Brazing

Brazing (not to be confused with the cooking technique braising), is a joining process whereby a non-ferrous filler metal and an alloy are heated to melting temperature (above 450°C / 800 °F) and distributed between two or more close-fitting parts by capillary action. At its liquid temperature, the molten filler metal interacts with a thin layer of the base metal, cooling to form an exceptionally strong, sealed joint due to grain structure interaction. The brazed joint becomes a sandwich of different layers, each metallurgically linked to each other. Common brazements are about 1/3 as strong as the materials they join, because the metals partially dissolve each other at the interface, and usually the grain structure and joint alloy is uncontrolled. To create high-strength brazes, sometimes a brazement can be annealed, or cooled at a controlled rate, so that the joint's grain structure and alloying is controlled.

Related Topics:
Braising - Ferrous - Metal - Alloy - °C - Capillary action - Metallurgically

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If silver alloy is used, brazing can be referred to as silver brazing. Colloquially, the inaccurate terms "silver soldering" or "hard soldering" are used.

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Brazing is similar to soldering but higher temperatures are used and the filler metal has a significantly different composition and higher melting point than solder.

Related Topics:
Soldering - Solder

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In the more common, more specific usage, brazing is the use of a bronze or brass filler rod coated with flux, together with an oxyacetylene torch, to join pieces of steel. The American Welding Society prefers to use the term "Braze Welding" for this process, as capillary attraction is not involved. Braze welding takes place at the melting temperature of the filler (e.g., 1600–1800 °F or 870–980 °C for bronze alloys) which is often considerably lower than the melting point of the base material (e.g., 2900 °F (1600 °C) for mild steel).

Related Topics:
Bronze - Brass - Flux - Oxyacetylene - Steel

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A variety of alloys of metals, including silver, tin, zinc, copper and others are used as filler for both processes. There are specific brazing alloys and fluxes recommended, depending on which metals are to be joined. Metals such as aluminum can be brazed though aluminum requires more skill and special fluxes. It conducts heat much better than steel and is more prone to oxidation. Some metals, such as titanium cannot be brazed because they are insoluble with other metals, or have an oxide layer that forms too quickly at intersoluble temperatures.

Related Topics:
Silver - Tin - Zinc - Copper - Titanium

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Although there is a popular belief that brazing is an inferior substitute for welding, this is false. For example, brazing brass has a strength and hardness near that of mild steel, and is much more corrosion-resistant. In some applications, brazing is indisputably superior. For example, silver brazing is the customary method of joining high-reliability, controlled-strength corrosion-resistant piping such as a nuclear submarine's seawater coolant pipes.

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In order to work properly, parts must be closely fitted and the base metals must be exceptionally clean and free of oxides. For capillary action to be effective, joint clearances of 0.002 to 0.006 inch (50 to 150 µm) are recommended. In braze-welding where a thick bead is deposited, tolerances may be relaxed to 0.02 inch or about 0.5 mm. Cleaning of surfaces can be done in a couple of ways. It is vitally important to remove all grease, oils, and paint. For custom jobs and part work, this can often be done with fine sand paper or steel wool. In pure brazing it is important to use sufficiently fine abrasive. Coarse abrasive can lead to deep scoring that interferes with capillary action and final bonding strength. Residual particulate from sanding should be thoroughly cleaned from pieces. In assembly line work, a "pickling bath" is often used to chemically dissolve oxides. Dilute sulfuric acid is often used. Pickling is also often employed on metals like aluminum that are particularly prone to oxidation.

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In most cases, flux is required to prevent oxides from forming while the metal is heated. The most common fluxes for bronze brazing are borax-based. The flux can be applied in a number of ways. It can be applied as a paste with a brush directly to the parts to be brazed. Commercial pastes can be purchased or made up from powder combined with water (or in some cases, alcohol). Alternatively, brazing rods can be heated and then dipped into dry flux powder to coat them in flux. Brazing rods can also be purchased with a coating of flux. In either case, the flux flows into the joint when the rod is applied to the heated joint. Using a special torch head, special flux powders can be blown onto the workpiece using the torch flame itself. Excess flux should be removed when the joint is completed. Flux left in the joint can lead to corrosion. During the brazing process, flux may char and adhere to the work piece. Often this is removed by quenching the still hot workpiece in water (to loosen the flux scale) and then wire brushing the remainder.

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Brazing is different from welding, where even higher temperatures are used, the base material melts and the filler material (if used at all) has the same composition as the base material. Given two joints with the same geometry, brazed joints are generally not as strong as welded joints. Careful matching of joint geometry to the forces acting on the joint, however, can often lead to very strong brazed joints. The butt joint is the weakest geometry for tensile forces. The lap joint is much stronger, as it resists through shearing action rather than tensile pull and its surface area is much larger. To get joints roughly equivalent to a weld, a general rule of thumb is to make the overlap equal to 3 times the width of the pieces of metal being joined.

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The "welding" of cast iron is usually a brazing operation, with a filler rod made chiefly of nickel being used although true welding with cast iron rods is also available.

Related Topics:
Cast iron - Nickel

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