Stainless steel shielding gas - again

[GrahamGKD]

I was just musing, as you do, and was wondering why it is that you weld stainless with TIG using pure argon but you need 2-3% Co2 in argon or an argon/helium/Co2 mix to MIG stainless? I guess a similar question could be asked for mild steel as well, although I understand you 'can' MIG mild steel using argon but will get a different bead shape?

Is it something to do with chemicals released from the MIG wire?

BTW, I understand that you have to use pure argon for TIG to protect the tungsten.

 

[shenion]

The argon cools the weld pool. Pure argon MIG will have tall welds that don't "wet" out. Adding CO2 or O2 will add heat to the weld pool.

CO2 absorbs heat from the wire and transfers it to the weld pool via association/disassociation. Also, some of the O2 burns the weld pool. The main problem with CO2 is it can add carbon to the weld.

O2 will burn the surface of the weld pool, it is about 8 times as effective as Co2.

Helium has a very low heat transfer so it reduces the cooling effect of argon. It probably could be used on mild steel but it is expensive.

Pure argon gets cathode effects where the weld pool starts acting like a diode and blocks the current and makes the arc unstable. CO2 or O2 help this.

 

[GrahamGKD]

Thanks for that detailed explanation Scott.

So, we are saying that the small quantity of CO2 is needed for stainless MIG for heat transfer but that too much (as in the case of argoshield light) puts too much carbon into the weld pool?

I’m not sure what you mean by association/disassociation in this context?

 

[shenion]

Yes, too much CO2 will add carbon to stainless. Turns it black and can make it brittle.

Mild steel has suck low carbon content, it can stand a bit extra from CO2.

Disassociation is where CO2 breaks down to C + O + O at the electrode (that absorbs heat). Association is where it combines back at the weld pool and releases the heat back. Some of the oxygen never re-combines and burns the weld pool adding even more heat.

 

[hotrodder]

Pretty much all been said but FWIW... http://www.boconline.co.uk/en/products-and-supply/industrial-gases/shielding-gases/argoshield/argoshield.html

Helium has a higher ionisation potential and higher thermal conductivity than argon i.e. the arc voltage increases slightly and it's hotter. On the flip side arc stability isn't as good with helium and it's harder to strike an arc hence it usually being mixed with argon. As said helium is also expensive being effectively a finite resource etc

FWIW there are flavours of 'stainshield' specifically for TIG... typically argon + small amount of hydrogen, the hydrogen increases the arc voltage and creates a 'reducing atmosphere' which actively scavanges O2 i.e. improves shielding quality. Pretty much limited to austenitic stainless and some nickel alloys AFAIK due to hydrogen porosity (ally and Ti) or hydrogen embrittlement

Most of the common stainless steels are purposely extremely low in carbon because at elevated temps the chromium reacts with carbon forming chromium carbides... http://www.azom.com/article.aspx?ArticleID=1177#_Intergranular_Corrosion

 

[GrahamGKD]

Yes, too much CO2 will add carbon to stainless. Turns it black .....

Yes, I see that when I MIG stainless with Argoshield Light (for non-critical stuff, I should add). I have TIG but sometimes use MIG for stainless (even though I don't have the correct gas) mainly because MIG is quick and not so finicky about the need for backpurging etc..

Thanks for the explanation of association and disassociation - also in the link provided by Hotrodder - why didn't I think of looking on the BOC website

I have a much better understanding of it all now - thank you both.