additional metal is added as necessary to fill the vee.
more thoroughly preheated. This relieves stresses,
speeds up the braze welding operation, and requires less
Tinning must, at all times, continuously precede filling
oxygen and acetylene.
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
it completely fills the vee and a full-sized braze weld is
the weld and protect the surrounding surfaces with
made. Be sure that the tinning action takes place
insulating cloth. Large castings should be postheated to
continuously, just ahead of the puddle. Good braze
relieve stresses. Flux is essential in braze welding with
welding makes one continuous operation of the tinning
an oxyacetylene flame for two reasons: (1) it removes
action and the building up of the braze weld to the
oxide that forms ahead of the welding zone due to the
oxygen in the air, and (2) it dissolves the oxides formed
The inner flame cone is kept from oneeighth to
in the braze welding operation. Use plenty of flux in the
one-fourth inch away from the surface of the metal.
tinning operation, but use flux sparingly to fill the vee.
Usually the flame is pointed ahead of the completed part
When you see that more flux is required, add it carefully.
of the weld at an angle of about 45°, with the puddle
The puddle should not be made mirror clear, but it
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.
position, but the flat position should be used if
Make periodic checks to be sure that this adjustment is
maintained. A slightly oxidizing flame makes a better
Bright spots on the metal in the puddle mean that
bond between the filler metal and the base metal. It also
keeps a slight film of oxide over the puddle. This film
worked out with the torch flame or with flux. Don't use
protects the weld metal from the oxygen in the air.
too much flux; it is wasteful and prevents making the
After the parts have been properly prepared--
best joint. Use just enough to get a good tinning action
cleaned, fluxed, aligned, and preheated if necessary--
between the filler metal and the base metal. The proper
they should be tack welded together. Heat the metal with
rate of braze welding is controlled by the rate of tinning;
a torch at the point where the braze weld is to start. Play
never allow the puddle to get ahead of the tinning action.
the torch flame over the part to be heated, using a
If it is necessary to deposit filler metal in layers, as
circular motion. As the base metal gets hot, test its
when braze welding heavy materials, the tinning of the
temperature with a drop of metal from the welding rod.
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
After you have finished the braze welding
when the conditions are just right for it. If the base metal
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
side of the weld for several inches. The size of the weld
too hot, the filler metal will form little balls like drops
and the size of the casting determine the size of the area
of water on a hot stove. If the temperature of the base
that should be heated. Continue to heat until all sections
metal is right and the tinning is done properly, the
of the part have been brought to even heat. If the
molten filler metal will spread over the surface like
repaired part is small, bury it in dry slaked lime. If it is
water spreading over a clean, moist surface. Tinning is
large, cover it with an insulating-type fire-resistant
the most important step in the braze welding process.
cloth. Always cool the repaired part slowly. The part
This free-flowing film of deposited filler metal forms
should be protected from drafts and cold air, which
the intimacy of contact necessary for coalescence
cause uneven cooling. Never subject a braze-welded
between the base metal and the filler metal. When the
immediate area under the torch flame is tinned,
joint to stress until it has completely cooled.