Why should flux be left on the weld deposit during cooling?

Leaving flux on the weld deposit during cooling primarily serves to slow down the cooling process and protect the hot metal from exposure to air. This is crucial because rapid cooling can lead to the development of stress and cracks within the weld. Flux creates a protective barrier that not only retains heat but also chemically interacts with any potential contaminants, ensuring that the weld joint remains free from oxidation and other detrimental reactions as it solidifies. Maintaining a controlled cooling rate is essential for achieving the desired mechanical properties of the weld, such as toughness and ductility, which are critical for the long-term durability of the welded structure.