NO, for most steel pure CO2 is fine, but with argon mixed in is MUCH better, esp with hardened, high carbon, or allloyed steel. The more argon the better, argon is many times more expensive than CO2 though. For stainless pure argon or 75% argon 25 CO2 is reccomended.
Aluminum, titanium etc must use pure argon!!!!
Read up on it I am not the ultimate authority
Jay