Yeah, pure argon is for TIG welding aluminums and stainless,or the exotics like chromemolly. Be CAREFULL! the way it works is by displacing oxygen. It can suffocate you quick in a confined area.
With an ER70 series solid wire on mild steel (1040's) CO2 is just fine and least expensive. 75/25 and TriMix generate more heat but don't figure in to the strength given the same penetration and bead profile. Largely dependent on deposited weld metal strenght (ER70=70,000psi tensile) and profile (width, height, depth....)
Several manufactures offer quality Flux-cored wires, in various diameters.
As with most welding proceedures, the final "strength" (quality) is proper proceedure by the operator (skill).
Proper equipment, materials, and application = success!
If you'd like, give me model machine, dia wire, mat'l thickness, etc. and I'd be glad to give you specific pointer's .