What is the purpose of preheating in welding?

The purpose of preheating in welding is primarily to prevent cracking by controlling temperature. When metal is welded, particularly thick sections or certain types of materials, there is a significant thermal gradient created due to the heat of welding. This thermal gradient can lead to high residual stresses, and if the material cools too quickly, it increases the risk of cracking, especially in hardenable steels or other susceptible alloys.

By preheating the base material before welding, the overall thermal gradient is mitigated. The preheat raises the temperature of the surrounding metal, reducing the cooling rate after welding. This controlled cooling helps to relieve stress and allows for a more uniform distribution of temperature throughout the weld zone, which is crucial in preventing cracking.

Furthermore, preheating can help to improve the overall quality and integrity of the weld, ensuring that defects are minimized and the bond between the filler material and the base material is strong.