Question

Brazing temperature is

• A. above 450\(^{\circ}\)C
• B. below 200\(^{\circ}\)C
• C. above melting point of base metal
• D. room temperature

Hint:

Keep this temperature scale clear in mind:
- Soldering \(\rightarrow \lt 450^{\circ}\text{C}\)
- Brazing \(\rightarrow \gt 450^{\circ}\text{C}\) (base metal does not melt)
- Welding \(\rightarrow \gt T_m\) of base metal (base metal melts)

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question asks for the standard metallurgical temperature range that defines the brazing process.
Brazing is a liquid-gas-solid state joining process that is distinguished from soldering and fusion welding based on its operating temperatures.

Step 2: Key Formula or Approach:
The definition of brazing is based on the melting point of the filler metal:
- The filler metal must have a liquidus temperature greater than \( 450^{\circ}\text{C} \) (\( 840^{\circ}\text{F} \)).
- The filler metal must have a liquidus temperature below the solidus temperature of the base metal(s) being joined.

Step 3: Detailed Explanation:

• Distinction from Soldering: Soldering is very similar to brazing as it also relies on capillary action to distribute liquid filler metal.
However, soldering utilizes filler metals (solders) that melt at temperatures below \( 450^{\circ}\text{C} \) (typically between \( 180^{\circ}\text{C} \) and \( 300^{\circ}\text{C} \)).

• Distinction from Welding: Fusion welding involves melting and coalescing the base metals themselves, which requires extremely high temperatures matching or exceeding the base metal's melting point.
In contrast, brazing does not melt the base metal, preserving metallurgical structures and minimizing thermal distortion.

• Capillary Action: In brazing, the joint clearance is tightly controlled (typically \( 0.025-0.1\text{ mm} \)) so that the molten filler metal can be drawn smoothly between the close-fitting surfaces by capillary forces.


Step 4: Final Answer:
By international metallurgical standards, brazing filler metals melt at temperatures above \( 450^{\circ}\text{C} \) but below the melting point of the base metal.
Thus, option (A) is the correct choice.