Brazing is a process similar to soldering, the main difference being in the use of a harder filling material and one requiring a higher melting temperature. Gold, silver, copper, brass, and iron may be brazed and the process consists broadly in melting a filling material called "spelter" into the joint to be made. The spelter used for-; brazing varies with the nature of the work, the hard alloys consisting of copper and zinc, and the soft ones consisting of tin and copper, or tin and antimony. Hard spelter gives a stronger joint than soft spelter, of course, but in all cases the spelter must amalgamate with the metal joined in order to make a good joint.
The flux for brazing is made of borax or boracic acid, the latter being cheaper. Borax will swell up under the flame, blister, and run off unless the water of crystallization has been burned out, whereas good borax will melt and run over the surface and clean it nicely. The surfaces should be cleaned carefully before brazing, the same as when soldering. Salammoniac, zinc chloride, salt, and various acids have been tried as brazing fluxes but none are so good as borax.
The equipment required for brazing consists principally in having suitable means for heating the pieces to the proper temperature and a supply of the proper spelter. Beyond this there are many good things which assist in making brazing easier, such as special fixtures for holding the pieces while working, torches or furnaces of various sizes, and similar things which will suggest themselves to the worker from time to time. As stated, the things which are necessary are but few in number.
A torch is used when brazing small and moderate-sized pieces and a forge or furnace such as shown in Fig. 42, for heating large pieces. Gasoline or kerosene give more heat than gas, but a blacksmith's forge provides the best means for heating and cooling. The pieces to be brazed should be preheated, brazed, and cooled slowly, and care must be taken to see that any sulphur in the coal or any soot from the fire is kept away from the pieces to be welded. The parts should not touch the fuel. A reducing or nonoxi-dizing flame is required and the brazing is done at a high temperature. Iron and steel require almost a white heat and a Bunsen flame with a blue cone is generally used.
The process of brazing varies according to the work to be done and is a somewhat more expensive and complex process than soldering. The surfaces to be brazed must be cleaned, of course, by scraping and washing, and then brushing with a wire brush. The borax or other flux is then applied and the pieces placed together ready for brazing. The tighter the parts are clamped, the stronger will be the joint. If no regular furnace is available, a rough one should be built of bricks, Fig. 43, so as to enclose the articles on all sides but one, in order to retain the heat.
Fig. 42. Double-Jet Braser.
Courtesy. of Turner Bross Works.
There is considerable variation in the practice of brazing, for the details of the operation depend upon the nature of the work to be done, the size and the shape of the pieces, the material, etc. For the production of pieces in quantities, especially articles made of sheet metal, the spelter is melted in a ladle or pot and the pieces are dipped therein. In this operation the flux is floated on top of the spelter and the pieces are cleaned before dipping. The flux then does its work as the articles are dipped into the spelter.
Fig. 43. Temporary Brick Furnace for Braing.
For castings and other articles which require brazing, but which cannot be dipped because of the nature or location of the joint, it is customary to prepare the joint as previously described; place the pieces together in proper position; build a temporary furnace around the piece, or place it in a suitable furnace; and then heat it by playing a flame on it. Gas torches with air blast, Fig. 44, make good heaters and the temperature may be regulated to suit the size of the piece. When the article is properly heated and fluxed, the spelter will flow into the space between the parts, wet both surfaces fully, and make a tight joint.
Fig. 44. Hand gas Torch for Brasing Courtesy of Turner Bross Works.
Care should be taken to see that the article does not cool too quickly, especially iron castings, or unequal shrinkage will cause cracking. Heating should also be gradual to prevent unequal expansion for the same reason and burning must be avoided. Sometimes the pieces may be covered with graphite paint, except where the brazing is to be done, thus preventing the flame affecting those parts. This is especially important when brazing brass as it should prevent the zinc from volatilizing and passing out of the alloy, thus leaving it spongy. The brazing of gold and silver requires special alloys; this is usually put in the hands of a jeweler and, as it is generally a small job, it is usually done with a blowpipe.
Brazed joints are often stronger than the original metal but are not so good as welded joints, although they are cheaper and easier to make. Cast-iron pieces will not break at the joint, when properly brazed, and such joints are about 15 per cent stronger than the casting. One of the greatest objections lies in the possibility of leaving flux or rust in a brazed joint and weakening it. There is also danger of electrolytic action between the spelter and the material brazed and brazed joints do not always stand up well under repeated shocks. On the other hand, on articles of steel there are probably more joints made by brazing than by any other process (except riveting), thus proving it to be a generally reliable commercial process.