Question

Question was provided via image upload.

Explanation:

Identify the target materials and gases

The question asks for two shielding gases that can be used when welding steel. The options provided in the image are:

1. 75% Argon / 25% Carbon Dioxide
2. 100% Helium
3. 100% Carbon Dioxide
4. 100% Argon

Evaluate the options for steel welding

Using the Carbon Dioxide Shielding Effects knowledge point

• 75% Argon / 25% Carbon Dioxide (C25): This is the most common shielding gas mixture used for Gas Metal Arc Welding (GMAW/MIG) on carbon steel, providing excellent arc stability and minimal spatter.
• 100% Carbon Dioxide (\(\text{CO}_2\)): This is a common, low-cost active shielding gas used for welding carbon steel, offering deep penetration.
• 100% Argon: Pure argon is not typically used for GMAW on carbon steel because it causes a narrow, finger-like penetration profile and poor wetting at the weld toes, though it is used for GTAW (TIG).
• 100% Helium: Pure helium is not standard for steel welding due to high cost and poor arc stability on steel.

Therefore, the two primary shielding gases/mixtures from the options that are standard for steel are 75% Argon / 25% Carbon Dioxide and 100% Carbon Dioxide.

Answer:

• 75% Argon / 25% Carbon Dioxide (Correct answer)
• 100% Helium
• 100% Carbon Dioxide (Correct answer)
• 100% Argon