Which statement about preheating and hydrogen-induced cracking is least accurate?

• A. Preheating slows hydrogen diffusion and cooling, reducing cracking risk.
• B. Low preheat temperatures can increase the risk of hydrogen cracking.
• C. Preheating is not used for high-strength steels when hydrogen cracking is a concern.
• D. Preheating should be used to control hydrogen diffusion and cooling.

Answer: (C)

Preheating helps manage the weld’s thermal cycle to reduce hydrogen-induced cracking by keeping the base metal and weld area ductile longer and giving hydrogen a chance to diffuse out as cooling proceeds. This is especially important for high-strength steels, which are more susceptible to hydrogen cracking; in practice, preheating is a common and effective measure for those steels when hydrogen cracking is a concern.

The statement that preheating is not used for high-strength steels is not accurate because, rather than avoiding preheat, engineers often apply it to mitigate hydrogen cracking in these very steels. Low preheat temperatures do increase cracking risk by allowing the weld to cool rapidly and harden, trapping hydrogen and creating brittle conditions. Preheating is indeed used to control the cooling rate and help hydrogen diffuse out; while hydrogen diffusion itself rises with temperature, the overall effect of preheating is to reduce peak hardness and residual stresses, lowering cracking tendency.