Don't worry about what welders think. They always think they know more than anyone else. Thats just welders. I was one for almost 20 years (giving away my age a bit).
In my current shop I'm lucky if all I get is condescending smiles upon offering advice.
Its just the nature of the way things are.

Hey Ray,

great topic, great answers, great thread!

Perhaps you can imagine why I have stated "Welder" as my occupation and not "Consultant" or "Surveyor"?

Fine, please allow me to add on some additional words.

Sometimes I am a bit surprised. Often I can recognize that people are talking about the "how" in handling specific sequences in welding. Asking questions to "smart" people and - in best case - receiving answers for being able to continuing their work. I have experienced these situations personally when I have discussed different issues of welding with welding engineers working in the field of industry where I have to execute my job. Problems and questions were discussed for finding solutions and answers and to improve specific circumstances within the production. Not seldom it happens that a few months or years later I am confronted to the same questions and problems. Sometimes due to the engineers, I have cooperated with, have made their "career" and went away, but not seldom I have to discuss the same issues again with the same people having the same problems and questions, been already replied months or years ago. I guess this is one of the main reasons Ed Craig's website is so successful. He is providing information to the people whenever they need it.

However, what I personally miss - although the  w e l d e r s  are the ones who have to transform, what the engineers have fixed within the WPS' or whatever - is the question "Why?".

"Why do things work as they work?" "Why is the arc "looking" different under different circumstances?" "Why does the arc act different on different materials?" "Why have different shielding gases different properties?" "When can we observe a different arc-behaviour when changing the peripheral conditions of welding?" etc. etc. Too many questions to be answered in only one lifetime and certainly enough to remain for next generations of Welders and of course - Researchers.

I am honest in some cases the "welder" in relation to his job probably wants only a short answer to continue his work. But not only the welders do so. When I speak to the heads of welding shops having stringent "inquiries" and "demands" in terms of improving their welding-quality, I often try to show them that welding is much more than what can be observed when the arc is ignited and "burns". Also these people often tell me "Please, Stephan stop talking about physics! We do only want to weld, and... we want to weld faster!"

Funny? But that's the way it - unfortunately - goes...

Well, I would like to ask for being allowed to add some real elementary information about the use of shielding gases and in particular for adding some elementary physical information to your topic. I am sure, it will be better to convince your welders of what you are willing to do, than to trust in their natural character of knowing always everything better and - heaven forbid - to be hated by them not only now but also in the future. I think they would have deserved it, to talk to them about the "Pro's" and "Contra's" of your decision to use an Ar/2%CO2 shielding gas mixture.

Well, I have truly considered a longer while to write something on your topic, since actually by having the Internet today - I guess - nobody would really need a hint in the usage of welding shielding gases via the AWS-Forum. Since everyone is having the chance to gather every imaginable information on every imaginable question - as far he or she may have the patience of "googling" long enough. There are Shielding-Gas Manufacturers having excellent websites and providing excellent information. But nonetheless, the AWS-Forum is being used as a platform for asking questions and receiving answers, being used by welders, engineers or even academics, having the pleasure to share their knowledge and experience. This is, what no kind of company-website can provide and this is, what makes the forum so unique. Thanks God and the American Welding Society!

I request your understanding for that I would like to deal subsequently only with the field of shielding-gases for high-alloyed steels and here again, only with the field of the physical differences between Ar/O2 and Ar/CO2 mixtures (I guess for everything more I probably would meet with criticism again). I hope that these information may be helpful to show your welders that you are on the right way when considering to use Ar/CO2 but also to see, that your welders are not entirely wrong when they are considering to use Ar/O2.

Firstly I would like to mention (once again) that we are far away from "knowing" what really occurs within the arc. By the way, I have attended a national arc-physics-conference only a few days back and there, where we have also discussed about the influences of shielding gases on the arc, this predication could be confirmed again. Actually we do have some theoretical models being only parts of a puzzle and only survivable under very specific peripheral conditions. Lots of work to do for the future and what we can do currently is just, to having a deeper look into the in general known physical properties of gases and their behaviour at elevated temperatures. Hereby, many of the required replies to those questions, being asked in terms of shielding gases and their influence on welding properties, can be approached in a sufficient amount. For having an idea of how the shielding gas "reacts" when changing the peripheral conditions. Normally the coherences within an arc column are not quite easy to recognize, among others due to the arc as we can see it, by using our visual senses, is not the arc as it really exists in the world of plasma-physics. This means, the optical spectrum of the arc as we see it does not match with the spectrum of its real nature (our eyes do visualize the arc "linear" but its nature is "logarithmical"). The measuring of what occurs within the arc column is intricate to perform, and thus the physicists today do use a great amount of mathematical formulae to express the sequences. No, I won't use any kind of these formulae herein, the reply should stay comprehensible.

