What is the primary purpose of the slag formed by the flux coating?

• A. It provides shielding gas
• B. It protects the weld deposit
• C. It increases weld speed
• D. It strengthens the weld

Answer: (B)

Slag from the flux coating acts as a protective blanket for the weld deposit as it cools. In shielded metal arc welding, the flux decomposes in the arc to create a layer of slag on top of the molten weld metal. This slag shields the hot metal from the atmosphere, preventing oxidation and contamination, which helps reduce porosity and other defects. It also helps trap impurities and can improve the surface before any additional passes. After the weld solidifies, the slag is chipped away to reveal the finished bead. It isn’t meant to provide shielding gas, increase weld speed, or strengthen the weld’s metal—its main job is protecting the weld deposit.