What is the purpose of preheating a metal before welding?

Preheating a metal before welding is primarily done to reduce the potential for cracking, particularly in high-carbon steels and other alloys that are prone to cracking when exposed to extreme temperature changes. When welding, the heat generated can create thermal stresses due to rapid cooling. Preheating helps to raise the overall temperature of the metal, thus minimizing the temperature differential between the weld area and the surrounding metal. This gradual transition in temperature can significantly reduce the likelihood of formation of hard, brittle microstructures that are more likely to crack.

While preheating might have some minor effects on cleaning the metal or potentially speeding up the process under certain conditions, those are not the primary reasons for the preheating practice. The main focus is on enhancing the integrity of the weld by minimizing stress and controlling the cooling rate, thus emphasizing the importance of option B in the context of welding procedures.