Although brazing as applied to tools is generally for repairs, there are many applications in which brazing is used to make composite tools by joining hard, tough metal to a softer steel. Whether the application is repairing or building a tool, the same general brazing processes are used. Brazing with a silver-base filler metal is probably the most widely used of the processes; this is particularly true for high-speed steel cutting tool reclamation, and for applying cemented carbide tips. See BRAZING, Carbide Tools.
Repair Technique
Brazing with a silver-base filler metal has been successfully used to reclaim tools that had broken and appeared ready for the scrap heap. An example is a gear cutter which had broken into three pieces.
The equipment needed to restore this cutter consisted of an oxyacetylene torch, a small length of 0.8 rnm (0.031 in.) diameter silver alloy brazing wire, a jar of flux, a few ounces of solvent, and a small length of nichrome wire for holding the parts while brazing.
Cleanliness is the first law of good brazing practice. In making this repair, the first step was to carefully clean the pieces individually with a solvent to make certain that each piece was scrupulously clean and bright. All surfaces of the pieces were then fluxed. It is important to use a silver brazing flux which is entirely liquid and active in dissolving oxides at the exact temperature required for silver alloy brazing.
Joint surfaces of each broken piece were precoated (pretinned) with a silver brazing alloy in the following process:
(1) An oxyacetylene torch was used to bring the surfaces to be joined to brazing temperature, or approximately 635°C (1175°F). The flame was kept moving to assure uniform heating and to avoid hot spots. Enough silver brazing alloy was applied to cover the surface completely.
(2) While the heat was maintained, the molten brazing alloy was puddled, rubbed into the surface with a metal rod until the surface was completely covered with a thin, even coat. After pretinning, all joint surfaces were again liberally covered with flux to get rid of oxides which tend to form during final heating. Pieces were then assembled upright so that the weight of the upper piece would cause the parts to settle into intimate contact during brazing. The parts were carefully aligned. With irregular breaks this is seldom a problem as the parts tend to align themselves naturally.
Heating. Heat was applied to the cutter with an oxy-acetylene torch; the torch was chosen primarily because of its speed in heating the tool steel to 635°C (1175°F), the brazing alloy melting point, or flow point.
When the metal glowed a dull red and the flux was completely liquid and clear, the joints had to be only touched with silver alloy wire, and the alloy was pulled by capillary action throughout all joint areas. The alloy was fed into the joints until it became visibly evident that all breaks had been completely filled. While every step is important, this dual heating is the payoff. Heat must be uniformly distributed and the torch kept moving to avoid affecting tool temper and setting up strains. After brazing, the cutter was covered with several layers of insulating blanket and left in position to cool slowly. See American Welding Society Welding Handbook, Volume 4, 8th Edition, Miami, Florida: American Welding Society.