How does preheating reduce hydrogen cracking?

• A. Slows cooling, allowing hydrogen to diffuse out
• B. Increases cooling rate
• C. Reduces weld pool size
• D. Prevents oxidation

Answer: (A)

Preheating slows the cooling rate, which gives hydrogen atoms more time to diffuse out of the weld area before the metal hardens and tensile stresses lock hydrogen in. At higher temperatures, hydrogen diffusion is faster, so hydrogen can escape from the weld and heat-affected zone instead of getting trapped and forming microcracks as the steel transforms to a brittle structure. This reduced diffusion time and lower peak hardness/grade of the transformed phase lessen the likelihood of hydrogen-induced cracking.

If cooling were faster, hydrogen would be trapped more readily and cracking risk would rise. Reducing the weld pool size or focusing on oxidation prevention doesn’t address the diffusion and stress mechanisms behind hydrogen cracking, so they’re not the primary factors here.