What is the purpose of preheating?

• A. Slow the cooling rate
• B. Increase welding speed
• C. Reduce porosity
• D. Improve paint adhesion

Answer: (A)

Preheating serves to slow the cooling of the weld area. By raising the base metal temperature before welding, the heat dissipates more gradually as the weld cures, lowering thermal gradients. This slower cooling helps hydrogen escape from the molten metal, reduces residual stresses, and lowers the risk of cracking and porosity—especially in thicker sections or certain steels. It’s not about increasing welding speed or directly improving paint adhesion; those effects come from different factors like technique or surface preparation. So the primary purpose is to slow the cooling rate.