Aufhauser Oxyacetylene Braze Welding copper alloy filler rods and fluxes enable the joining of many base metals. They are especially useful on steel and cast iron.
Braze welding is similar to torch brazing, except that joint openings may be wider and the distribution of filler metal takes place by deposition rather than by capillary flow. Equipment and some filler metals used in braze welding are the same as those used in torch brazing.
In braze welding of ferrous metals, the base metal is not melted. The
filler-to-base-metal bond is the same as in torch brazing. Flux is applied to
the joint surfaces, which, together with the surrounding area, are preheated to
the point where the filler metal will wet or "tin" these surfaces.
During welding, tinning precedes the weld puddle.
Aufhauser
Braze welding products
Braze welding resembles brazing in two
respects: (a) nonferrous filler metals are used, and (b) bonding is achieved
without melting the base metal. On the other hand, braze welding resembles
welding because it can be used for filling grooves and for building up fillets
as may be required.
Drawbacks of the braze welding process include :(a) weld and base-metal colors do not match; (b) weld strength, while usually adequate, is limited by the strength of the copper alloy used, and thus to service below 500 F; (c) joints are subject to galvanic corrosion and to differential chemical attack. (Many of these disadvantages are overcome in the silver brazing of these alloys).
For carbon steels, the oxyacetylene flame for braze welding is adjusted to the neutral condition. For cast irons, the flame is adjusted to a slightly oxidizing condition by increasing oxygen or decreasing acetylene flow, which reduces the telltale acetylene "feather". When the feather disappears and the inner cone becomes slightly necked, the flame is oxidizing. This type of flame removes the graphite from surfaces of cast iron.
Air is not used as the combustion agent
in braze welding, because it results in slow heating.
Aufhauser
Braze Welding Filler Metals
All-weld-metal tensile strengths of
these alloys range from approximately 50,000 to 70,000 psi. Melting
temperatures of these alloys range from about 1600 to 1800 F.
Aufhauser Low
Fuming Bronze (RCuZn-C) rods are also low-fuming bronze rods, and are of the
same composition as Aufhauser LFB_B (RCuZn-B) except they do not
contain nickel. RCuZn-C rods provide as-welded mechanical properties that are
higher than those obtained with RBCuZn-A, and are widely used as
general-purpose rods for braze welding of both steel and cast iron.
Aufhauser
Nickel Silver (RBCuZn-D) rods have lower copper and higher nickel contents than
those of the three other filler metals. Because of this difference in
composition, the deposit from an RBCuZn-D rod is whiter and is thus used for
braze welding when closer color match is important. RBCuZn-D provides the
highest as-welded strength of the four braze welding filler-metal alloys
that are discussed here.
Aufhauser Naval Brass (RBCuZn-A): contains up to 1% tin, which improves strength and corrosion resistance. These filler-metal rods are especially suited for use with oxyacetylene, and are considered as general-purpose rods for braze welding of steel and the various grades of graphitic cast iron.
Joint Properties.
Joints braze welded with one of the
RCuZn filler metals have tensile strength at room temperature that usually
ranges from 40,000 to 60,000 psi, depending on the filler metal used. The
strength of the joint drops off very quickly at temperatures above 500 F. Color
match with the base metal is not usually obtained, but where color is
important the RBCuZn-D (nickel silver) rod is used. The bimetallic joint is
subject to galvanic corrosion and is less resistant to alkaline solutions than
the ferrous base metal.
Aufhauser
Nickel Silver (RBCuZn-D) is often the superior choice because:
The filler metal is less susceptible to
attack during cleaning; voids and roughness are reduced; cleaning time is less
critical, giving the operator more latitude in removing smut; the filler metal
is harder and less likely to feather during grinding; the roughness resulting
from buildup of nickel is greatly reduced; color match is improved, making the
joint less noticeable. In addition, the joint strength is higher.
Fluxes for braze welding are not the
same as those used for capillary brazing. Because the temperatures used in
braze welding (often higher than 1800 F) are higher than those used in most
capillary brazing, and because the time of exposure to elevated temperature is
longer in braze welding, the flux used must have a higher melting point and be
able to withstand sustained exposure at the higher temperatures.
Three types of flux are in general use:
1) A basic type of flux, which simply
facilitates braze welding by cleaning the base metals and aiding in the tinning
operation.
2) A flux, available in paste form, that
performs the functions of the basic flux described above and, in addition,
suppresses the formation of zinc oxide fumes from the filler metal.
3) A flux, sometimes called a tinning
flux, that is formulated expressly for use in braze welding of gray or ductile
iron.
The first two of the above-listed fluxes
are generally satisfactory for braze welding of steel and malleable iron. The
second type is sometimes used with copper-zinc filler metals in capillary
brazing. The third type contains iron oxide or manganese dioxide, either of
which combines with the surface carbon of the gray and ductile irons;
consequently, this type of flux, is preferred for braze welding these cast
irons.
