25.
BRAZING GRAY CAST IRON.
a. Gray cast iron can be brazed with very little or no preheating. For this
reason, broken castings that would otherwise need to be dismantled and preheated
can be brazed in place, A nonferrous filler metal such as naval brass (60 percent
copper, 39.25 percent zinc; 0.75 percent tin) is satisfactory for this purpose. A
sufficiently high strength bond cannot be obtained with silver brazing alloys. The
melting point of the nonferrous filler metal is several hundred degrees lower than
the cast iron; consequently, the work can be accomplished with a lower heat input,
the deposition of metal is greater, and the brazing can be accomplished faster.
Because of the lower heat required for brazing, the thermal stresses developed are
less severe and stress relief heat treatment is usually not
required.
b. The preparation of large castings for brazing is much like that required
for welding with cast iron rods. The joint to be brazed must be clean and the part
must be sufficiently warm to prevent chilling of the filler metal before sufficient
penetration and bonding are obtained. When possible, the joint should be brazed
from both sides to insure uniform strength throughout the weld. In heavy sections
the edges should be beveled to form a 60 to 90 V.
26.
SILVER (BRAZING) SOLDERING.
a. Silver brazing, frequently called "silver soldering," is a low temperature
brazing process with rods having melting points ranging from 1,145 to 1,650F.
This is considerably lower than that of the copper alloy brazing filler metals.
The strength of a joint made by this process is dependent on a thin film of silver
brazing filler metal.
b. Silver brazing filler metals are composed of silver with varying
percentages of copper, nickel, tin, and zinc. They are used for joining all
ferrous and nonferrous metals except aluminum, magnesium, and other metals which
have too low a melting point.
c. It is essential that the joints be free of oxides, scale, grease, dirt, or
other foreign matter. The surfaces, other than cadmium plating, can be easily
cleaned mechanically by wire brushing or an abrasive cloth; chemically by acid
pickling or other means. Extreme care must be used to grind all cadmium surfaces
to the base metals since cadmium oxide formed by overheating and melting of the
silver brazing alloys is highly toxic.
d. Flux is generally required. The melting point of the flux must be lower
than the melting point of the silver brazing filler metal so that it will clean the
base metal and properly flux the molten metal. A satisfactory flux should be
applied by means of a brush to the parts to be joined and also to the silver
brazing filler metal rod.
e. When silver brazing by the oxyacetylene process, a slightly reducing flame
is desirable. The outer envelope of the flame, not the inner cone, should be
applied to the work. The cone of the flame is too hot for this purpose. Joint
clearances should be between 0.002 and 0.005 inch for best filler metal
distribution. A thin film of filler metal in a joint is stronger and more
effective; a fillet build up around the joint will increase its strength. Some
joints which can be used are shown in figures 18 and 19.
f. The base metal should be heated until the flux starts to melt along the
line of the joint; the filler metal is not subjected to the flame but is applied to
the heated area of the base metal just long enough to flow the filler metal
completely into the joint. If one of the parts to be joined is heavier than the
other, the heavier part should receive the most heat. Also, parts having high heat
conductivity should receive more heat.
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