Why are preheat and interpass temperatures used in welding?

• A. To reduce hydrogen cracking, control hardening and distortion, improve weld quality, and promote proper fusion.
• B. To increase welding speed above the limit.
• C. To reduce the need for shielding gas.
• D. To decrease the energy input of the arc.

Answer: (A)

Preheating and keeping a controlled interpass temperature are about managing heat flow to the weld area so the metal doesn’t cool too quickly. When the base metal is heated before welding, hydrogen that’s present in the weld metal has a better chance to diffuse out instead of getting trapped and causing hydrogen cracking, which is a major concern in high-hardenability steels and thicker sections. The slower, more gradual cooling also reduces the formation of brittle microstructures in the heat-affected zone, lowers peak hardness, and helps control residual stresses and distortion. Maintaining a safe interpass temperature between passes ensures each subsequent weld pass fuses properly with the previous layer and isn’t quenched too aggressively, further improving fusion and weld quality. These temperature controls specifically aim at reducing cracking, controlling distortion, and promoting sound fusion, rather than speeding up welding, reducing shielding gas needs, or lowering arc energy.