Often the brazing shop is not aware of how some of their suppliers are making the sub-components that will be subsequently brazed. Then when there is a problem brazing some of the assemblies containing these sub-components, the brazing shop may try to “solve the problem” by trying to find out what is wrong with their own in-house brazing operations, getting very frustrated when an “in-house cause” for the problem can't be found.
Many suppliers are not aware that their own manufacturing processes can have a negative impact on brazing. Unless you have talked with them extensively about how certain processes will, or will not, hurt brazing, they will continue to do what works best for them in supplying a nice looking product for you in as cost-effective a manner for themselves as possible.
So, as part of any good, ongoing quality-control program for brazing, you need to make sure that any sub-components coming in from your suppliers are fully brazeable! To that end, you need to know (1) the actual chemistry of the base-metals you're bringing in from the outside, (2) how they've been manufactured by your suppliers, and (3) what, if any, material (oil, wax, rust preventative, etc.) is on the surface of the parts you are receiving from them. And since any of these items can potentially cause problems for your brazing, you need to work with your suppliers to understand each of these issues, perhaps making recommendations of alternate processes to use, etc.
Let's take a little closer look at these items.
Base-metal chemistryThe chemistry of the base-materials you receive from your supplier should be easily brazeable as-received, and if that cannot realistically be done (e.g., certain super-alloy base-metal chemistries contain small amounts of aluminum and/or titanium that can interfere with brazing), then you have to be aware of this, and you may need to have the surfaces nickel-plated or fluoride-ion cleaned prior to brazing. This is YOUR responsibility to know this. No excuses, please.
Manufacturing processesThere are a couple of manufacturing processes that I'll mention here that need to be known and understood by the brazing shop (as well as by their suppliers) because of their potential impact on brazing: (a) the type of lubricants used by the supplier in the manufacture of their parts and (b) whether or not any tumble-deburring or grit-blasting has been done on the parts.
Lubricants can be water-based, mineral-oil based (petroleum-based) or synthetic silicon-based. Synthetic silicon-based lubricants are great lubricants (such as WD-40®), but if they are allowed to dry on the parts (usually after 8 hours or so) they may not be removable by normal cleaning operations in your shop and could then render the part non-brazeable. Synthetic silicon-based lubricants may have to be mechanically removed or burned off in a furnace (preferably before the parts are then assembled for later brazing).
As the world goes “green,” water-based lubricants are gaining favor more and more. That’s fine, but appropriate solvents to remove these water-based lubricants prior to brazing may be different from the standard solvents many shops have used over the years for removing petroleum/mineral-oil based lubricants. Many people still use acetone and alcohol as a cleaning bath in trays or tanks at parts-assembly desks because they have always done so, not realizing that acetone and alcohol, and even vapor degreasing with hydrocarbon-based solvents, may not effectively remove water-based lubricants!
If this is not understood in your shop, then parts still contaminated with non-removed lubricants may be heading into your brazing operations, even though you and your people think that they have been cleaned!
This process is designed to remove burrs from manufactured parts by tumbling them in a large vibrating tub usually containing a liquid and a tumbling "media" to "soften the tumbling action" and thus minimize any potential surface damage to the parts. Some of the common tumble-deburring media used today are: aluminum oxide (in pressed cone/pyramid form), marble chips, stone, walnut shells and even corn cobs.
All such "non-metallic" tumbling media can negatively impact the brazeability of the surfaces of the parts being tumbled in them. Because all tumbling media leaves a residue on the parts being tumbled, it's important that such residue not interfere with subsequent brazing. Non-metallic tumbling media leaves a residue on the surface of parts that can actually prevent brazing or cause voids in the brazed joints.
It is therefore strongly recommended that only metallic media should be used, such as small stainless steel balls, screws or nuts. Or, if preferred, the parts may be gently tumbled on themselves (self-tumbling). The liquid in the tank should be an alkaline cleaner of some type (which is effective for both water-based oils and mineral-based lubricants).
Your suppliers should not grit blast with aluminum-oxide if you will subsequently be brazing those parts. Aluminum oxide is a common ingredient in many commercial stop-off compounds and can very effectively stop brazing. Aluminum-oxide is also a common material used in grinding wheels and emery-paper. None of these should be used in braze prep since residues from these surface-prep operations can hinder or prevent brazing. Cubic-boron nitride, another good stop-off material, is also a common ingredient in many grinding wheels used to prepare parts for brazing.
It is recommended that you use stainless steel wire wheels and brushes to prepare surfaces since any "residue" from them can easily be brazed.
Silicon-carbide wheels, emory-paper, etc., are marginally acceptable as a braze prep, but because it is not a metallic material, be careful about using it. The same applies to "Scotch-Brite" scouring pads that many folks use to clean surfaces prior to braze. Try to stay with metallic materials instead, such as steel wool (available in carbon steel, stainless, etc.).
Surface coatingsIt is not uncommon to have parts coated with rust inhibitors, waxes or "vanishing oil" compounds for shipment or storage prior to braze. Many people have told me that they do not remove these prior to braze, thinking or saying instead, "Don't worry about that, the furnace will clean that up," or "Don't worry about that, more flux on the part will take care of that." Wrong thinking! Furnaces used in brazing will effectively clean the outside surface of assemblies, but they do not effectively deep-clean INSIDE the assembled braze joint areas, and thus leaks often result when these surface coatings are not removed prior to brazing.
Paste-razing fluxes used in many flame-brazing and induction-razing operations will not remove surface oils or lubricants either. Fluxes are only designed to prevent, or react with, metallic oxides on surfaces.