1 Furnace Brazing - Johnson Matthey Metal Joining

FURNACE BRAZING
A Survey of Modern Processes and Plant
J. D. Boughton and P. M. Roberts
Johnson Matthey Metals Limited, London
Reprinted from
Welding and Metal Fabrication
March and April 1973
JOHNSON MATTHEY METALS LIMITED
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FURNACE BRAZING
A Survey of Modern Processes and Plant
J. D. Boughton and P. M. Roberts
Johnson Matthey Metals Limited, London
There is no simple and concise answer to the
question “What is furnace brazing ?”. The term
“furnace brazing” can be applied to any brazing
process where a furnace is the heat source used to raise
the parts to be joined to their brazing temperature.
Consequently, in its simplest form it can be the brazing
of two pieces of brass, where heating is applied by an
air-containing, gas- or oil-fired muffle furnace, the joint
being produced with a silver-copper-zinc-cadmium
alloy in association with a fluoride-base flux; at the
other end of the scale, it can be the brazing of alumina
to niobium with titanium in a vacuum furnace, where
the end product is intended to be used as a component
in an inter-planetary vehicle.
Thus, furnace brazing can reasonably be described
as the most versatile of all the brazing processes
in common use, encompassing as it does brazing
applications from the mundane to the highly esoteric.
While this survey is primarily concerned with
protective atmosphere furnace brazing techniques, no
review of furnace brazing processes can be considered
to be complete without reference to brazing in muffle
furnaces where brazing is carried out with conventional
fluxes in an atmosphere of air. Muffle furnaces are
discussed later in this survey in the section entitled
“Types of Furnaces”.
Although many different types of protective
atmosphere can be used for brazing, for convenience
they can be divided into two broad categories:
Gaseous atmospheres.
Vacuum.
1. Brazing in Gaseous Atmospheres
There are two distinct types of gaseous atmospheres in use for brazing applications:
Chemically inert atmospheres which protect
the parts being brazed from coming into
contact with other gaseous elements which might
react with the metals being joined to produce
surface films which might inhibit flowing and
wetting by the molten brazing alloy.
Chemically active atmospheres which will react,
during the brazing cycle, with any surface films
present on either the parts to be joined, or the brazing
alloy preform, removing them in the process.
In either instance, the partial pressure of any
oxygen present in the protective atmosphere will play
an important part in the removal of any contaminating
film which may be present on the component parts of
the assembly when they are placed into the furnace.
During the brazing cycle, the chemical activity of
the atmosphere employed can promote removal of
the solid films (particularly oxides) from the surface
of the parts to be brazed either by decomposition, or
by combining with the elements which make up the
film to produce easily removable compounds. Such
films may be simple or complex compounds of sul
phides, borides, phosphides, oxides and other organic
substances. However, for the purposes of this survey
the formation and decomposition of oxide films only
will be considered in detail.
One of the prime requirements for any successful
furnace brazing operation is to ensure that the surfaces
of the metals being brazed are free from oxide, or
other films, which may inhibit wetting when the
brazing alloy becomes molten. Thus, the ease with
which surface oxides can be removed from any
given material is a function of the ease with which
the oxygen ions can be detached from the metallic
ions present in the oxide. Naturally, the degree of
difficulty that is encountered when such separation
is required to be accomplished depends upon the
strength of the chemical bond existing between the
oxygen ions and the metal involved. The strength
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of such a bond may be assessed in several different
ways:
By the heat of formation (∆H) of the
particular oxide in question. (However, this
will provide an approximate guide only) or,
more accurately,
By the change in free energy (∆F) in the
system during the reaction. From the
maximum energy obtainable from the
general chemical reaction:
m
nMe + —O2
2
+MenOm
where Me = metal
and m = 1 Mole of oxygen.
Table 1 presents some data on the heat of formation
for a series of different oxides. As shown in the table,
metals like gold, silver and palladium possess low
values for heat of formation of their oxides. Thus,
from what has been said above, it is clear that these
oxides can be considered to be unstable and hence
can be readily decomposed. The oxides of metals
such as copper, cobalt, nickel, or cadmium, are
higher on the stability scale and are hence more
difficult to reduce. Higher still on the stability scale
will be found the oxides of chromium, tantalum,
aluminium, and beryllium. In fact, the various oxides
of beryllium have a far higher degree of stability than
those of virtually any other element.
