Jump to: navigation, search

Brazing is a metal-joining process whereby a filler metal is heated above melting point and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting (liquidus) temperature while protected by a suitable atmosphere, usually a flux. It then flows over the base metal (known as wetting) and is then cooled to join the workpieces together. It is similar to soldering, except the temperatures used to melt the filler metal are higher for brazing.


Brazing Materials

By LBSC, The Miniature Locomotive, July-August 1953

Your humble servant's idea of a properly brazed joint is one that is made with brass wire, granulated spelter, or an easy brazing strip with a high brass content. Doing a job with silver-solder is not strictly brazing, though some silver-solders, such as Johnson-Matthey's B6 alloy, will make a joint equal in strength to a brass brazed one. Sifbronze is a kind of "super-brazing" material; joints made with this are practically surface welded and the material can be used with an oxy-acetylene blowpipe without any risk of burning out or partly vaporizing any of the contents and rendering the residue brittle or spongy. I use it in all my boiler work--and as the S.I.F. Company's advertisements say, "Sifbronze is darn good stuff." I say it is! Brass wire can also be freely used with oxy-acetylene heating; but with easy-running strip, the intensity of the heat tends to "destroy its nature," in a manner of speaking, and renders the joint either brittle, spongy, or both. However, easy running strip works find with oxy-coal-gas heating, and ocourse is OK with a blowlamp or air-gas blowpipe. One of the best I ever used was of French origin, and call "Latitte".

Pur silver-solder--that is, an alloy of silver and brass--will make a sound joint when used with an air-gas blowpipe, or a paraffin blowlamp. It melts at a dull red heat, and the exact temperature depends on the proportions of silver, the less heat is needed. Silver solder should never be used with an oxy-acetylene flame, and only skilled workers should use even oxy-coal-gas; too much heat just "boils up" the molten metal and you get a spongy joint with little, if any, mechanical strength.

There is very little mechanical strength in the "ersatz" silver solders, and the same applies to the "tectics". These are the materials which are self-fluxing. Joints made with them, even by low heats, are brittle; and "Silbralloy", for example, should never be used for any joints in a boiler, unless the said joints are mechanically able to stand the pressure required, without any aid from the material. It is intended by the makers, to be used for different purposes altogether, for which it is suitable. It can be used for stopping up the crakcs, in the same way as ordinary soft solder; but even then, it needs a mild heat, such as a blow-lamp or air-gas blowpipe. If subjected to intense heat, as provided by an oxy-acetylene or oxy-coal blowpipe, its nature is change, and it becomes brittle, porous, and spongy. Same remarks apply to all other alloys of a like nature. A joint made by applying this material to it, by aid of an oxy-acetylene flame, is just the reverse to being "properly brazed"; even if the joints are flanged they must be riveted as well. One correspondent tells me that he made up one of my boilers for a 2-1/2 inch gauge pacific, flanging all the plates exactly as specified, and "brazed" it with "Silbralloy". It simply split up on the hydraulic test. He scrapped it, starte from scratch again, and used B6 and "Easyflo", as specified in my notes as an alternative to brass or Sifbronze. The boiler stood the test perfectly, and is still in use.