more thoroughly preheated. This relieves stresses, additional metal is added as necessary to fill the vee.
speeds up the braze welding operation, and requires less Tinning must, at all times, continuously precede filling
oxygen and acetylene.
the joint.
Preheating can be done with a torch, or with an
As the tinning action is in progress, continue to feed
improvised firebrick furnace covered with insulating-
the brazing filler metal into the molten puddle to build
type fire-retardant cloth. At times, castings attached to
the weld up to the desired size. The puddle should be
a machine may be welded in place. These castings often
small in size, but increased as it is moved forward, until
can be preheated if you play the flame along the line of
the weld and protect the surrounding surfaces with
it completely fills the vee and a full-sized braze weld is
insulating cloth. Large castings should be postheated to
made. Be sure that the tinning action takes place
relieve stresses. Flux is essential in braze welding with
continuously, just ahead of the puddle. Good braze
an oxyacetylene flame for two reasons: (1) it removes
welding makes one continuous operation of the tinning
oxide that forms ahead of the welding zone due to the
action and the building up of the braze weld to the
oxygen in the air, and (2) it dissolves the oxides formed
desired size.
in the braze welding operation. Use plenty of flux in the
The inner flame cone is kept from oneeighth to
tinning operation, but use flux sparingly to fill the vee.
one-fourth inch away from the surface of the metal.
When you see that more flux is required, add it carefully.
Usually the flame is pointed ahead of the completed part
The puddle should not be made mirror clear, but it of the weld at an angle of about 45°, with the puddle
should be left slightly clouded with oxide. Where the under and slightly behind the flame. You vary the angle,
braze welding is rapid, coat the rods entirely with flux.
however, when welding in flat, overhead, and vertical
If the operation is slow, as with heavy castings, dip the
positions. Also, the angle will vary with the size of the
hot end of the welding rod into the flux and add to the
puddle being carried, the nature of the surface, and the
puddle as required.
speed of welding. Braze welding can be done in any
Use a slightly oxidizing flame for braze welding.
Make periodic checks to be sure that this adjustment is
position, but the flat position should be used if
maintained. A slightly oxidizing flame makes a better
possible.
bond between the filler metal and the base metal. It also
Bright spots on the metal in the puddle mean that
keeps a slight film of oxide over the puddle. This film
oxides and impurities are present. They should be
protects the weld metal from the oxygen in the air.
worked out with the torch flame or with flux. Don’t use
After the parts have been properly prepared—
too much flux; it is wasteful and prevents making the
cleaned, fluxed, aligned, and preheated if necessary—
best joint. Use just enough to get a good tinning action
they should be tack welded together. Heat the metal with
between the filler metal and the base metal. The proper
a torch at the point where the braze weld is to start. Play
rate of braze welding is controlled by the rate of tinning;
the torch flame over the part to be heated, using a
never allow the puddle to get ahead of the tinning action.
circular motion. As the base metal gets hot, test its
If it is necessary to deposit filler metal in layers, as
temperature with a drop of metal from the welding rod. when braze welding heavy materials, the tinning of the
When the base metal is at the right temperature, molten base metal is particularly important. If the tinning is
filler metal spreads evenly over the surface. This
good and fusion between the layers is complete, a strong
produces a tinning coat on the base metal.
braze weld is assured.
You can tin the base metal in braze welding only
when the conditions are just right for it. If the base metal
After you have finished the braze welding
operation, the internal stresses will need to be relieved.
is too cold, the filler metal will not run out and spread
To do this, play the torch over the weld and on either
over the heated surface as it should. If the base metal is
too hot, the filler metal will form little balls like drops
side of the weld for several inches. The size of the weld
of water on a hot stove. If the temperature of the base
and the size of the casting determine the size of the area
metal is right and the tinning is done properly, the
that should be heated. Continue to heat until all sections
molten filler metal will spread over the surface like
of the part have been brought to even heat. If the
water spreading over a clean, moist surface. Tinning is
repaired part is small, bury it in dry slaked lime. If it is
the most important step in the braze welding process.
large, cover it with an insulating-type fire-resistant
This free-flowing film of deposited filler metal forms
cloth. Always cool the repaired part slowly. The part
the intimacy of contact necessary for coalescence
should be protected from drafts and cold air, which
between the base metal and the filler metal. When the cause uneven cooling. Never subject a braze-welded
immediate area under the torch flame is tinned,
joint to stress until it has completely cooled.
9-14
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