Repair-brazing is an essential part of the brazing industry, and will be looked at in two common situations:
1) In-house braze repair before release of parts to customers
2) Repairs done to brazed components that have “worn out” in service
An understanding of both is very important and their differences understood. The former involves in-house QA programs designed to catch braze defects before they get out the door. The latter situation involves those types of brazed components that literally “wear out” in service from corrosion, erosion or fatigue (thermal and/or mechanical) in cyclic service. The result is surfaces that are cracked, pitted or eroded. Let’s look at these situations in more detail.
Situation 1: In-house Brazing RepairShrinkage Cracks
Sometimes it has been seen that parts that appear to have been brazed correctly still show cracks in the joint after brazing when visually inspected. The most common cause of this is that there is a large difference in thermal-expansion characteristics between the two metals being brazed, and as the parts cool from brazing temperature and the two component parts try to return to their room temperature dimensions, a lot of strain is placed on the BFM. Since no BFM can realistically “stretch” more than 50%, the BFM may be literally ripped apart as the brazed assembly is cooled and the two base metals contract at different rates.
The cause of the problem is the big difference in expansion rates of the base metals, and in tubular assemblies it is invariably seen when the higher-expansion material is the inner member of the assembly. That’s because the inner member wants to pull away from the outer member when cooling, literally pulling the joint apart. To eliminate this problem to begin with, always strive to make the higher-expansion material the outer member of the tubular construction so that the joint goes into compression on cooling.
But if the situation requires that the higher-expansion material be the inner member and cracking occurs as a result, how can those cracks be repaired? The easiest way is to apply a lower-melting BFM over the cracks and re-braze the assembly at the lower temperature required to melt this second BFM. Since the first BFM will not re-melt, the cracks may be “healed” by this technique. It is essential, however, to be sure that the parts to be re-brazed are still very clean and that the cracks have not been contaminated. If these cracks were only discovered because a penetrant inspection had been used, it will be virtually impossible to repair the cracks because the penetrant compounds cannot be adequately removed from the cracks. This repair technique will only work when the cracks were discovered during visual inspection of the parts.