What is the proper shielding gas to use for penetrating welds on steel in MIG welding?

In MIG welding, using carbon dioxide as a shielding gas for penetrating welds on steel is appropriate due to its ability to produce deep penetration and a strong arc. Carbon dioxide has a lower ionization potential than other gases, which helps to stabilize the arc and allows for excellent heat transfer to the base material. This is particularly beneficial when working with thicker materials or when deeper weld penetrations are required.

Additionally, carbon dioxide is cost-effective, making it a popular choice for welding steel in various applications. While argon and helium can be effective shielding gases in other contexts (such as aluminum welding), they do not provide the same level of penetration and heat generation when welding steel. Oxygen, on the other hand, is not used as a shielding gas in MIG welding due to its potential to create oxidizing conditions that can lead to poor weld quality and defects. Thus, carbon dioxide stands out as the most suitable option for producing strong, penetrative welds in steel using MIG welding techniques.