Why is preheating used in welding certain steels?

• A. To accelerate cooling
• B. To slow cooling, reduce hardness and cracking risk, especially in high-carbon or alloy steels
• C. To remove preheat from the base metal
• D. To increase weld cooling speed for all steels

Answer: (B)

Preheating raises the base metal temperature before welding, which slows the cooling rate after the weld solidifies. Slower cooling reduces thermal stresses and the formation of brittle microstructures, such as martensite, that can make steel crack-prone—especially in high-carbon or alloy steels. It also gives diffusible hydrogen a chance to escape from the weld area, lowering the risk of hydrogen-induced cracking. Because of these benefits, preheating is used on certain steels to improve weld quality and reduce cracking risk, particularly for high-carbon or alloy steels and thicker sections. The other options describe effects that are opposite or nonsensical for preheating, such as speeding cooling, removing preheat, or claiming it increases cooling for all steels.