Application of flux is done in any of
four ways: (a) dipping the heated filler-metal rod in the flux, (b) brushing
flux on the joint before brazing, (c) using flux-coated filler-metal rods, or
(d) fluxing through the gas flame.
The use of flux-coated filler rods or
gas fluxing eliminates the fluxing operation and ensures uniform application.
Gas fluxing is used chiefly on steel in production.
In base metals thicker than about 2
inches, the edges of butt joints for braze welding are prepared with a 90° or a
120° V-groove, to provide a wide bonding area. Fillet, plug and slot welds present naturally open faces. Edge preparation in base
metals less than about 2 inches thick can be optional; either a square groove
with a root opening comparable to the thickness may be used, or a V-groove may
be cut. The V-groove makes it possible for the braze
welding operator to see whether the joint is properly filled; this is not
always possible in torch brazing.
In applications involving thin joints
with parallel-side joint surfaces and relatively close clearances, it is
sometimes difficult to determine whether the joining process qualifies as
brazing or braze welding, because there is some capillary action.
Cleaning Before Braze Welding
For satisfactory results in braze
welding, joint edges must be as clean as possible before the operation begins.
In order to obtain maximum bond strength, the joint surfaces must be bright and
free of oil, rust or other foreign matter. Also, the metal surrounding the
joint edges must be cleaned, both on bottom and on top.
The use of a salt bath is best for
cleaning any of the cast irons prior to braze welding, just as it is for
brazing. If a salt bath is not available, however, following procedure is
reasonably satisfactory.
If the surface of cast iron has been
ground, the graphitic smear can be removed by quickly heating the surface until
it is dull red in color and, after cooling, going over the surface with a wire
brush. If greasy or oily cast iron is ground, some of the grease or oil may
penetrate the surface. If this occurs, the resulting film should be removed by
painting the surface with chemically pure hydrochloric acid. After 15 minutes,
the surface must be scrubbed with a wire brush and cold clean water.
Although it is not always necessary,
iron castings may be preheated before being braze welded, to ensure success of
the operation. The preheating may be local or general, depending on the size of
the casting. Large castings require extensive preheating. A black preheat, or a
low red heat visible in darkness (obtained at approximately advantageous in
some applications to preheat to as high as 1650 F, but temperatures above 1000
F may have an adverse effect on the wetting action of the filler metal, because
of oxidation of the surfaces before braze welding.
No post-heating is necessary after braze
welding of cast iron. However, cooling of the braze
welded assemblies is preferably retarded by wrapping them with asbestos or by
the use of similarly effective methods.
Braze welding of steel is faster than
gas welding, because braze welding requires less heat. Overheating of the base
metal must be avoided, to prevent the filler metal from failing to wet the
joint surfaces. Low-carbon steels are heated no higher than 1350 F before the
filler metal is deposited. Although the filler-metal alloys used in braze
welding melt at temperatures between 1600 and 1800 F, the only rise in the
temperature of the base metal is that incidental to deposition.
The low peak temperatures in braze
welding reduce the probability of distortion and avoid the problem of
melt-through in thin metals.
Production braze welding of metal
furniture and similar products that are to be plated requires joints that are
free of surface oxidation, overheated flux and smut film. In addition to
pickling, it may be necessary to grind, brush, buff and polish the joint for
satisfactory plating. These problems may
be largely avoided by using Aufhauser Speed Flux (gas flux).
Unlike other welding processes, braze
welding with Aufhauser copper alloy filler metals is effective on
any type of cast iron. Peak temperatures of the base metal in braze welding can
be low enough to avoid, or at least to cause very little, transformation during
the heating cycle. Brittle transformation products in the heat-affected zone of
the joint, therefore, can be largely prevented. The weld-metal strength is in
the strength range of the gray irons, the ferritic
malleable irons and the lower-strength ferritic
ductile irons. Tension tests of braze welded joints have sometimes shown a
disturbingly frequent occurrence of parting at the bond line, but this is
generally attributed to improper cleaning, fluxing or tinning, and it
emphasizes the need for care and skill.
The low temperature requirements of
braze welding make the process particularly suitable for joining malleable
iron. Cast iron base metal rarely needs to be preheated to more than 1000 F,
and lower temperatures are often used. In fact, the base metal itself
determines the correct preheating temperature; if too hot or too cold, the
filler metal will not wet (or "tin") the joint. The heat added during
braze welding is normally of short duration.
The principal drawbacks of braze welding as applied to cast iron are: a)the color of the copper alloy filler metal does not match that of the iron; b)the corrosion resistance of the weld metal differs from that of the base metal, being particularly low when the weld metal is exposed to strong alkalis; c) galvanic corrosion, due to dissimilar metals, may be a problem; and (d) the strength of a braze weld falls off rapidly with increasing temperature, so that the service temperature of the casting is limited to 500 F max.
Braze welding is used to join cast iron
to itself or to other metals for production assemblies, and for repairing worn
or broken castings.
Chief advantages over gas welding with
cast iron rods: