Shielding Gas Considerations for Welding Carbon Steels
GMAW (MIG)

Current - DC, Reverse Polarity
Shielding - While CO₂ gives deeper penetration at faster welding speeds than Ar-2%O₂, it does produce a more violent arc and more spatter. Adding argon to CO₂ will benefit. For example:  25% CO₂ - 75% Ar is popularly used for short circuiting transfer in welding mild steels. This mixture is excellent when welding out-of-position on thin gauge steel; and also for high current welding. Spatter is minimized.

50% CO₂ - 50% Ar: Frequently used in pipe welding, this mixture gives greater penetration than the above with a quieter arc and more strength than CO₂.

Other mixtures of 3% to 10% CO₂ with argon can be considered for good penetration with minimal porosity, e.g. 90% Ar-10% CO₂ is excellent for spray transfer, as is 92% Ar-8% CO₂ also, on both mild and low alloy steels.

GTAW (TIG)

Current - DC, Straight Polarity (while AC may sometimes be used, DCSP is generally most preferable). Reverse Polarity is never used.

Shielding - Argon. In some instances, the addition of helium might be considered for deeper penetration or on large sections, but He-Ar or He are generally restricted to automatic welding.

GMA (MIG) Welding Parameters Mild and Low-Alloy Steel

Short circuit transfer:

Settings based on CO₂ shielding gas for mild steel, Ar-CO₂ for low-alloy steel.

Spray transfer:

Settings based on Ar, 5 O2 shielding gas.

File Test to Estimate Hardness of Steel