What is "slag" in welding?

In welding, slag refers to a byproduct formed from the materials melted during the welding process. When the weld pool solidifies, impurities, and non-metallic materials combining with molten metal can create a layer on the surface of the weld known as slag. This accumulation occurs due to the chemical reactions between the flux used in some welding processes and the base metals being joined.

Slag plays a dual role; it helps to protect the molten weld pool from atmospheric contamination, while also aiding in the stability and quality of the weld. After the welding is complete and the joint has cooled, the slag can typically be removed to reveal the final weld bead beneath. Understanding the nature of slag is essential as it indicates the cleanliness of the welding process, and a proper understanding of it can help in improving welding techniques and ensuring the quality of welded joints.