HI, Thanks Ray for this good information on brazing... This helps me out a lot.
Now maybe I can try some brazing and hope it will work..
THANKS ROB from Minnesota----- Original Message -----
From: Ray Boyce
To: [email protected]
Sent: Tuesday, July 10, 2007 5:24 PM
Subject: [BlindHandyMan] Brazing Explained.
Hi Everyone
Brazing is a joining process whereby a non-
ferrous
filler metal
or
alloy
is heated to melting temperature above 450
°C
(842°F), or, by the traditional definition that has been used in the United
States, above 800°F (425)
°C
and distributed between two or more close-fitting parts by
capillary action.
At its liquid temperature, the molten filler metal and
flux
interacts with a thin layer of the base metal, cooling to form an
exceptionally strong, sealed joint due to grain structure interaction. With
certain metals,
such as Nitinol (Nickel Titanium) and Niobium, a low temperature
eutectic
can form. This leads to the bonding of the two metals at a point that can be
substantially lower than their respective melting temperatures. The brazed
joint becomes a sandwich of different layers, each
metallurgically
linked to the adjacent layers. Common brazements are about 1/3 as strong as
the materials they join because the metals partially dissolve each other at
the interface and usually the grain structure and joint alloy is
uncontrolled. To create high-strength brazes, sometimes a brazement can be
annealed,
or cooled at a controlled rate, so that the joint's grain structure and
alloying is controlled. It is also at 1/3 strength because the metal used to
braze
is usually weaker than the substrate metal because it melts at a lower
temperature, ensuring the substrate does not melt.
Common Techniques
Silver brazing
If silver alloy is used, brazing can be referred to as 'silver brazing'.
Colloquially, the inaccurate terms "silver soldering" or "hard soldering"
are used,
to distinguish from the process of low temperature
soldering
that is done with solder having a melting point below 450
°C
(842
°F),
or, as traditionally defined in the United States, having a melting point
below 800°F or 425
°C.
Silver brazing is similar to soldering but higher temperatures are used and
the filler metal has a significantly different composition and higher
melting
point than
solder.
Likewise, silver brazing often requires the prior machining of parts to be
joined to very close tolerances prior to joining them, to establish a joint
gap
distance of a few micrometres or
mils
for proper capillary action during joining of parts, whereas soldering does
not require gap distances that are nearly this small for successful joining
of parts. Silver brazing works especially well for joining tubular
thick-walled metal pipes, provided the proper fit-up is done prior to
joining the parts.
Braze welding
In another similar usage, brazing is the use of a
bronze
or
brass
filler rod coated with flux together with an
oxyacetylene
torch, to join pieces of
steel.
The American Welding Society prefers to use the term Braze Welding for this
process, as capillary attraction is not involved, unlike the prior silver
brazing
example. Braze welding takes place at the melting temperature of the filler
(e.g., 870 °C to 980 °C or 1600 °F to 1800 °F for bronze alloys) which is
often
considerably lower than the melting point of the base material (e.g., 1600
°C (2900 °F) for mild steel).
In Braze Welding or Fillet Brazing, a bead of filler material reinforces the
joint. A braze-welded tee joint is shown here.
In Braze Welding or
Fillet
Brazing, a bead of filler material reinforces the joint. A braze-welded tee
joint is shown here.
Cast iron "welding"
The "welding" of
cast iron
is usually a brazing operation, with a filler rod made chiefly of
nickel
being used although true welding with cast iron rods is also available.
Vacuum brazing
Vacuum brazing is another materials joining technique, one that offers
extremely clean, superior, flux-free braze joints while providing high
integrity
and strength. The process can be expensive because it is performed inside a
vacuum chamber vessel; however, the advantages are significant. For example,
furnace operating temperatures, when using specialized vacuum vessels, can
reach temperatures of 2400 °C. Other high temperature vacuum furnaces are
available
ranging from 1500 °C and up at a much lesser cost. Temperature uniformity is
maintained on the work piece when heating in a vacuum, greatly reducing
residual
stresses because of slow heating and cooling cycles. This, in turn, can have
a significant impact on the thermal and mechanical properties of the
material,
thus providing unique heat treatment capabilities. One such capability is
heat treating or age hardening the work piece while performing a
metal-joining
process, all in a single furnace thermal cycle.
Brazing Fundamentals
In order to work properly, parts must be closely fitted and the base metals
must be exceptionally clean and free of oxides for achieving the highest
strengths
for brazed joints. For capillary action to be effective, joint clearances of
50 to 150 µm (0.002 to 0.006 inch) are recommended. In braze-welding, where
a thick bead is deposited, tolerances may be relaxed to 0.5 mm (0.020 inch).
Cleaning of surfaces can be done in several ways. Whichever method is
selected,
it is vitally important to remove all grease, oils, and paint. For custom
jobs and part work, this can often be done with fine sand paper or steel
wool.
In pure brazing (not braze welding), it is vitally important to use
sufficiently fine abrasive. Coarse abrasive can lead to deep scoring that
interferes
with capillary action and final bond strength. Residual particulates from
sanding should be thoroughly cleaned from pieces. In assembly line work, a
"pickling
bath" is often used to dissolve oxides chemically. Diluted
sulfuric acid
is often used. Pickling is also often employed on metals like aluminum that
are particularly prone to oxidation.
