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 2: Repairing "worn out" parts (continued)B. Application of BFM for wide gaps – To repair cracks in worn surfaces, it is often necessary to try to braze gaps that may be 0.030 inches wide or more. This is extremely difficult with only BFM and, therefore, often requires the addition of special gap-filling compounds (GFC). These GFCs may be packed directly into the crack prior to application of the BFM or, in some cases, may be mixed into the BFM paste ahead of time. It is important to note that these GFCs are NOT intended to be melted, but merely to fill up the space in a gap or crack and have the BFM then flow around it and bind them in place in the gap. These GFCs are usually superalloy base-metal powders such as Inconel or Hastelloy. They may often be regular stainless steel powders or even carbon steel.
The type of powder should be chosen to match the base metal being repaired in light of the corrosive conditions to be encountered in service. The packing of the GFC into the crack ahead of time is most desirable since more GFC is available to fill the gap (50% or more of the gap volume) than when the GFC is mixed in with the BFM ahead of time (usually no more than about 20% max by weight is possible when put in the BFM paste). Sufficient BFM must be applied to fill the faying surfaces inside the gap.
C. Heating and flowing the BFM – For adequate repair of parts, the BFM must melt and completely fill the gap to be repaired, with no excess. Excess BFM will merely run down the part by gravity and necessitate extra cleanup procedures and can, in some instances, ruin parts by flowing into critical areas such as threads or special wear surfaces.
NOTE: Since BFM flows toward the hottest surface areas, it is important to be sure that heating techniques are such that the interior of the cracks draw the BFM into the joint by capillary action before the BFM “flashes out” over the hot external surfaces of the part being repaired. This may require heating cracked surfaces from the underside of the part to draw the BFM down through the crack toward the source of heat. In furnace brazing, it may require special masking or methods of BFM application.
After the joint has been packed with the non-melting GFC, the BFM, if applied separately, is carefully applied to the top of the gap. The quantity of BFM should be about 50% of the volume of the gap or crack in its “empty” state. When the heat is applied to the joint during the braze repair process, the BFM will melt and flow around the GFC particles in the joint, forming a strong joint in only one repair cycle (instead of the two or three cycles that may be required were the GFC not used). The repaired gap or crack will also be much more sound (stronger) than large gaps repaired without using a GFC.
D. Inspection of repaired joints – This is brought up again briefly at this point to remind the reader once again that the most important inspection technique at this stage is visual. A lot can be determined about completeness of braze coverage, that all cracks have been filled and that all surfaces show good wetting by the BFM. To verify integrity of the inside of the joint, either radiographic or ultrasonic inspection may be required.