American Welding Society Forum
We are considering changing our shielding gas to a 90% argon/7.5% N2/ 2.5% C02 mix for GMAW welding of 304 and 316 sch 10 stainless pipe. We currently use a helium base shielding gas for our root pass. I have some concerns from a metallurgical standpoint. Based on information I've been reading, introducing nitrogen may reduce ferrite levels in the material we are welding that could lead to the potential of hot cracking. I also have concerns about nitriding affects. We weld to B31.3 and also have concerns about having to re-qualify our procedure. I understand why we are looking to change as helium prices have sky-rocketed just not sure if it's worth the risk.
Any thoughts/opinions would certainly be appreciated. Thanks
Most common blends used for GMAW of SS alloys is Argon + small amounts of oxygen (1-2%). Addition of Nitrogen is unusual. Is there a special reason that you are interested in using a ternary blend with Nitrogen?
Yes, I agree it is unusual except in super austenitic or duplex stainless steel. We like the helium blended gases because of the arc energy which we find helps "wet out" the weld better than dual blends. My main concern is ferrite depletion as all our work is code work and we can't afford to have any in field failures. Any comments on ferrite levels I'm certainly not a metallurgist and can only go by what I've read. Thanks for response.
It is more and more common for fabricators to select shield gasses without Helium for GMAW of stainless.
For Ferritic, Martensitic or Standard Austinitic stainless steels the Avesta Welding Manual says that Argon with 1-2% Oxygen or Argon with 2-3% Carbon Dioxide are recommended equally.
The shield gas selection you mention seems to be overly rich in active gas. What is the reason you have selected it?
Folks have found that the 2-3% Co2 has provided good GMAW-P wetting on stainless and often less oxidation than with the 2% Oxygen mix.
Ed Craig is also a proponent of Co2 in certain Stainless GMAW spray transfer and GMAW-P applications. He has published extensively both online and in text....... www.weldreality.com I strongly reccomend purshase of his Mig Parameters and other texts, and to dig all the free data you can from his very informative website...
As far as B31 goes.... I would suspect that even if you did have to requalify because of the change in gas you would be saving a bunch of money if your production levels are high.
there has been interst (not mine) in this mix from a cost standpoint and according to our supplier (Ed would scream) the arc energy is similar to Helium mixes which we like due to the fact that they seem to wet out the weld better than dual mixes. My biggest concern is ferrite depletion and the potential for weld cracks. As indicated all our welding is code work and any in-field failures is un-acceptable. I appreciate your response
I dont have any direct experience in using Nitrogen for arc welding so the following discussion is primarily theory that will have to be validated with experiments.
Ionization potential of Nitrogen is similar to that of Argon and hence the gas may not ionize and might not be very reactive. Even gases such as Oxygen that ionizes and helps stabilize the arc does not get absorbed much into the weld metal. Additionally, a 7% concentration of Nitrogen is quite low. My guess would be that Nitrogen does not get absorbed into the weld metal in any significant amount. You should be able to measure any changes in Nitrogen content in the weld and compare it to that in the base metal with a Microprobe/SEM analysis. A ferrite number meter reading will provide supporting evidence.
I recently used Nitrogen as a shielding gas in laser welding Aluminum (which is not the recommended procedure) and was pleasantly surprised that there was no measurable Nitrogen pickup in the weld. Arc welding is an arc-based process and the results could be quite different.
You will find in the new AWS D10.18 document for Welding of Ferritic/Austenitic Duplex Stainless Steel Piping and Tubing (I just got an advanced copy so I'm not sure its actually being offered yet-but it will be soon if not-and I highly recommend)that nitrogen in low levels is recommended in shielding gases for duplex SS's as a replacement for N in those alloys that uses N as an element. I know it is not uncommon in GTAW, though I have no personal experience in GMAW. The doc does state that its better with machine applications since the parameters can be controlled better, but doesn't claim it is exlusive to machine applications.
""Based on information I've been reading, introducing nitrogen may reduce ferrite levels in the material we are welding that could lead to the potential of hot cracking""
Regarding nitrogen, the above statement is absolutely correct. Nitrogen contamination in 3xx series and duplex stainless steel weld deposits will reduce ferrite formation and promote solidification cracking. The reason for the above; nitrogen is an austenite stabilizer when dissolved and thus decreases ferrite content, which would promote solidification cracking.
Source: "Welding Metallurgy" by S. Kou
I have to disagree with Kou to a certain extent. Introducing nitrogen in duplex is used for the very purpose of reducing ferrite to get the ferrite/austenite balance as close to the 50% as possible, which is desirable. Thats why its called duplex. If your getting below 50% ferrite you don't need nitrogen in your shielding.
The idea that introducing nitrogen can lead to hot cracking in duplex is almost unimaginable, in my opinion, considering that with 300 series SS's you have to get almost to 0% ferrite (some studies have microfissuring manifest at = or < 2% for most 300 alloys if memory serves) before its considered a serious problem. Many specifications require ~4% mins. and thats with a considerable margin of error to accomodate things such as section thicknesses and restraint, and other contributing factors like high residuals. If your getting down that low in ferrite with duplex you have far more serious problems than nitrogen. Like maybe the wrong alloy all together. And once you get below 30% you're probably going to fail at least 90% of the specs out there. If you're remaining in the duplex ferrite region you are not anywhere near close to 'hot cracking' range.
If you're out of that region your corrosion and strength will most likely not hold up in service, and therefore microfissuring is the least of your problems, considering microfissuring has existed in countless weldments for years/decades without failure (quite common in NiCrMo-3 weldments in AL6XN where CbN contribute to microfissuring).
In fact, if your ferrite is that low, you don't have duplex AT ALL. You have an austenitic.
And another thing, its not really hot cracking in the sense that most people understand that is the problem with low ferrite. Its microfissuring. Which is hot cracking to be sure, but a form that is not visible. Often not even by PT. Hot cracking, usually manifest as centerline cracking or crater cracking (very visisble by PT) is caused more by too high of welding parameters, to high of depth to width ratio, restraint, or insufficent fill on the craters.
The real problem with too much nitrogen is the formation of chromium nitrides which can be selectively attacked in some services.
Bottom line, if you're achieving your phase balance you don't need it. If you need for phase balance don't worry about hot cracking since you have to stay over 30% ferrite by many specs anyway (I think some go as low as 25% but most I believe are actually at ~35% if memory serves).
If you want a definitive source on duplex try Duplex Stainless Steel by Gunn. It ain't cheap, But if your doing lots of duplex I would highly recommend it.
Some despicable bastad went and stole mine, and I really wish I still had it sometimes.
It is fine to disagree.
Well, thats not too condescending. :>)
Did a quick brouse of the Mo Assoc doc. Looks pretty good. Already have it printed up.
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