Which of the following is a direct consequence of preheating a brazing joint?

• A. Slow heat-up and reduced thermal gradient.
• B. Faster heat-up.
• C. Increases distortion.
• D. Eliminates need for flux.

Answer: (C)

Preheating a brazing joint is done to raise the base metal temperature before the filler is added, so the temperature difference between the joint and the surrounding metal is smaller. This makes heat flow more uniform and lowers the thermal gradient, which reduces the residual stresses that cause distortion in the part. It also allows for a more controlled, gradual rise to the brazing temperature rather than a sharp, rapid heat spike. Flux is still needed to protect the surfaces from oxidation and to help wetting, so preheating doesn’t eliminate flux. So the direct consequence is a slower, more controlled heat-up coupled with reduced distortion, not an increase in distortion.