Here is the third and final part of Dan Kay's blog series about the purpose of a braze fillet. Part 2 can be found here.
How should a braze fillet be inspected?
The best way to check the quality of a fillet is to simply look at it with your eyes. Is the fillet concave in shape? Does it go completely around the joint? Is it clean and smooth? These are the readily noticeable characteristics of any good brazed joint. Since the braze fillet (meniscus) is supposed to be very small, simply observing these few qualities of the meniscus should be totally adequate to verify the “goodness” of a brazed joint.
If there are any noticeable cracks in the fillet/meniscus, then that would be justification to reject any brazed assembly until further evaluation can be made to determine the nature and extent of the crack. If it is superficial and can be ground away, then perhaps that cracked area may be repaired by a localized braze (such as by a torch or induction coil).
NOTE: Fluorescent penetration inspection (FPI) is not recommended. Many people still use FPI on braze fillets to accept or reject parts. This can be a big mistake. FPI may be fine for use in weld-repairs, but it is not at all useful if you wish to repair a brazed joint for two primary reasons. First, FPI merely shows that there may be surface imperfections on the outside of the fillet, but it tells absolutely nothing about the quality of the inside of the brazed joint where capillary action has caused much of the molten BFM to flow. Second, FPI chemicals inside the crack in a brazed joint cannot be fully removed since nobody completely grinds out the inside capillary surfaces of a brazed joint and the external fillet in order to completely eliminate the crack. Therefore, such cracks cannot be re-brazed. FPI merely presents a scrap option to brazers if cracks are found. It does not present any repair options.
Braze Fillets Should Never be Dimensioned on Drawings or in Specs
Fillets are not a dimensionally controlled part of any brazing process. The external braze fillet (meniscus) is merely the “after effect” of a brazing process. Because its size can never be completely controlled (especially in furnace brazing), the size of a braze fillet should never be specified on a drawing or in a spec.
Post-braze measurement of a fillet’s size becomes a meaningless waste of time in the shop, and it can be very costly to accurately perform. If a drawing specified that a fillet needed to be a minimum of 0.0625 inch in size, and it was found that the fillet was only about 0.050 inch upon inspection after brazing, what then? Theoretically, it is supposed to be rejected and re-brazed to add more BFM to the fillet. This is a time-consuming waste, especially since it has absolutely no bearing on the filled capillary space in the actual brazed joint itself, where all the “goodness” of a brazed joint resides.