Brazing, not to be confused with Braising, is a joining process whereby a non-ferrous filler metal and an alloy are heated to melting temperature (above 450°C) and distributed between two or more close-fitting parts by capillary action. At its liquidus 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. If silver alloy is used, brazing can be referred to as Silver Brazing. Colloquially, the inaccurate terms "Silver Soldering" or "Hard Soldering" are used.
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 for Bronze alloys) which is often considerably lower than the melting point of the base material (e.g., 2900 F for mild steel) and therefore less likely to distort the work piece or induce thermal stresses. The lower heat input associated with brazing vs. welding can increase joining speed and reduce fuel gas consumption. Brazing can be easier for beginners to learn than welding. For thin workpieces (e.g., sheet metal or thin-walled pipe) brazing is less likely to result in burn through. Brazing can also be a cheap and effective technique for mass production. Components can be assembled with preformed plugs of filler material positioned at joints and then heated in a furnace or passed through heating stations on an assembly line.
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.
In order to work properly, the base metals must be exceptionally clean and free of oxides. In most cases, flux is required to prevent oxides from forming. Some metals, such as Titanium cannot be brazed.
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.
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.
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.
With all things being equal, brazed joints are stronger than soldered joints but weaker than welding.
Twin Carbon Arc Brazing