Define preheat and interpass temperature; why are they used?

• A. Preheat raises the base metal temperature before welding; interpass temperature is the maximum allowed temperature between passes; used to control hydrogen diffusion, reduce cracking, and control cooling rate.
• B. Interpass temperature is the maximum temperature between passes; Preheat raises the base metal temperature before welding; used to control hydrogen diffusion, reduce cracking, and control cooling rate.
• C. Preheat lowers the temperature; interpass is the minimum.
• D. They are unrelated to hydrogen diffusion.

Answer: (A)

Preheating raises the base metal temperature before welding, which slows how fast the weld and surrounding metal cool once welding begins. That slower cooling gives hydrogen formed during welding time to diffuse out of the weld metal and heat-affected zone, so you’re less likely to get hydrogen-induced cracking. It also lowers thermal stresses and helps produce a more favorable microstructure in susceptible steels.

Interpass temperature is the maximum temperature allowed between passes in a multi-pass weld. Keeping the work within a controlled temperature window between passes prevents overly rapid cooling or overheating, which helps the hydrogen already present diffuse out between passes and avoids brittle microstructures or excessive grain growth. In short, it harmonizes the cooling rate and reduces cracking risk, contributing to more consistent weld quality.

So, both practices are used to manage heat input in a way that reduces hydrogen diffusion-related cracking and gives you better control over the cooling rate and the resulting metallurgical properties.