Table 1
Heat of Formation of certain Oxides1
Type
Of
oxides
Au2O3
Ag2O
PdO
CuO
Cu2O
CoO
Co3O4
H2O (liquid)
H2O (gaseous)
NiO
CdO
FeO
Fe2O3
Fe3O4
MnO
ZnO
Cr2O3
Ta2Q5
TiO2
ZrO2
A12O3
BeO
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Reaction
of formation
of oxides
2Au + 3/2O2
2Ag + 1/2O2
Pd + 1/2O2
Cu2O + 1/2O2
2Cu + 1/2O2
Co + 1/2O2
3Co +2O2
2 H + 1/2O2
2 H + 1/2O2
Ni + 1/2O2
Cd + 1/2O2
Fe + 1/2O2
2Fe + 3/2O2
3Fe + 2O2
Mn + 1/2O2
Zn + 1/2O2
2Cr + 3/2O2
2Ta + 5/2O2
Ti + O2
Zr+ O2
2Al + 3/2O2
Be + 1/2O2
Heat of formation
of oxides, related
to 1 mole 02:
in kcal
-7.3
14.6
42.0
75.0
82
115.0
98.2
115.6
136.6
116.8
124.4
129.0
130.0
133.8
185.0
166.8
179.6
199.6
219.0
258.5
266.6
294.6
From this, it follows that metals and their alloys
may be classified in groups according to the difficulty
of separation of oxygen ions from the respective
metallic ions. This in turn, of course, is directly
related to the degree of difficulty that one might
expect to experience when attempting to carry out a
brazing operation when these materials are employed
as the parent metals. For instance, the brazing of
noble metals does not present any special difficulties
while copper, cobalt or nickel are somewhat more
difficult. Chromium, tantalum, or beryllium are
extremely difficult to braze with furnace techniques
due to the very great stability of their oxides.
To complicate matters, it must be remembered
that certain metals form more than one oxide and
such oxides have varying stability. Consequently, it
is necessary to consider not only the oxide present
on the surface of the parent material when attempting
to assess the difficulty of brazing, but also which
particular oxide, or group of oxides, is present.
Oxides which are formed on the surfaces of alloys
are usually solid solutions of the oxides of the metals
which comprise the alloy and not just a single oxide.
Moreover, the heat of formation of oxides on the
surface of pure metals will not necessarily be the
same as that of oxides produced on alloys of the same
metals. In consequence, it does not follow that, if it is
relatively easy to braze a particular alloy, the brazing
of the individual metals which go to make up that
alloy will be equally easy to achieve.
During any oxidation cycle, the surface of an
alloy becomes covered with a heterogeneous oxide
film, which frequently consists of layers of different
oxides, having differing compositions. The type
and composition of oxides present in such layers is
dependent upon the temperature and time for which
the component has been exposed to the oxidising
environment. A typical example may be found in
the range of chromium-bearing steels which are
generally considered to be difficult items to furnace
braze. The oxide of chromium, Cr2O3, forms a
strong bond with the steel surface and, in addition,
is not readily reduced to metallic ions. However, it
is recognised that other more complex oxides such
as FeCr2O3 and FeOCr2O3, will be produced on the
surface of the steel during the oxidation cycle and
that the ionic bonds of these oxides will be weaker
than in the Cr2O3 layer, and hence easier to reduce.
The diagram shown in Fig. 1 gives data on
the temperature dependence of the free energy of
certain oxides. Variation of free energy has been
related to one mole of oxygen at a pressure equal to
one atmosphere combined with the corresponding
quantity of metal in the oxides.
Fig. 1 Variation with temperature of the free energy of
certain oxides
2. Dissociation of Oxides
With increasing temperature, the free energy
associated with the formation of oxides decreases.
Hence dissociation of oxides becomes easier to
accomplish as the temperature within the furnace
increases (Fig. 2). The temperature at which the
oxides dissociate depends directly on the partial
pressure of the oxygen in the environment. If
the partial pressure of oxygen in the surrounding
atmosphere is 0.21 atmosphere or more, the
dissociation pressure of the oxide will, in most
known metals and alloys, exceed their respective
melting points. It is clear, therefore, that under these
conditions (where dissociation of the oxide coating
is relied upon to provide an oxide-free surface over
which the molten brazing alloy can wet and flow),
brazing will be impossible.
There are, fortunately, exceptions to this rule.
