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 RepairThe first situation necessitating braze repair is when defective parts are found after the initial brazing process but before they are shipped to the customer. This is the most desirable place to track them down! It is never acceptable to rely on your customer to do the inspection of your parts for braze defects after he receives them. Make sure your own in-house inspection process is robust and thorough enough to catch any braze defects before the parts are shipped. This is accomplished by visual inspection first and foremost (it’s the simplest, and probably the best, inspection method to use) and, if necessary, by one or more non-destructive testing (NDT) procedures such as pressure testing, ultrasonic testing or radiographic inspection.
Note: It is not recommended to ever use penetrant inspection to check brazed joints. Such an inspection technique is ineffective for braze joints and should only be used with fusion welding.
Three leading causes of in-house braze defects are poor part fitup, poor joint design and shrinkage cracks. Defects resulting from poor fitups may be repairable if a proper understanding is gained of how to go about it. Similarly, repairing shrinkage cracks is possible. However, defects caused by poor joint design can lead to futile attempts at repair, the results of which may be questionable at best. Therefore, if it is ever suspected that poor joint design is the reason for a braze defect, it would be best to have these parts redesigned using the design guidelines discussed in detail in the Brazing Handbook, an excellent book on brazing available from the American Welding Society (AWS). Let’s look at the first of two types of defects that can be repaired.
Poor fitup of parts
Some people erroneously believe that BFMs will bridge gaps of almost any size. Good braze practice calls for 0.001-0.005” clearance in the braze joint at brazing temperature. I’ve seen joints in which the gaps are many times this, some even approaching 0.030” or more! These wide joint gaps can be caused by carelessness or poor manufacturing practices or just by an improper understanding of what is required in a good braze. Rarely can BFMs fill such wide gaps the first time through the brazing cycle, and the common method of repair is then to merely send the parts through the brazing cycle a second or even a third time until enough BFM is finally “cast” into the joint to bridge the gap so that it can finally pass visual inspection.
Many brazing specs allow for these multiple runs through brazing operations in order to fill a braze gap. This is not only poor practice but also a waste of time and money since rarely does a customer pay for multiple runs through a braze cycle. The cost for extra brazing cycles must be borne by the brazing shop!
The answer to this problem is to make sure that parts have proper fitup and gap clearance to begin with. Then the applied BFM will completely fill the joint the first time through the brazing cycle with no need for re-running the parts. By keeping brazing gaps to 0.001-0.005”, a lot of brazing problems can be eliminated.