What is the main purpose of applying a minimum preheat when welding?

The primary purpose of applying a minimum preheat when welding is to avoid cracking. Preheating the base material before welding helps to elevate the temperature of the metal, which reduces the risk of cracking in the heat-affected zone and the weld metal. When welding, molten metal cools rapidly, which can lead to the formation of hard, brittle microstructures that are more susceptible to cracking, especially in certain steel grades or when welding thicker sections. By preheating, you ensure a more gradual cooling rate, allowing the alloying elements in the metal to remain in a more ductile state, thereby reducing residual stresses and the likelihood of cracking.

While preventing warping, reducing fatigue, and maintaining weld quality are important considerations in welding, they are secondary effects of preheating. The primary focus is on managing the thermal constraints during the welding process to minimize the risk of cracking in the welded joint and surrounding material.