These are the oxides of gold, silver and platinum
or oxides of their alloys, where dissociation takes
place at temperatures below their respective melting
points. Thus, a decrease in the partial pressure of
oxygen in the surrounding atmosphere tends to
favour decomposition of oxides on these metals
and their alloys, and so increases the likelihood that
brazing can be completed successfully.
Reduction of the partial pressure of oxygen
contained in the gas atmosphere may be achieved
in two ways:
By the formation of a vacuum in the vicinity
of the parts to be brazed; or
By filling the space surrounding the part to be
brazed with an oxygen-free inert or reducing
gas.
In the first instance, the partial pressure of
oxygen is reduced without altering the composition
of the atmosphere, while, in the second instance, the
composition of the gas atmosphere is changed.
This second method of the reduction of partial
pressures of oxygen mentioned above is being
increasingly used in brazing operations. The
joining of titanium and its alloys with silver-bearing
brazing materials is a typical example. Difficulties
associated with the brazing of titanium arise not
only because titanium and its alloys have oxide
films of a high stability but also because they tend
to absorb both nitrogen and hydrogen from any
atmosphere in their vicinity. Both titanium hydride
and titanium nitride will embrittle titanium, and it
is, therefore, of fundamental importance to ensure
that when brazing titanium or its alloys one employs
only vacuum brazing or an inert atmosphere of
argon which is free of both these gases. Freedom
from contamination by hydrogen and nitrogen may
be achieved by passing the argon gas stream through
titanium chips or sponge, heated to 900ºC, prior to
the gas being fed into the brazing retort.
Fig. 2 Variation with temperature of the dissociation
pressure of certain oxides.2
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A further problem must also be overcome before
brazing of titanium or its alloys can be achieved. It
has been established that beneath the oxide films on
titanium, a gas-rich layer (the so-called alpha layer)
may be found. Brazing should only be carried out
after the removal of this film. The surface of the
titanium should be either mechanically or chemically
cleaned prior to beginning the brazing operation.
Once the material has been cleaned, usually by
pickling, brazing should be carried out within fortyeight hours.
3. Brazing in Inert Gases
Brazing in an atmosphere of inert gas is normally
carried out in special retorts, although in some
specialised work muffle-type furnaces may be used.
In these instances, the muffle itself is completely filled
with an inert gas such as argon or helium.
When argon is introduced into an air-filled retort,
a certain amount of air is displaced. In consequence
the partial pressure of the residual oxygen is reduced.
By then evacuating the retort and again refilling with
argon a further reduction in the partial pressure of
oxygen can be achieved. By successive evacuation
and refilling with argon, low values for partial
pressure of oxygen may be readily achieved. It is this
procedure which allows the brazing of such materials
as stainless steel (even where the difficult-to-remove
oxide of chromium, Cr2O3, is present) to be carried
out under an atmosphere of argon.
However, it must be remembered that if the inert
gas atmosphere remains stagnant during the brazing
cycle then the partial pressure of oxygen, attributable
to the decomposition of the surface oxide films, can
build up in the joint area. In such work, it is not
unknown for this local build-up of oxygen to arrest
the process of dissociation despite the otherwise
favourable mean partial pressure of oxygen within
the atmosphere. It is, therefore, essential to prevent
accumulation of oxygen near the surface of the parts
during the brazing cycle. This can be readily achieved
by ensuring that there is a continuous flow of the
inert gas over the parts being treated. By this means,
complete dissociation of the oxide film can occur, and
brazing can be completed.
oxygen, it is also required that hydrogen, or some other
reducing gas, is present in the furnace atmosphere to
assist in the decomposition of the oxides.
One of the most widely used gases in industry today
is hydrogen and this will reduce most metal oxides
in accordance with the following general chemical
reaction:
MeO+H2 Me+H2O
Since this is a reversible reaction it follows
that the presence of too much water vapour in the
furnace atmosphere will result in oxidation of the
metal occurring. Consequently, the decomposition
of the oxides, and hence the success of the brazing
process, will be favoured if the partial pressure of
water vapour contained in the hydrogen atmosphere is
reduced below a definite value. The amount of water
vapour present in an atmosphere may be conveniently
expressed in terms of its “dew point”. Thus, if the gas
has a dew point of, say, —40ºC (representing a level
of 130 parts per million of water vapour in the gas),
this level of water vapour will be present in that gas
at all temperatures above —40ºC. An understanding
of this fact is essential in recognising the principles
which govern the successful brazing of metal parts in a
reducing atmosphere.
