Process - Six Basic Steps
- Good Fit And Proper Clearance
The best clearance between the two metal parts being brazed is a
close one – usually .001" to .003", and generally
not more than .005". If the clearance is smaller than .001",
there may be no room for the molten filler metal to flow through
the joint. And if the clearance is wider than .005" or .006",
capillary action is less effective and the filler metal may fail
to fill the joint completely.
However, you do have to give some care to the preparation of the
tubes for brazing. For example, if you're brazing a tube to a fitting,
you'll generally start by cutting the tube to the length you want.
Make sure it's cut square, by using a sawing vise or tube cutter,
and remove the burrs. Slide the tube end into the fitting and check
that it has the proper close fit – an easy slip fit.
|In the fabrication
of HVAC equipment, achieving the proper clearance for brazed
joints is seldom a problem. The design of the joint is predetermined
– it's almost always a "cup" design, in which
one tube is inserted into the expanded end of another tube.
As long as you start with tubing that's round and manufactured
to the correct dimensions, you can anticipate no clearance
A well brazed joint begins with a
proper fit between components,
.001"–.003" of clearance.
The majority of brazed joints HVAC units are copper-to-copper joints.
That means you don't have to worry about the "coefficients
of expansion" of the base metals – as the two copper
parts will expand at the same rate. However, if you're brazing joints
of dissimilar metals (copper to steel) you have to take into consideration
the different expansion rates. You know that non-ferrous metals
(copper, brass) expand more during heating than ferrous ones (steel,
stainless steel). So, be sure to adjust the initial joint clearance
accordingly. Where heating will reduce the clearance, for example
brazing a copper tube to a steel header, start with a relatively
loose fit (approx. .010"). Where heating will increase the
clearance, start with a close or even force fit.
- Cleaning The Metals
Capillary action, the basis of all brazing, can't work properly
on dirty surfaces. If the surfaces of the metals are contaminated
(with oils, dirt, etc.) the brazing filler metals will not adhere
to them and the joint will fail.
In fabricating HVAC units, you're generally working with factory
new tubing and parts. But if for any reason those parts have become
dirty or greasy, you will have to clean them to insure a leak tight
joint. Oil and grease can be removed with a solvent, then oxides
can be removed by an abrasive like an emery cloth. Once the parts
are clean, it's recommended to braze as soon as possible so the
parts won't have time to become re-contaminated.
- Fluxing The Parts
Flux is a chemical compound, usually made in the form of a liquid
or paste which is applied to the joint surface prior to brazing.
Its purpose is to protect the joint surface from the oxides that
form during heating. Flux melts and becomes active during brazing,
absorbing oxides and assisting in the flow of the brazing alloy.
Although fluxing is generally an important part of the brazing process,
it plays only a minor role in brazing HVAC components because most
of these joints are copper-to-copper. When brazing copper-to-copper
you can eliminate the flux by using a phosphorus bearing filler
metal like Sil-Fos® or Fos Flo® because the phosphorus acts
as a fluxing agent.
Flux can be applied on to the parts being brazed with a brush, or
it can be incorporated directly into the filler material in the
form of flux cored or flux coated wire or rod. However the flux
is applied, it is important that it does not get inside the parts
where it could contaminate the system.
|However, when brazing
copper to steel (in compressor or valve assemblies) you can't
use phosphorus bearing filler metals as they form brittle
phosphides and the joint could fail. Instead use a phosphorus-free
filler metal (like the Braze™ family of alloys) but
then you will have to use a flux. When brazing brass, you
could use a phosphorus bearing alloy, but you will have to
apply flux here as well.
- Assemble For Brazing
|Before the parts
can be heated and brazed, they have to be aligned and then
supported. This is typically not an issue when brazing HVAC
components since virtually all the joints are tubular, slip-fit
assemblies. By their nature, they are self-aligning and self-supporting
during the brazing process.
When assembling, make sure there is full insertion of one
tube into the other before brazing. The distance of insertion
should be equal to the inside diameter of the inner tube section.
A reducing flame is
recommended when brazing
- Brazing The Assembly
|The actual brazing operation is a two-part process: first,
heating the base metals and second, applying the filler metal
to the joint.
