What is the purpose of preheating when welding thick steel sections with GMAW?

• A. Reduces hydrogen diffusion, lowers cooling rate, and minimizes cracking
• B. Increases cooling rate to speed up welding
• C. Makes oxidation harder to remove
• D. Has no effect on cracking tendency

Answer: (A)

Preheating thick steel sections before GMAW slows the cooling of the weld and the heat-affected zone, which keeps the metal ductile longer and reduces the buildup of residual stresses. That slower cooling also gives hydrogen that may be present in the weld area a better chance to diffuse out before cracking can occur, so the likelihood of hydrogen-induced cracking is minimized. In thick sections, this combination is especially important because rapid cooling would promote hard, brittle microstructures and crack formation. The most important idea is the controlled, slower cooling and improved hydrogen management, not an increase in cooling rate or an inability to remove oxidation.