Flux
In most cases, flux is required to prevent oxides from forming while the
metal is heated and also helps to spread out the metal that is used to seal
the
joint. The most common fluxes for bronze brazing are
borax-
based. The flux can be applied in a number of ways. It can be applied as a
paste with a brush directly to the parts to be brazed. Commercial pastes can
be purchased or made up from powder combined with water (or in some cases,
alcohol). Brazing pastes are also commercially available, combining filler
metal
powder, flux powder, and a non-reacting vehicle binder. Alternatively,
brazing rods can be heated and then dipped into dry flux powder to coat them
in
flux. Brazing rods can also be purchased with a coating of flux, or a flux
core. In either case, the flux flows into the joint when the rod is applied
to the heated joint. Using a special torch head, special flux powders can be
blown onto the workpiece using the torch flame itself. Excess flux should
be removed when the joint is completed. Flux left in the joint can lead to
corrosion. During the brazing process, flux may char and adhere to the work
piece. Often this is removed by
quenching
the still-hot workpiece in water (to loosen the flux scale), followed by
wire brushing the remainder.
Brazing strength/Joint geometry
Brazing is different from
welding,
where even higher temperatures are used, the base material melts and the
filler material (if used at all) has the same composition as the base
material.
Given two joints with the same geometry, brazed joints are generally not as
strong as welded joints but if properly designed & executed, a brazed joint
is stronger than the parent metal. Careful matching of joint geometry to the
forces acting on the joint & properly maintained clearance between two
mating
parts however, can lead to very strong brazed joints, too. The butt joint is
the weakest geometry for tensile forces. The lap joint is much stronger, as
it resists through shearing action rather than tensile pull and its surface
area is much larger. To get braze joints roughly equivalent in strength to
a weld, a general rule of thumb is to make the overlap equal to 3 times the
thickness of the pieces of metal being joined.
Filler materials
A variety of alloys of metals, including
silver,
tin,
zinc,
copper
and others are used as filler for brazing processes. There are specific
brazing alloys and fluxes recommended, depending on which metals are to be
joined.
Metals such as aluminum can be brazed, although aluminum requires more skill
and special fluxes. It conducts heat much better than steel and is more
prone
to oxidation. Some metals, such as
titanium,
cannot be brazed because they are insoluble with other metals, or have an
oxide layer that forms too quickly at high temperatures.
Brazing filler material is commonly available as flux-coated rods, very
similar to stick-welding electrodes. Typical sizes are 3 mm (1/8") diameter.
Some
widely available filler materials are:
. Nickel-Silver: Usually with blue flux coating. 600 MPa (85,000 psi)
tensile strength, 680 - 950
°C
(1250-1750°F) working temperature. Used for carbon and alloy steels and most
metals not including aluminum.
. Bronze: Available with white borax flux coating. 420 MPa (60,000 psi)
tensile strength. 870
°C
(1600°F) working temperature. Used for copper, steel, galvanized metal, and
other metals not including aluminum.
. Brass: Uncoated plain brass brazing rod is often used, but requires the
use of some type of additional flux.
Advantages of brazing
Although there is a popular belief that brazing is an inferior substitute
for welding, it has advantages over welding in many situations. For example,
brazing
brass has a strength and hardness near that of mild steel and is much more
corrosion-resistant. In some applications, brazing is highly preferred. For
example, silver brazing is the customary method of joining high-reliability,
controlled-strength corrosion-resistant piping such as a nuclear submarine's
seawater coolant pipes. Silver brazed parts can also be precisely machined
after joining, to hide the presence of the joint to all but the most
discerning
observers, whereas it is nearly impossible to machine welds having any
residual slag present and still hide joints.
. The lower temperature of brazing and brass-welding is less likely to
distort the work piece, significantly change the crystalline structure
(create a
heat affected zone)
or induce thermal stresses. For example, when large iron castings crack, it
is almost always impractical to repair them with welding. In order to weld
cast-iron
without recracking it from thermal stress, the work piece must be hot-soaked
to 870
°C
(1600 °F). When a large (more than 50 kg (100 lb)) casting cracks in an
industrial setting, heat-soaking it for welding is almost always
impractical. Often
the casting only needs to be watertight, or take mild mechanical stress.
Brazing is the preferred repair method in these cases.
. The lower temperature associated with brazing vs. welding can increase
joining speed and reduce fuel gas consumption.
. Brazing can be easier for beginners to learn than welding.
List of 1 items
. For thin workpieces (e.g., sheet metal or thin-walled pipe) brazing is
less likely to result in burn-through.
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. Brazing can also be a cheap and effective technique for mass production.
Components can be assembled with preformed plugs of filler material
positioned
at joints and then heated in a furnace or passed through heating stations on
an assembly line. The heated filler then flows into the joints by capillary
action.
List of 1 items
. Braze-welded joints generally have smooth attractive beads that do not
require additional grinding or finishing. The most common filler materials
are
gold in colour, but fillers that more closely match the color of the base
materials can be used if appearance is important.
edit]
Possible problems
A brazing operation may cause defects in the base metal, especially if it is
in stress. This can be due either to the material not being properly
annealed
before brazing, or to thermal expansion stress during heating.
An example of this is the silver brazing of copper-nickel alloys, where even
moderate stress in the base material causes intergranular penetration by
molten
filler material during brazing, resulting in cracking at the joint.
Any flux residues left after brazing (inside or out) must be thoroughly
removed; otherwise, severe corrosion may eventually occur.
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