A Guide to Successful Brazing

The purpose of this guide is to provide basic information for correct silver brazing.

Brazing is a joining process of two or more metal parts into a single assembly by heating the assembly to at least 800° F, and using a filler metal that will melt above 800° F, but below the melting point of the metals to be joined. Keep in mind, brazing should not be confused with soft soldering where the filler metal melts below 800° F. Too often, silver brazing is called silver soldering, which is incorrect.

The importance of brazing cannot be over-emphasized in the air conditioning and refrigeration industry. Manufacturers spend up to forty hours in classrooms and on-the-job training to qualify as competent brazers since it is the process used to join major components in a refrigeration system into a closed circuit. Also, since the closed circuit contains a refrigerant, every brazed joint must be leak free.

The following information is primarily for brazing copper-to-copper assemblies. The procedures are applicable for copper-to-brass copper-to-steel, brass-to-steel and steel-to-steel operations.

Correct Oxygen and Fuel Gas Mixture -
- With Excessive Gas Mixture, this type of flame indicates excessive fuel gas. The gas mixture contains a slightly greater amount of fuel gas than oxygen and produces a reducing flame that heats and cleans the metal surface for faster and better brazing.
- A Balanced Gas Mixture, the gas mixture contains an equal amount of oxygen and fuel gas. It produces a flame that only heats the metal.
- With a Excessive Oxygen Mixture, the gas mixture contains an excessive amount of oxygen. It produces a flame that oxidizes the metal surface. A black oxide scale will form on the metal surface.
Cleanliness
- General cleanliness is vital for reliable brazing. All metal surfaces to be brazed must be completely free of all dirt and foreign matter. In repairs, the metal surfaces to be joined must be wire brushed and / or cleaned with sand paper. Keep oil, paint, dirt, grease and aluminum from the surface of the metals to be joined. The contaminants will: Keep brazing materials from flowing into the joint and prevent brazing materials from adhering to the metal surfaces.
Insertion and Clearance of Parts
- There should be .003" - .005" clearance between the walls of the inner and outer tubes.
- The inner tube should be inserted into the joint at a distance equal to the diameter of the inner tube.
Correct Heating
- Heat evenly over the entire joint circumference and joint length. Use enough heat to get entire joint hot enough to melt the brazing material.
- Do not overheat the joint to the point where the metal to be joined begins to melt. Always try to use the correct size torch tip, and a slightly reducing flame.
Application of Brazing Rod
- Use the brazing rod as a temperature indicator. If the brazing rod melts when placed on the hot joint, the tubing is hot enough to begin brazing.
- For best results, preheat the rod slightly with the outer envelope of the flame. The hot copper tubing (not the entire flame) should melt the brazing rod.
Capillary Action
- The brazing material is drawn into the joint which is caused by the attraction between the molecules of the brazing material and the molecules of the metal surfaces to be joined. This will only work if: The surface of the metal is clean, the clearance between the metal surfaces is correct and the metal at the joint area is hot enough to melt the brazing material.
Simplified Rules for Good Brazing
- Keep oil, dirt, grease, paint or aluminum from the metal surface.
- Examine metal parts for proper joint clearance and insertion of the male tube.
- Use a slightly reducing flame.
- Apply heat correctly to the metal surfaces.