Monatomic (Inert) and Diatomic (Active) Shielding Gases

In general one has to distinguish between monatomic and diatomic shielding gases. Monatomic gases in terms of welding include Argon and Helium, which are also called "inert", due to their ability of non reacting with the molten weld-pool. Diatomic gases in terms of welding are among others and as already stated by you, CO2 (Carbon dioxide) or O2 (Oxygen). Diatomic gases have, compared with the first group, the ability to "react" with the molten weld pool, and are named - as already well known - "active" gases. Both - monatomic and diatomic - can be mixed and thus forming shielding gas mixtures, as the ones you have mentioned in your topic. It is important to know that there is - of course - a physical difference between the inert gas(es) and the active constituents of a shielding gas mixture. When we are talking about GMAW of high alloyed steels in grades you have mentioned (mainly using solid wire electrodes) intermediately two main kinds of shielding gases have proven to work properly. Ar/CO2 and Ar/O2 mixed gases. Both types of shielding gas containing oxygen. Both types can be used for GMAW of these base materials but where - so must from my point of view the question be - where are the specific differences as far as there even are some.

To (simplified) answer this elementary question I would like to carry out a short description of the mixed gas constituents and their crucial specific physical properties.

Ar (Argon):

-  Inert, i.e. no chemical reaction between the arc atmosphere and the molten metal
-  Heavier than atmospheric air, i.e. good shielding of the molten pool
-  Compared with Helium easy to ionise, i.e. improved arc ignition properties

Differently to GTA-Welding, pure Argon is normally not being used for GAMW, although it would be possible in general, and there was a time also pure Argon was used for welding high-alloyed steels. Due to Argon has a relatively low thermal conductivity, please see also the attachment Heat_Conductivity_jpeg, the heat input into the base material is relatively low. One has to know that the ability of conducting heat is one of the crucial properties of a shielding gas in general, since hereby the energy being generated within the arc column is transferred into the base- and filler material. Since high alloyed steel molten beads have in general relatively high surface tensions (somebody may correct me if I'm wrong) and the heat transfer via the arc column is relatively low, the molten pool has a high surface tension gradient which leads finally to bad wetting properties of the molten metal. The droplet transfer is being hindered, a higher amount of spatter can be observed, the seam appearance is comparably poor with varying depth of fusion, and the ability of using the process in all positions is restricted.

O2 (Oxygen):

-  Chemical "active" i.e. strong oxidizing reactions with the molten weld pool (2...3 times higher than CO2!)
-  Arc stabilizing
-  Strongly reducing the weld pool's surface tension (improved wetting)
-  Susceptible for increasing the weld pool's gas content (larger amounts of porosity)

By adding Oxygen to the Argon (for welding "normal grade" high alloyed steels) in an amount of 1... 3%, the arc stabilization is improved. Furthermore the mentioned contents of Oxygen cause a reduction of spatter. By having a look onto the diagram (Heat_Conductivity_jpeg) one can recognize that O2 has a relatively low thermal conductivity. An additional aspect in terms of using different shielding gas constituents (inert and active) is the fact, that different physical mechanisms do occur when the arc firstly is ignited. These different mechanisms are called

