What is the purpose of preheating in welding hydrogen-sensitive materials?

• A. To accelerate cooling.
• B. To slow cooling and reduce hydrogen diffusion/cracking.
• C. To produce more porosity.
• D. To reduce required filler metal.

Answer: (B)

Preheating hydrogen-sensitive materials before welding is about controlling how the weld cools and how hydrogen behaves in the weld area. By raising the metal’s temperature before welding, the cooling rate after welding is slowed. This slower cooling reduces the formation of hard, brittle microstructures in the heat-affected zone and lowers residual stresses. With less brittleness and stress, hydrogen has fewer driving forces to migrate into cracks, and any hydrogen that is present has a better chance to diffuse out rather than become trapped in the metal. In short, preheating slows cooling to minimize hydrogen-related cracking and improve weld integrity.