Regardless of the heating equipment being used, be sure to
heat both base metals broadly and evenly, so the filler metal
will wet equally well on both metal surfaces and completely
fill the joint. Because of the wide variety of joints and
joint locations, the gas-air torch is frequently used as the
heat source. A soft flame provides the best type of heat.
When brazing a tube to a fitting, or into another tube of
larger diameter, the following procedures should be followed:
Adjust the torch for a reducing flame (one that contains more
fuel gas than oxygen). The flame should be soft enough and
large enough to envelop both the tube and fitting.
Start heating the tube about an inch away from the end of
the fitting, then shift the heat to the fitting. Sweep the
heat steadily back and forth from tube to fitting, with most
of the heat being applied to the heavier (and slower to heat)
Heat the assembly until it reaches brazing temperature. If
the part has been fluxed with Handy Flux®, the flux will become
clear or transparent at this temperature. If you have not
used flux, you'll know you're at brazing temperature by the
dull red color of the metals being heated.
At this point, pull the flame back a little and apply the
filler metal firmly against the tube at the junction of the
tube and fitting. If you are using a phosphorus-bearing alloy,
lay the rod on and wipe it around the joint, as these alloys
tend to flow sluggishly. If the joint has been properly heated,
the filler metal will melt, penetrate and completely fill
After the joint has been completed, make one final pass of
the flame at the base of the joint, and even twist the joint
if possible, to expel any entrapped gas or flux and to provide
maximum wetting by the filler metal.
Bring both pieces being
brazed up to temperature
evenly and quickly
Once the assembly has reached
brazing temperature the filler
metal is applied.
A well brazed joint – Proper
alloy flow reaches inside the joint
Improper alloy flow does
not penetrate into the joint and
produces a weak bond.
- Cleaning the Brazed Joint
Generally speaking, brazed joints in HVAC units require no post
braze cleaning operations. However, in the minority of cases where
flux has been used, it may be necessary to remove the flux residues
after the joint has set. A hot water wash, assisted by brushing
or swabbing usually does the job. If necessary, you can remove more
stubborn residues with a wire brush.
Some Things To Consider
you're heating an assembly for brazing, you want to heat the joint
area as rapidly and as uniformly as possible. So in those instances
where you're joining metals of unequal mass and thickness you'll have
to apply some extra heat to the heavier section which heats more slowly.
And where you're joining dissimilar metals with differing heat conductivity
(copper to steel), you'll have to apply proportionally more heat to
the copper, since copper is a better conductor and carries heat away
more rapidly to the colder sections. In no case, of course, should
the metals be heated to the point where they begin to melt.
Three Common Joints
The brazing techniques described will vary somewhat depending upon
the kind of joint you're making. The three common tube-to-fitting
joints used in HVAC components are the vertical down, the vertical
up and the horizontal joint. The following procedures are recommended
for each of these joints:
Vertical down joints: Bring the entire joint area up to temperature
quickly and uniformly, heating the tube first, then the fitting. When
the joint area has reached brazing temperature, apply a little extra
heat to the fitting, since this is the direction in which you want
the filler metal to flow.
Vertical up joints: Start by heating the tube. When it has
reached a temperature of about 800° F (425° C) transfer the
heat to the fitting. Then sweep back and forth from fitting to tube,
all around the joint area. Be careful not to overheat the tube below
the fitting, as this would encourage the filler metal to run down
the tube and out of the joint. When brazing temperature is reached,
touch the filler metal to the joint with the flame on the wall of
the fitting. This heating pattern will draw the filler metal up and
completely through the joint area.
Horizontal joints: Preheat the tubing and fitting quickly
and evenly. When brazing temperature is reached, apply the filler
metal to the top of the joint. The combination of gravity and capillary
action will draw the filler metal completely around the tube to its
bottom. You can apply a slight excess heat to the bottom of the fitting
to insure that the filler metal totally penetrates the joint. Check
the joint face to be sure filler metal is visible all around it. In
particular, make sure that filler metal shows at the top of the joint.
If it does not, apply some additional filler metal until it is visible
all around the joint.