Troubleshooting Brazing Joints
The art of brazing is relatively simple, and the rules are common sense rules. Occasionally, however, things do go wrong, and the brazing process fails to do its job satisfactorily. The check lists below will assist in such instances. They are intended to give practical tips on what to try and what to look for.
If brazing alloy does not flow into the joint, even though it melts and forms a fillet
1. The outside of the joint is hot, but the inside is not up to brazing temperature. Review correct heating procedures. Remember to heat the tube first to conduct heat inside the fitting.
2. There is a flux breakdown due to excessive heat. If overheated, the flux can become saturated with oxides, and the brazing alloy won’t flow. Try using a softer flame and/or applying a heavier coating of flux. On thick sections where heating is prolonged or on stainless steel, Harris Stay-Silv® black flux is recommended.
If brazing alloy does not wet surfaces but balls up instead of running into the joint
1. Review heating technique
a. The base metals are not up to brazing temperature, and the alloy has been melted by the torch flame.
2. Base metals have not been properly cleaned.
If brazing alloy flows away from instead of into the joint
1. Make sure fitting is up to temperature and the flame is directed towards the fitting.
If the filler metal cracks after it solidifies
1. When brazing dissimilar metals, the different coefficient of expansion may put the filler metal in tension just below the liquidus temperature during cooling. This sometimes occurs in a copper-into-steel joint. The copper expands and contracts at a greater rate than the steel. Brazing alloys are stronger in compression, so a steel-into-copper assembly would help alleviate the problem.
2. Brazing steel (or other ferrous metals) with an alloy containing phosphorus can lead to formation of a brittle phosphide prone to cracking. Braze ferrous metals with non-phosphorus content alloys.
3. Excessive joint clearance can lead to filler metals cracking under stress orvibration. Make sure clearances are held to .002” - .006” at brazing temperature (depending on alloy).
4. Too rapid quenching can sometime cause cracking. Let joint cool more before washing off flux residue.
If joint leaks in service
90% of “leakers” are due to incorrect brazing technique. The most common causes are:
1. Improper (uneven) heating of joint. The effect of this is inadequate or incomplete penetration by the filler metal. Review proper brazing technique.
2. Over heating, causing volitalization of elements (phosphorus, zinc, etc.)
3. Incorrect torch flame adjustment leading to deposition of carbon orcausing excessive oxidation.