Buying Guide

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7. Brazing is started by applying heat to the parts to be joined. The
preferred method is by the oxyacetylene flame. Propane and other
gases are sometimes used on smaller sizes. A slightly reducing
flame should be used, with a slight feather on the inner blue cone;
the outer portion of the flame, pale green. Heat the tube first,
beginning at about one inch from the edge of the fitting. Sweep
the flames around the tube in short strokes up and down at right
angles to the run of the tube. It is very important that the flame be in
continuous motion and should not be allowed to remain on any one
point to avoid burning through the tube. Generally, the flux may be
used as a guide as to how long to heat the tube, continuing heating
after the flux starts to bubble or work, and until the flux becomes
quiet and transparent, like clear water. The flux will pass through
four stages:
A. At 212°F (100°C) the water boils off.
B. At 600°F (315.6°C) the flux becomes white and slightly puffy and
starts to work.
C. At 800°F (426.7°C) it lays against the surface and has a milky
appearance.
D. At 1100°F (593.3°C) it is completely clear and active and has the
appearance of water.
8. Now switch the flame to the
fitting at the base of the cup.
Heat uniformly, sweeping
the flame from the fitting to
the tube until the flux on the
fitting becomes quiet. Avoid
excessive heating
of cast fittings.
9. When the flux appears liquid
and transparent on both the
tube and the fitting, start sweeping the flame back and forth along
the axis of the joint to maintain heat on the parts to be joined,
especially toward the base of the cup of the fitting. The flame must
be kept moving to avoid burning the tube or the fitting.
10. Apply the brazing wire or rod at a point where the tube enters the
socket of the fitting. The
temperature of the joint should
be hot enough to melt the
brazing alloy. Keep the flame
away from the rod or wire as
it is fed into the joint. Keep
both the fitting and the tube
heated by moving the flame
back and forth from one to the
other as the alloy is drawn
into the joint. When the proper
temperature is reached, the alloy will flow readily into the space
between the tube outer wall and the fitting socket, drawn in by
the natural force of capillary attraction. When the joint is filled, a
continuous fillet of brazing alloy will be visible completely around
the joint. Stop feeding as soon as the joint is filled.
NOTE: For tubing one inch and larger, it
is difficult to bring the whole joint up to
heat at one time. It frequently will be
found desirable to use a double-tip torch
to maintain the proper temperature over
the larger area. A mild pre-heating of
the whole fitting is recommended. The
heating then can proceed as in steps 7,
8, 9, and 10. If difficulty is encountered
in getting the whole joint up to heat at
one time, then when the joint is nearly
up to the desired temperature the alloy
is concentrated in a limited area. At the
brazing temperature the alloy is fed into
the joint and the torch is then moved to an
adjacent area and the operation carried on
progressively all around the joint.
HORIZONTAL JOINTS — When making
horizontal joints, it is preferable to
start applying the brazing alloy at
the 5 o’clock position, then move
around to the 7 o’clock position and
then move up the sides to the top
of the joint, making sure that the
operations overlap.
VERTICAL JOINTS — On vertical
joints, it is immaterial where the
start is made. If the opening of the
cup is pointed down, care should be taken to avoid overheating the
tube, as this may cause the alloy to run down the tube. If this condition
is encountered, take the heat away and allow the alloy to set. Then
reheat the solder cup of the fitting to draw up the alloy.
After the brazing alloy has set,
remove residual flux from the
joint area as it is corrosive and
presents an unclean appearance and
condition. Hot water or steam and
a soft cloth should be used. Wrot
fittings may be chilled; however it
is advisable to allow cast fittings to
cool naturally to some extent before
applying a swab. All flux must be
removed before inspection and pressure testing.
TROUBLE SPOTS
If the alloy fails to flow or has a tendency to ball up, it indicates
oxidation on the metal surfaces, or insufficient heat on the parts
to be joined. If work starts to oxidize during heating, it indicates
too little flux, or a flux of too thin consistency. If the brazing alloy
refuses to enter the joint and tends to flow over the outside of either
member of the joint, it indicates this member is overheated, or the
other is underheated, or both. In both cases, operations should be
stopped and the joints disassembled, recleaned and fluxed.