When a part fails to meet service requirements, the brazing personnel are surprised and often can’t understand why the parts failed because the outside surfaces came out of their brazing furnace looking shiny, clean and bright. Based on the nice-looking outside surfaces of the parts, should they not assume that the inside of the brazed joints were also similarly cleaned and therefore probably brazed well? Unfortunately, no!
The inside surfaces of a brazed joint are called the “faying surfaces” and are usually assembled with very little gap clearance between them – typically in the range of 0.001-0.005 inch (0.025-0.125 mm). Such a clearance should be very good for capillary flow of the molten brazing filler metal (BFM) into that gap between those faying surfaces if they are clean.
However, such gap clearances are actually far too tight to allow any gaseous atmosphere (or vacuum dissociation process or brazing flux) to get into that very thin gap to clean those surfaces. Because of this, any lubricants or oxides inside the joint will not be removed, so the inner faying surfaces will not be clean enough to allow the molten BFM to be drawn into the joint by capillary action. Such cleaning of pre-assembled, dirty parts is virtually impossible.
That is why I use the expression “cleanliness is next to godliness” when I talk or teach about brazing preparation. The faying surfaces must be cleaned thoroughly prior to being assembled for brazing.
The BFM will not bond to or flow over oils, dirt, grease, lubricants or oxides. If any of these contaminants are present on the faying surfaces during a brazing process, the molten BFM will not respond properly and braze failures will be common.
It is important for any brazing shop to check the cleanliness of parts coming to them for brazing, irrespective of whether those parts were shipped to them from an outside supplier or from another part of their own shop. Parts must be cleaned to remove the following:
- Lubricants/compounds used for rolling, drawing, forming and machining of metals
- Oils, greases, waxes
- Synthetic machining-oil residue, when allowed to dry on surface
- Scale, oxides and smut
- Abrasive and grinding residues
- Tumbling and vibratory polishing residue
- Developer pigments from penetrant inspection
- Rinse water and hard-water residues (Ca, Mg, etc.)