What is the purpose of preheating a joint before brazing?

• A. To slow heat-up rates, reduce thermal gradient, minimize thermal stress, drive off moisture, and promote even brazing.
• B. To increase the melting point of the base metal.
• C. To cause oxidation for better bonding.
• D. To keep the joint near room temperature.

Answer: (A)

Preheating sets up heat flow so the joint remains stable as brazing occurs. By bringing the parts up to an elevated, controlled temperature, heat is added more gradually when the filler metal is introduced. This lowers the temperature gradient between the joint and the surrounding metal, reducing thermal stresses, distortion, and cracking as the assembly cools. It also drives off moisture and volatiles and helps the filler metal flow and wet the surfaces more evenly for a consistent joint. The goal isn’t to change the base metal’s melting point or to promote oxidation; oxidation is typically undesirable and flux is used to manage oxides. Keeping the joint near room temperature would prevent proper brazing.