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.
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
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).
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.
When the sand blast is not available, a grind-
ing wheel, wire buff, or a file may be used.
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
Heat joint. The joint should be brought
to a full red heat so that flux will melt and flow when
(d) Brazing metal. Add sufficient brazing
metal to sweat through joint.
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.
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
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.
Solder. General purpose silver solder is
procured under Federal Specification QQ-B-654, Grade
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.
Cleaning. Clean dirt, grease, oil, oxide, and
paint from metal to be soldered.
flame in the following manner for soldering:
Adjust flame to a neutral or a very slight excess
The tip of central cone should be held about 1/2
inch from work and kept in motion to avoid over-
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