What is slag in welding and what is its role?

• A. A flux-derived protective layer that shields the molten weld; it must be removed after welding.
• B. A type of insulating coating applied after welding.
• C. A color change of the molten metal.
• D. A tool used to thin the weld seam.

Answer: (A)

Slag is the solidified layer that forms from the flux used in flux-based welding. As the weld pool melts, the flux melts and floats on top, creating a protective shield that keeps air and moisture away from the hot metal while it cools. This helps prevent oxidation and contamination of the weld during the critical cooling phase. After welding, the slag crust is chipped or brushed away to reveal the clean weld surface and to prepare the weld for any additional passes or inspection. If slag is left on the weld, it can trap slag inclusions or porosity and interfere with fusion or inspection. This behavior is characteristic of flux-based processes like shielded metal arc welding or flux-cored welding; welding processes that use shielding gas (MIG/TIG) don’t produce slag in the same way.