What is the purpose of preheating metal before welding?

Preheating metal before welding serves a crucial role in reducing the risk of cracking. When metal is welded, the sudden heat from the welding arc can create a thermal gradient that leads to rapid cooling, especially in thicker materials or specific metal types. This rapid temperature change can create stresses that may result in cracking, particularly in high-strength steels and other materials that are susceptible to hydrogen-induced cracking or solidification cracking.

By preheating the metal, you help to minimize the temperature differential between the weld area and the surrounding material. It allows the entire workpiece to reach a more uniform temperature, reducing the thermal shock that can lead to cracking. Furthermore, preheating assists in driving off any moisture or contaminants on the metal's surface, thereby contributing to a stronger bond and cleaner weld. This practice is particularly important in critical applications, where the integrity of the weld joint is of utmost importance.

While enhancing the appearance of the weld, saving energy, or increasing the melting point of the metal are relevant considerations in welding, they do not capture the primary purpose of preheating as effectively as the reduction of cracking does.