Inert Gas Clarification

Date 08-30-2004 14:54

Again, I feel like everyone knows something about welding gasses that I don't. Some time ago I posted a question about CO2 and how/why a reactive gas is used for welding. The best response I got was concerning cost and arc stability - really not alot of response.

Now reading a topic recently posted by Ter, someone posts that "straight argon won't work correctly on mild steel". That's news to me...I'm not saying that's wrong, but I've never heard that. It was my understanding that the purpose of using gas in welding was to remove oxygen from the weld area...most commonly Argon and Helium are used...then I discovered someone invented 75-25, and the more I read the less I understood why it was used (I recently read an Ed Craig Q&A where he was reluctant to advise someone that 100% Ar was OK for MIG)...hence my original post about CO2...so here are my new questions:

Are there more reasons for using CO2, in any mixture, than just cost and "arc stability"?

Is it really true that straight Ar won't work "correctly" when MIG welding mild steel? if so, please explain why not

By sparx

Date 08-30-2004 15:30

I have taken this information from the "Praxair shielding gases selection manual". The reason Argon isn't used as a stand alone GMAW gas is due to the phenomenon of surface tension.

"In any liquid there is an attractive force exerted by the molecules below the surface upon those at the surface. An inward pull, or internal pressure is created, which tends to restrain the liquid from flowing. Its strength varies with the chemical nature of the liquid.
In welding, the surface tension between molten steel and its surrounding atmosphere has a pronounced influence on bead shape. If surface tension is high, a convex, irregular bead will result. Lower values promote flatter beads with minimum succeptibility for undercutting.
Pure ARGON shielding when used with GMAW is usually associated with high interfacial energy, producing a sluggish puddle and a high-crowned bead when mild steel welding is considered. This is partially attributed to the high surface tension of liquid iron in an inert atmosphere. For this reason, it is not recommended for use in MIG welding of mild steel. Iron oxides, however, have a considerably lower surface tension and thus promote good wetting to the parent metal. Therefore, the addition of small percentages of oxygen or carbon dioxide to argon when performing GMAW result in a more fluid weld puddle."

This basically explains that steel needs the oxygen from CO2 or straight oxygen to form iron oxides during the welding process for puddle fluidity.

hope this helps.

By Muskwa Date 09-02-2004 11:29

CO2 is used to weld carbon steel (Mig and Tig) for one reason and one reason only..........cost......I will take a bottle of argon anytime, as long as you are buying it. CO2 is used on Carbon steel because iron oxide melts below the melting temperature of the parent material and because it is cheap. A little O2 does help with puddle fluidity, but straight argon=no splatter so there is a savings in cleanup. And straight argon will have a deeper penetrating bead which is an advantage on anything .250 and thicker.

By backpurge (*)

Date 09-07-2004 09:14

Have you ever seen macrosections on carbon steel plate welded with different CO2/argon mixtures? The higher the level of CO2 the better the penetration profile, MAG welding carbon steel with straight argon will give a very smooth spatter free weld but you will also suffer from "arc wander" and the mechanics of the weld, particularly on thicker plate, will suffer drastically.
CO2 additions up to a maximum of around 20% (dependant upon plate thickness and welding position) will increase penetration depth, side wall and inter-run fusion and because CO2 is capable of carrying more heat across the arc than argon deposition rates will be increased also. It's also a good idea to add a couple of percent O2 to reduce the surface tension of the weld pool which helps reduce spatter generation but in reality any welder who has the slightest idea what he is doing should be able to set up a MAG welder to ensure the minimum amount of spatter is produced anyway

Also where did you get the idea you can TIG weld with CO2???????

Cheers - Bob