than 3% nitrogen during purging to avoid introducing porosity.

Sandvik & Outokumpu are two world leaders in S/S welding and yet they have totally different views on the use of Nitrogen.

The effect of PURGING GAS on 308L TIG root pass FERRITE CONTENT
by Eva-Lena Bergquist, Tapio Huhtala & Leif Karlsson
Argon is commonly used as root shielding gas in TIG-welding of stainless steels. As argon world-wide is in short supply and prices are rising, it is of interest to investigate what effects changing to alternative root shielding gases might have on weld metal microstructure. Manual TIG welding was used for root and hot pass welding in 304L pipe and plate material using Ar2 as shielding gas and Ar2, N2+10%H2 or N2 as root shielding gas. A significant increase in the nitrogen content of root beads was found when changing the root shielding gas from pure argon to a nitrogen-rich gas. Typically the root bead nitrogen content increased from 500–600 ppm for argon shielding to 1 100 ppm with nitrogen as root shielding gas. As a result the ferrite content decreased with up to 5 FN from about 8–9 FN to 3–5 FN. However, no indication of hot cracking was found and all root beads solidified as predicted by the WRC-92 diagram with ferrite as the leading phase. It is suggested that typical 304L steel and 308L consumable compositions will permit use of nitrogen-rich gases for root shielding without a significantly increased risk of hot cracking. However, the increased nitrogen level must be considered in the choice of steel and consumable. It is advised to use the WRC-92 diagram to make sure there is sufficient safety margin for actual compositions and, if possible, check root bead ferrite content.

Just to clarify - seems to be different terminology in different parts of the world.
Shielding gas - the gas coming out of the torch is the same all over the world.
However, Backing gas, Purging gas or Root Shielding gas (as noted above) are the same thing but different terminology.

Ed, I appreciate your qualifications and have seen multiple highly informative posts from you over the years so I certainly do not want to get into an arguement over who is right and who is wrong.
I think I could find half a dozen research papers stating nitrogen is an acceptable purging / backing gas.
Then I could find half a dozen research papers saying it is not acceptable.
Look at the Sandvik / Outukumpu situation as an example.
Where does that leave me as the Clients Rep when the Contractor wants to replace 100% Argon with 100% Nitrogen for purging / backing gas ?

State what you find acceptable for FN. If this doesn't matter to you, you'll also have no interest in what gas the contractor uses for backing/purging.

If the FN is acceptable, and the NDT will catch porosity then go for it.
But the NDT is critical. Are these welds going to have 100% ID visual inspection?

The only alloys that I have used straight N2 on have been ones with 0.50% N2 content to start with.
Increasing FN in these alloys can significantly lower the corrosion resistance of the welds in even mildly acidic environments.
And the high N welds will be much harder than base metal.
I don't get the cost issue with Ar.
After all you are only purging a few inches of tube for each weld.
OK, in pipe it may be a foot of pipe.
This is what backpurge dams are made for, you don't purge the whole line.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

I always figured that if the Owner can afford to pay the premium to use stainless steel instead of carbon steel, they can afford the cost of argon to shield the root side of the joint.
If I recollect, nitrogen is a strong promoter of austenite. A small increase in nitrogen in the root bead is going to promote a higher Nickel Equivalent. Potentially, that could result in an undesirable morphology.
While one can “get away” with a nitrogen purge with certain stainless steel alloys, it may not be used with the same level of success with other stainless alloys. Welders, not being metallurgists, often base what they do on the current job on what they did on the last job. Yes, they are supposed to follow a WPS, but what do they when the specified purge gas isn’t available? That’s when schedule slips and the trouble starts.


Best regards - Al

Kingnero / Ed / GTAW,
I am not in any way advocating the use of Nitrogen.
Just promoting discussion regarding the conflicting information on the internet - a lot of it from seemingly very knowledgable people and companies.
Just Google - "Is Nitrogen an inert gas ?" and you have Yes, No, Sometimes.

Universal Industrial Gases
Gases Which Are Valued for Inertness:
Inertness is a somewhat relative term and concept. Some industrial gases (helium, neon, argon, krypton and xenon) are almost totally inert. Helium and argon are commercially available in relatively large quantities. The more expensive inert gases such as neon, krypton and xenon have much more limited availability.

Many applications requiring an "inert gas" use nitrogen or carbon dioxide for that purpose. While not truly inert, they have very little reactivity under normal pressure and temperature conditions and are much less expensive than the other "inert" gases.

The naturally inert or "noble" gases are members of "Group 18" of the Periodic Table. They have their outermost, or valence, electron shell complete (with two electrons for helium and eight for the other gases). The "noble" gases are all monatomic.

Nitrogen (N2) and Argon (Ar) are commonly used in the gaseous form to shield potentially reactive materials from contact with oxygen. Nitrogen will react with oxygen at very high temperatures, as in furnaces, but it is inert under most other circumstances. Argon, helium, neon, krypton and xenon are "noble gases" that are extremely inert under all conditions.

My understanding is Nitrogen is inert under "normal atmospheric conditions" but becomes reactive at a certain heat (no idea what that temperature is).
I presume that is why it is not used for shielding gas.
But would the heat of a weld cause a purging gas or backing gas to become reactive ?

Example, we would anneal 304L at 2000F in 85N/15H and get a little N2 uptake. 316L would get 1950F in same gases.
The surface in about 1/3 of a grain would look sort of sensitized (CrN in GB), but there were no corrosion issues because of the high temp allowing Cr diffusion so there was no local Cr depletion.
With other alloys (321, 316L, AL-6XN) we had to be more careful either using less N2 or lower temps.
321 we often annealed in 50/50 gas at 1850-1900. (Cr-C-N compounds would form)
The issue with AL-6XN was that the anneal temp was 2200F and it needed at least 85H/15N (we often ran 100%H2 as it was easy) to prevent too much N2 uptake. (all sorts of weird intermetallics)
This all in solid materials.
In liquid these alloys will absorb a lot of N, much more than can stay in solution as a solid.
This is where problems arise.
In duplex SS or PH SS you have real issues because they rely on delicate phase balancing.
Any slight favoring of austenite by added N2 will likely destroy the corrosion/mechanical properties.
So yes, nitrogen is inert to a point.
The temp and alloy involved are both part of the balance.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

Ed,
My exposure to the use of nitrogen as a backing / purging gas over the years has been limited to basic 304 / 316 weld joints.
Not 100% conversant but I do understand a bit about required FN content of the completed weld - especially in DSS and SDSS.
Just find it very strange that Sandvik and Outukumpu are publishing recommendations that seem to be totally opposite to each other ?
Cheers,
Shane

Seeing two big time players disagree so clearly does make life difficult.
304L is likely the alloy that this would impact the least.
As you get to alloys with Mo the risk of intermetallics goes up.
And in DSS adding too much N2 will skew the A:F ratio and ruin corrosion resistance.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

I look at it this way. If nitrogen was inert, it shouldn't combine easily with other elements to form chemical compounds. So, the fact that we have ammonia, nitrous oxide, and numerous explosives containing nitrogen as a basic building block, it must not be inert.

Best regards - Al