-  Dissociation and
-  Ionisation

In order to keep my entire reply simple, I would like to avoid the in depth physical treatment of both mechanisms, which would need a specific amount of mathematical formulae. Therefore I would like to deal only with the both mentioned mechanisms by explaining their fundamentals. Normally a shielding gas - whether inert or active - has a chemical neutral character, i.e. it has no electrical conductivity (under normal conditions). When elevating the temperature of the gas, the movement of its particles is accelerated up to a point where a diatomic gas looses its molecular character and is being separated, which is called, the gas is being "dissociated". This means "nothing else" than that a molecule O2 (Dioxide) is split - dissociated - into two atoms of Oxygen (½  O2). The energy being necessary for providing this dissociation is called the specific Dissociation Energy and is a specific gaseous constant. It is measured in the unit eV/molecule, which means electronvolt/molecule (1 eV = 1,602·10^-19 Joule) and one can see that the unit "eV" is nothing more than another expression for Energy or Work (Joule). For separating one "O2" molecule into two Oxygen atoms, a specific energy of 5.1 eV is necessary. First when the dissociation of the O2 molecule has been performed, the second step can occur - the ionisation of the gas. Ionisation means, to make the formerly electrical neutral gas, electrical conductive. This is, separating one or more electrons from its outer "shells" and "use" them for carrying the electrical charge within the arc column and thus obtaining the physical state of a plasma. Every gas has also specific "Ionisation Energy Levels", which are likewise measured in unit eV/molecule (electronvolt/molecule). O2 has an Ionisation Energy (for the first stage = the first electron) of ~ 13.6 eV/molecule and one can see that the energy of ionisation is significantly higher than the energy for dissociating the molecule into its separate atoms. Argon however, has an Ionisation Energy of ~ 15.8 eV/molecule, whereas Helium, as another important shielding gas, has an energy level of ~ 24.6 eV/molecule which is significantly higher compared with Argon. Basically one can say that, the lower the Ionisation Energy of a shielding gas is, the better are the arc ignition and the arc-stability, which can simply been proven in case of using Helium, which needs a higher amount of arc-voltage to establish and stabilize the arc. Finally we can state that the » dissociation energy « levels for diatomic (active) shielding gas constituents are lower than their » ionisation energy « levels. Fractions of the arc-plasma's energy have thus to be used for dissociating the diatomic molecules before their ionisation can occur.

What can be observed when using Ar/O2-shielding gas mixtures for GMA-Welding high alloyed steels, is a relative (compared with CO2) increased susceptibility for porosity, higher amounts of surface oxidation (which can finally mean also a kind of alloy elements burn off and which must be removed again) and, compared with CO2, a relatively low heat input, by the comparably low thermal conductivity of Oxygen (see diagram).

CO2 (Carbon Dioxide)

-  Chemical "active" and thus reacting with the molten weld pool (Oxidation does increase with the amount of dissociation)
-  Arc stabilizing
-  Dissociation within the arc atmosphere: (CO + ½ O2) i.e. volume growth
-  Recombination of CO + ½ O2 to CO2 and thus release of recombination heat, i.e. improves depth of fusion
-  Able to reduce porosity

Particularly in Germany shielding gas mixtures of Argon + 2... 2.5% CO2 are intermediately those ones, being mainly used for GMA-Welding standard grades of high alloyed steels. And the success in using them, could confirm this evolution in that specific sector of welding. CO2 as an additional active constituent to the Argon combines different issues in a perfect manner. Higher thermal conductivity of the shielding gas (see diagram) compared with Oxygen and Argon, lower dissociation energy (CO2 = CO + ½ O2 at 4.3 eV/molecule vs. 5.1 eV/molecule for Oxygen), increased thermal input by the recombination of CO + ½ O2 = CO2 = 4,3 eV (~ 6,89 * 10^-19 Joule/molecule additional heat energy) reduced oxidation by having only approx. ½ of the CO2 as a free active constituent of Oxygen and, a reduced amount of - and susceptibility for porosity, respectively, compared with Oxygen.

Therefore, Ray, although there would be so many more interesting things to talk about in terms of shielding gases and their physical coherences when used in GMAW (I don't want to overwork your patience), finally I would reinforce you in your decision to try out 98Ar/2% or 3% CO2 shielding gas for GMA-Welding the steel grades you have mentioned. The benefits in using Ar/CO2 mixtures have been proved to work excellent in many applications. But nonetheless, as I have already mentioned at the beginning of this short reply, also your welders were not "wrong" when considering of using Ar/O2-mixtures. And as MDG Custom Weld has already mentioned, the Ar/2% O2 mixed gas is the "standard gas" for using it on 300- and 400- Stainless Steel welding applications. I guess, its only up to you now to decide what you will use for welding the mentioned base materials in the future. I truly hope I could give you some very elementary and hopefully new information about the gaseous properties of O2 and CO2 in Argon.

And if I were wrong in the way of explaining what I would have liked to explain?

Doesn't matter... one test says more than a thousand words - but I guess you'll know that anyhow!

Regards,
Stephan