What is the purpose of the slag covering on a completed weldment?

• A. To provide a decorative finish
• B. To protect the semi-molten weld from atmospheric contamination and slows down the cooling process of the weldment
• C. To increase weld penetration
• D. To reduce welding time

Answer: (B)

The main idea is that slag acts as a protective blanket while the weld finishes solidifying and helps control how fast it cools. The flux-derived slag forms a crust over the semi-molten weld, shielding it from the atmosphere. This barrier keeps air contaminants like oxygen and moisture from reacting with the weld metal, which reduces oxidation and defects such as porosity. At the same time, that slag layer provides some insulation, slowing the cooling rate. A slower, more controlled cooling helps the weld microstructure form soundly and reduces the risk of cracking in many steels. After the weld cools, the slag is chipped away so the weld can be inspected and finished. This purpose isn’t about a decorative finish, increasing penetration, or shortening welding time.