TM 1-1500-204-23-8 3-20           Change 1 3-9.  Brazing and Silver Soldering.    Brazing and silver soldering are methods of joining metals where strength is not important. In either process, a low melting point alloy  is  used  to  sweat  or  bond  metal  together  without fusing  the  base  metal.  The  term    brazing  is  generally understood to mean the joining of metals with a film of brass, while silver soldering indicates that a silver alloy is the medium. a.  Brazing  Brazing may be used to join copper, high melting point brass, bronze, Monel metal, plain carbon steels, and some  alloy steels. (1) Brazing mixture.    For metals having a melt- ing point of 1750°F (935°C) or more, use a mixture of alloy of copper, tin, and zinc which has a melting point of 1625°F   (884°C).   For   copper   and   low   melting   point brasses, use an alloy of 50 percent copper and 50 per- cent  brass.  This  alloy  has  a  melting  point  of  1560°F (848°C), and is used in a granulated form, referred to as brazing spelter. NOTE When the parts require heat-treating, an alloy of  80  percent  copper  and  20  percent  zinc should be used. This mixture has a melting point of 1832°F (999°C). (2) Cleaning.      Clean  joint  to  be  brazed  to  a bright surface using a hot 10 percent solution of sodium hydroxide, Federal Specification A-A-895, followed by a hot water rinse. Sand blast should be used for removal of oxide and  scale. NOTE When the sand blast is not available, a grind- ing wheel, wire buff, or a file may be used. (3) Blow pipe brazing.      Use oxyacetylene or Bunsen burner flame for blow pipe brazing . The follow- ing guidelines must be observed while blow pipe brazing. (a)    Flame adjustment.     The flame should be neutral and outer flame envelope used to supply heat. The tip of central cone should be held 1/4 to 3/8 inch from metal surface at joint. (b)    Application of flux.    Apply flux by either dipping hot end in dry powder, or by coating  rod with a flux solution. (c) Heat joint.    The joint should be brought to  a  full  red  heat  so  that  flux  will  melt  and  flow  when applied. (d)    Brazing metal.     Add sufficient brazing metal to sweat through joint. CAUTION Care must be taken to prevent overheating either the base metal or  brazing metal. Blue vapor is an indication of excess temperature. Damage to component may otherwise result. (4) Furnace brazing.     Accomplish this method of brazing  by setting up work with both  flux and brazing metal placed in the furnace, then adjust temperature to a point where brazing material will melt and adhere to the metal . b.  Silver Soldering   Silver soldering may be used for joining copper and its alloys, Monel  metal, nickel, and silver. It may also be used for joining thin steel parts. (1) Solder.      General purpose silver solder is procured under Federal Specification QQ-B-654, Grade 4. (2) Flux.     When prepared flux is not available, use a mixture of 12 parts of sodium borate (borax), Mili- tary Specification ASTMD-929, and 1 part of boric acid, Federal Specification O-C-265. The flux must melt at a slightly lower temperature than solder. When flux begins to flow, the  solder should be added. (3) Cleaning.    Clean dirt,  grease, oil, oxide, and paint from metal to be soldered. (4) Flame   adjustment.    Use      oxyacetylene flame in the following manner for soldering: Adjust flame to a neutral or a very slight excess of  acetylene. The tip of central cone should be held about 1/2 inch from work and kept in motion to avoid over- heating. (5) Application of solder.    After preheating both parts to an equal temperature, apply solder to surface of under or inner part at edge of seam, while the flame