Brazing onto a previously welded part is common in repair work: a cracked bracket, a leaking seam, a worn fitting land. The brazed layer does not replace the weld. It sits on top of it and depends on the surface it finds, which is a weld bead rather than clean parent metal.
That surface is the first problem. A weld bead is uneven, may carry slag or oxide, and has a different composition from the plate around it. Getting a brazed joint to flow evenly across it is harder than brazing new material.
Brazing heat is lower than welding heat, but it is still enough to alter a heat-affected zone that was already altered once. On a part that has seen service, the original weld may have aged or embrittled, and a second thermal cycle can finish it.
The risk is highest where the repair sits close to the original joint. If the brazed patch overlaps the weld toe, the two heat-affected zones stack and the region between them can crack. Keeping the patch clear of the original weld is usually better than trying to reinforce it.
Weld scale, spatter and any protective coating must come off before brazing, and the cleaning must not leave the surface polished. A wire brush on a grinder is effective; a flap disc that burnishes the surface is not, because it closes the texture the filler needs.
Check for cracks before brazing over anything. Dye penetrant or a careful visual inspection along the weld toe will show whether the original defect is a crack or a surface mark. Brazing over a live crack seals nothing, because the crack continues underneath and reappears beside the patch.
A brazed repair on a welded part is a composite: fusion at one interface, capillary filler at another. Load transfer across that combination is uneven, and the brazed side is the weaker path. The repair should therefore be treated as a seal or a fairing, not as a structural restoration.
Where the original part carried load, the brazed patch will not restore that capacity. It may hold for a while because the surrounding material still carries the load, but the margin that made the original design safe is gone, and the next overload finds the weak path first.
Brazing filler loses strength as temperature rises, and a repair on an exhaust component lives in a hot place. A patch that is sound cold may creep under sustained heat and vibration, and the failure mode is a slow opening of the joint rather than a sudden break.
This is the strongest argument against brazed repairs on hot sections. The TIG stainless steel welding route restores the section with filler that matches the parent's thermal behaviour, which is why it is preferred wherever the part will run hot.
If the defect runs through the wall, if the surrounding metal is thinned, or if the part has already been repaired in the same place, cut the section out and replace it. A patch over a patch has no reliable load path and no way to be inspected.
The same applies to titanium. Brazing titanium is rarely appropriate on a service part, and the atmosphere control it needs makes a field repair impractical, so the TIG titanium welding parts we produce rely on fusion in a shielded environment. Knowing when to stop repairing is part of the trade.
