Which of the following is true about preheating carbon steel before welding?

• A. It reduces hydrogen cracking risk by slowing cooling.
• B. It increases cooling rate to harden the weld.
• C. It eliminates the need for shielding gas.
• D. It has no effect on the heat-affected zone.

Answer: (A)

Preheating carbon steel before welding slows the cooling after the weld is completed. Hydrogen cracking happens when hydrogen absorbed during welding diffuses into the steel and the heat-affected zone becomes brittle as it cools and hardens. By starting at a higher temperature, the metal cools more slowly, giving hydrogen more time to diffuse out and reducing the peak hardness and residual stresses that promote cracking. This combination lowers the risk of hydrogen-assisted cracking in the welded joint. Shielding gas is still needed to prevent oxidation and porosity, and preheating affects the heat-affected zone by reducing its hardness and vulnerability, not by having no effect; it does not accelerate cooling.