How does preheating affect hydrogen-induced cracking in low-alloy steels?

• A. Preheating slows hydrogen diffusion and cooling, reducing the risk of hydrogen-induced cracking.
• B. Preheating speeds hydrogen diffusion, increasing cracking risk.
• C. Preheating has no impact on hydrogen cracking.
• D. Preheating eliminates the need for shielding gas.

Answer: (A)

Preheating helps by slowing how quickly the weld area cools. When steel cools rapidly after welding, hard, brittle microstructures can form and high residual tensile stresses develop in the heat-affected zone. Hydrogen in the weld or surrounding metal can then migrate to these stressed areas and grain boundaries, where it can cause cracking. By keeping the metal hotter for longer, preheating reduces these stresses and keeps the microstructure tougher, making it harder for hydrogen to initiate and propagate cracks. It also gives hydrogen more opportunity to diffuse away from potential crack sites before cracking can occur. This protective effect comes from the slower cooling and the more favorable metallurgical conditions, rather than any elimination of shielding gas or a change in diffusion dynamics at a basic level.