What is the purpose of preheating metal before welding?

The purpose of preheating metal before welding primarily revolves around reducing thermal stress and preventing cracking in the welded joint. When welding, the localized heating of the metal can cause significant thermal gradients, which can lead to the rapid cooling of the welded area compared to the surrounding material. This uneven cooling can result in thermal stresses due to the different rates of contraction, often leading to warping, distortion, or even cracking, especially in certain alloys or thicker sections.

By preheating the metal, you effectively raise the overall temperature of the workpiece, ensuring a more uniform distribution of heat. This helps to minimize the cooling rate after welding, which in turn reduces the potential for thermal stress and cracking during the solidification process. Additionally, it allows for better fusion between the base metal and the filler material, leading to a stronger weld joint.

While options such as increasing welding speed, improving the color of the weld, or enhancing the appearance of the metal may hold some relevance in certain contexts, they do not capture the primary technical reasons for preheating. The main objective is to ensure structural integrity and to manage the physical properties of the welded materials effectively.