What is the purpose of preheating metal surfaces before welding?

Preheating metal surfaces before welding serves to minimize thermal shock and reduce the likelihood of cracking. When welding, the heat generated can cause rapid temperature changes in the metal, especially at the weld joint, which can lead to thermal stresses. By preheating the metal, the temperature difference between the weld area and the surrounding material is reduced. This helps to evenly distribute the thermal energy, thereby lowering the risk of cracking due to sudden temperature changes. Preheating is particularly important for materials with high carbon content or those that are prone to hardening, as it can also improve the weld's mechanical properties and ensure a stronger, more durable joint.

While increasing the speed of welding can be an advantage in some cases, it is not the primary reason for preheating. Likewise, improving aesthetics and enhancing arc stability are not the main purposes of preheating; instead, they are affected by other factors during the welding process. Therefore, focusing on minimizing thermal shock and preventing cracks is the core rationale for the practice of preheating in welding applications.