Step 5: Brazing the assembly.
The fifth step is the actual accomplishment of the brazing joint. It involves heating the assembly to brazing temperature, and flowing the filler metal through the joint. First, the heating process. As we’ve seen in brazing, you apply heat broadly to the base metals. If you’re brazing a small assembly, you may heat the entire assembly to the flow point of the brazing filler metal. If you’re brazing a large assembly, you heat a broad area around the joint. The heating method most commonly used in brazing a single assembly is the hand held torch. A variety of fuels are available – natural gas, acetylene, propane, propylene, etc., combusted with either oxygen or air. (Most popular is still the oxy/acetylene mixture.) All you have to keep in mind is that both metals in the assembly should be heated as uniformly as possible so they reach brazing temperature at the same time. When joining a heavy section to a thin section, the "splash-off" of the flame may be sufficient to heat the thin part. Keep the torch moving at all times and do not heat the braze area directly. When joining heavy sections, the flux may become transparent – which is at 1100°F (593°C) – before the full assembly is hot enough to receive the filler metal. Some metals are good conductors – and consequently carry off heat faster into cooler areas. Others are poor conductors and tend to retain heat and overheat readily. The good conductors will need more heat than the poor conductors, simply because they dissipate the heat more rapidly. In all cases, your best insurance against uneven heating is to keep a watchful eye on the flux. If the flux changes in appearance uniformly, the parts are being heated evenly, regard- less of the difference in their mass or conductivity. You’ve heated the assembly to brazing temperature. Now you are ready to deposit the filler metal. In manual brazing, all this involves is carefully holding the rod or wire against the joint area. The heated assembly will melt off a portion of the filler metal, which will instantly be drawn by capillary action throughout the entire joint area. You may want to add some flux to the end of the filler metal rod – about 2" to 3" (51 mm to 76 mm) – to improve the flow. This can be accomplished by either brushing on or dipping the rod in flux. On larger parts requiring longer heating time, or where the flux has become saturated with much oxide, the addition of fresh flux on the filler metal will improve the flow and penetration of the filler metal into the joint area. However, there is one small pre- caution to observe. Molten brazing filler metal tends to flow toward areas of higher temperature. In the heated assembly, the outer base metal surfaces may be slightly hotter than the interior joint surfaces. So take care to deposit the filler metal immediately adjacent to the joint. If you deposit it
away from the joint, it tends to plate over the hot surfaces rather than flow into the joint. In addition, it’s best to heat the side of the assembly opposite the point where you’re going to feed the filler metal. In the example above, you heat the underside of the larger plate, so that the heat draws the filler metal down fully into the joint. (Always remember – the filler metal tends to flow toward the source of heat.) And if you’re using preforms – slugs, washers, shims or special shapes of filler metal – preplace them at the joint area before you heat the assembly.
After you’ve brazed the assembly, you have to clean it. And cleaning is usually a two-step operation. First – removal of the flux residues. Second – pickling to remove any oxide scale formed during the brazing process. Flux removal is a simple, but essential operation. (Flux residues are chemically corrosive and, if not removed, could weaken certain joints.) Since most brazing fluxes are water soluble, the easiest way to remove them is to quench the assembly in hot water (120°F/50°C or hotter). Best bet is to immerse them while they’re still hot, just making sure that the filler metal has solidified completely before quenching. The glass-like flux residues will usually crack and flake off. If they’re a little stubborn, brush them lightly with a wire brush while the assembly is still in the hot water. You can use more elaborate methods of removing flux as well – an ultra- sonic cleaning tank to speed the action of the hot water, or live steam.
Two tables here somehow
The only time you run into trouble removing flux is when you haven’t used enough of it to begin with, or you’ve overheated the parts during the brazing process. Then the flux becomes totally saturated with oxides, usually turning green or black. In this case, the flux has to be removed by a mild acid solution. A 25% hydrochloric acid bath (heated to 140- 160°F/60-70°C) will usually dissolve the most stubborn flux residues. Simply agitate the brazed assembly in this solution for 30 seconds to 2 minutes. No need to brush. A word of caution, however – acid solutions are potent, so when quenching hot brazed assemblies in an acid bath, be sure to wear a face shield and gloves. After you’ve gotten rid of the flux, use a pickling solution to remove any oxides that remain on areas that were unprotected by flux during the brazing process. The best pickle to use is generally the one recommended by the manufacturer of the brazing materials you’re using. (See the Handy & Harman recommendations for pickling solutions on the opposite page.) Highly oxidizing pickling solutions, such as bright dips containing nitric acid, should be avoided if possible, as they attack the silver filler metal. If you do find it necessary to use them, keep the pickling time very short. Once the flux and oxides are removed from the brazed assembly, further finishing operations are seldom needed. The assembly is ready for use, or for the application of an electroplated finish. In the few instances where you need an ultra-clean finish, you can get it by polishing the assembly with a fine emery cloth. If the assemblies are going to be stored for use at a later time, give them a light rust-resistant protective coating by adding a water soluble oil to the final rinse water.
Basic steps in brazing
Hidden treasure in your scrap.
There’s one last thing you should take into account, as part of your cleaning and finishing operations – the possible salvage value of your brazing scrap. Brazing filler metals may contain silver, often in fairly high proportions. So does the filler metal scrap. And that silver is reclaimable at a good price. It’s hard to believe that the amount of scrap you generate in your brazing operation is large enough to warrant salvaging. But consider this true story ... A Handy & Harman brazing representative, inquiring about scrap salvage, was told by a plant superintendent, "We don’t have any brazing scrap. We tack the rod stubs and coil ends together and use them up." The representative, however, noticed some brazing filler metal drip- pings hanging from the fixtures of a conveyorized brazing operation. He took a couple of samples for lab analysis. Some weeks later he presented the superintendent with a bright disc of pure silver. The silver had been refined from those few "worthless" drippings. From then on, those conveyor fixtures were cleaned regularly – and every bit of scrap accumulated for its silver value. Conveyor fixture drippings are just one source of reclaimable silver. There are others. For example, sup- pose you’re hand-cutting brazing filler metal strip to make custom-shaped shims for brazing carbide tool tips. The leftover scrap has just as high a silver content as the brazing shim itself. Depending on the nature of your brazing operations, there’s always the possibility that you’re generating enough scrap to make accumulation of it over a period of time very worth- while. The fact is – the refining of brazing filler metal scrap can often substantially reduce the cost of brazing operations. Your Handy & Harman/ Lucas-Milhaupt representative can help you spot the "hidden treasure" in your operation and implement the best salvage procedures.
We’ve discussed the six basic steps required in correct brazing procedures. And we’ve gone into a fair amount of detail in order to be as informative as possible. To get a more balanced picture of the overall brazing process, it’s important to note that in most day-to-day brazing work, these steps are accomplished very rapidly. Take the cleaning process, for example. Newly-fabricated metal parts may need no cleaning at all. When they do, a quick dip, dozens at a time, in a degreasing solution does the job. Fluxing is usually no more than a fast dab of a brush or dipping ends of the parts in flux. Heating can often be accomplished in seconds with an oxy-acetylene torch. And flowing the filler metal is virtually instantaneous, thanks to capillary action. Finally, flux removal is generally no more than a hot water rinse, and oxide removal needs only a dip into an acid bath. There are exceptions to the rule, of course, but in most cases a brazed joint is made fast – considerably faster than a linear welded joint. And, as we’ll see later on, these economies in time and labor are multiplied many times over in high production automated brazing. The pure speed of brazing represents one of its most significant advantages as a metal joining process.
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