Figure 19.
Silver brazing joints.
SECTION VI. SOLDERING
27.
GENERAL.
a. Soldering is a method of bonding together two pieces of metal by the use of
an alloy (solder) of comparatively low melting point. The type of alloy to be
used (soft or hard solder) depends upon the metals to be joined; however, the
solder must always melt at a lower temperature than the base metals. The nearer
the melting points of the solder and the metal being soldered, the stronger the
completed joint will be.
b. Soldering should not be confused with welding. In the welding process, the
two metals to be joined are actually fused together; in the soldering process, the
base metals are not melted but are merely coated (tinned) with the soldering alloy
and then "sweated" together or joined by the application of heat. The strength of
the bond between the solder and the base metal is both physical and chemical in
nature. The solder penetrates the pores of the base metal causing the physical
bond, and the heat used in the soldering process causes a surface reaction
(chemical bond) between the chemical elements of the solder and those in the metal
being soldered.
c. Soldered joints should not be relied upon for extra strength. However good
they may be, they must not be subjected to excessive vibrations, severe shocks, or
heavy loads. The primary purpose of a soldered joint is to hold two or more pieces
of metal in close contact, such as may be required for sealing and solidifying a
joint. Strength may be obtained by interlocking seams, crimping, riveting, etc.
28. TYPES OF SOLDERING. The two methods of soldering in common use are
soft soldering and hard soldering. Although the basic principles of each method
are the same, each serves its own special purpose.
a. Soft soldering. Soft soldering is a comparatively low-temperature method
of joining two pieces of metal; it is restricted to jobs subjected to temperatures
below approximately 400F. This method is used to join metals such as iron,
copper, nickel, lead, tin, zinc, and many of their alloys. Soft solders are tin-
lead alloys, with or without the addition of other elements, and may be applied
with either the soldering "iron" (copper) or a torch.
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