MIG Shielding--CO2 vs Argon 2

[swall]

For MIG welding of carbon steel, I have read that the use of CO2 shielding gas is preferred over Argon mixtures when good penetration is the primary concern. But I have yet to see an explanation as to why this is true.

 

[TheBlacksmith]

CO2 is known as an active shielding gas. In the presence of the arc, it will partially diassociate into CO and O2, which increases the heat and therefore the pentration. A very popular gas is C25, AKA 75% CO2 and 25% Argon, which exhibits less spatter that striaght CO2.

 

[metengr]

swall;
It depends on the arc transfer mode for GMAW (MIG). A good and reputable explanation for shielding gases and their use for welding processes is described in AWS SFA 5.32. CO₂ shielding gas is not used for the spray transfer mode of GMAW. It can be used for globular or short circuiting transfer modes.

Your best bet would be a combination of Ar-CO₂ for spray transfer to short circuiting mode. 20% or less CO₂ in Ar will support spray transfer mode using solid carbon steel wire.

 

[metengr]

swall;
While I was looking up some information on another welding project, I found this from ASM Handbook (Volume 6), related to gas shielding gas and affects;

Dissociation and Recombination. Shielding gases such as carbon dioxide, hydrogen, and oxygen are multiatom molecules. When heated to high temperatures within the arc plasma, these gases break down, or dissociate, into their component atoms. They are then at least partially ionized, producing free electrons and current flow. As the dissociated gas comes into contact with the relatively cool work surface, the atoms recombine and release heat at that point. This heat of recombination causes multiatomic gases to behave as if they have a higher thermal conductivity, similar to that of helium. Dissociation and recombination do not occur with gases, such as argon, that consist of a single atom. Thus, at the same arc temperature, the heat generated at the work surface can be considerably greater with gases such as carbon dioxide and hydrogen...


I believe the heat generated from recombination results in greater penetration.

 

[unclesyd]

Here is a short discourse on practical aspects of welding gases by a man who knows the industry, too well.

http://www.weldreality.com/MIG_welding_gases.htm

 

[swall]

Metengr--very interesting on that ionization with recombination reaction giving off heat. Unclesyd--I had been to that website before, but had forgotten about it. I always enjoy it.

 

[dbooker630]

WeldReality.com is a very good site. I use the 85Ar/15CO2 mixture when penetration is desired, in the spray transfer mode. We are about to take on a new customer that is requesting the use of C25 gas with E70T-1 wire, an unusual combination in my opinion.