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The primary goal of preheat treatment in welding is to minimize thermal strain in the welded materials. When welding takes place, the heat introduced can cause significant thermal gradients within the material, leading to the formation of stresses, particularly in materials with high carbon content or those that are thick. These stresses can result in cracking and other integrity issues post-welding.

By applying a preheat to the base material before the welding process begins, the temperature differential is reduced. This means that the heat introduced by the welding arc does not cause rapid local heating and cooling that can exacerbate thermal stresses. Preheating promotes an even distribution of heat within the material, thereby helping to alleviate the risk of cracking and enhancing the overall durability of the weld.

While other aspects such as reducing oxygen contamination and conserving energy are important in certain contexts, they do not address the core purpose of preheating related to minimizing thermal strain. Speeding up the welding process can sometimes be a secondary benefit of preheating if it allows for smoother welding transitions, but the fundamental reason remains focused on mitigating the negative effects of thermal stresses in the material being welded.