It is possible to produce individual curves for each
metallic element showing the relationship between the
dew point of the reducing atmosphere, temperature,
and the reducing or oxidising potential of that
atmosphere. Fig. 3 shows typical curves for several
elements.
Significance of the Curves
It has been the experience of the authors that users
of reducing atmosphere furnace brazing techniques
4. Brazing in Reducing Atmospheres
Successful brazing in a reducing atmosphere is
based upon the fact that a chemical reaction occurs
between the oxides on the surface of the metals to
be joined and the surrounding gas resulting in the
reduction of the oxide film. Unlike brazing in an
inert atmosphere, decomposition of the oxides is
not a spontaneous process that occurs by removal of
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Fig. 3 Relationship of dew point to equilibrium temperature
for the reduction of various metal oxides in hydrogen. At dew
points above the curve oxidising conditions prevail, at dew
points below the curve reducing conditions prevail.
rarely understand the significance of the curves shown
in Fig. 3. A proper understanding of their significance
will provide an answer to the problem of why, for
example, mild steel can be successfully brazed with
copper under an atmosphere of burnt propane while
stainless steel cannot.
Figure 4 shows the oxidation/reduction curve for
Cr2O3, the “hard-to-reduce” oxide normally present on
the surface of a stainless steel part when it is placed in
a furnace.
As has already been explained, for brazing to be
possible, it is necessary that the reduction potential of
the atmosphere is sufficiently high to ensure that the
reduction reaction proceeds only as follows:
Cr2O3+3H2
2Cr+3H2O
Reference to Fig. 4 indicates that there are only
three possible cases that need to be considered when
evaluating whether or not stainless steel can be
brazed under a specific set of dew point and furnace
temperature conditions, i.e., to the left of the Cr2O3
curve, on the curve, or to the right of it. Let us consider
each of these cases in detail:
Case A: Dew point of the gas atmosphere
—40ºC, the furnace operating at 750ºC.
In this particular case the reduction potential
of the gas atmosphere is not high enough to
cause reduction of the Cr2O3 and hence wetting
and flowing of the brazing alloy cannot occur.
Case B: Dew point of the gas atmosphere
—40ºC, the furnace operating at 950ºC.
In this case the conditions are such that the
atmosphere is neither truly reducing nor truly
oxidising. Consequently, it is possible that
wetting and flowing by the brazing alloy may
occur. However, only a marginal increase in the
level of water vapour present in the joint area,
due, for instance, to poor purging as a result of
too narrow a joint gap, or to an insufficient gas
flow rate into the furnace, will prevent brazing
occurring.
Case C: Dew point of the gas atmosphere
—40ºC, the furnace operating at 1,080ºC.
In this case, reducing conditions prevail and
brazing will occur.
These three cases serve to show the importance
of the oxidation/reduction potential curves since,
what is true for Cr2O3 is equally true for all other
oxides. By studying these cases, it is possible to
deduce one of the fundamentals of successful furnace
brazing, namely, that for wetting and flowing of the
brazing alloy to occur it is necessary that the point of
intersection of perpendicular lines drawn respectively
Fig. 4 Significance of dew point: temperature relationship
with regard to the successful brazing of stainless steel under
reducing atmosphere.
from the dew point value of the atmosphere employed
and the furnace operating temperature, must lie to the
right of the curve relating to the most “difficult-toreduce” oxide in the system being considered.
In general terms, only elements present in a
system at levels in excess of about 1 per cent need
be considered. Moreover, not only must one consider
the elements present in the parent materials but those
contained in the brazing alloy must also be taken into
account.
A second, and equally important, feature which
has emerged from this appraisal is that not only can
conditions in a furnace be made to change from
“oxidising” to “reducing” by raising the operating
temperature (assuming the dew point value of the
gas atmosphere is held constant), but that a similar
change can be effected by maintaining the furnace
temperature constant and reducing the dew point of the
atmosphere. (Point A will be in reducing conditions
when the furnace temperature is maintained at 750ºC,
and the dew point reduced to —60ºC.)
In practice, however, this latter option is only
selected when, for metallurgical reasons, it is either
impossible or undesirable to use a furnace temperature
above a certain value and where there is no chance of
the atmosphere in the furnace being reducing with
respect to the parts needing to be brazed due to the
available gas atmosphere having too high a dew point.
In general, it is easier, and far less expensive, simply
to increase the operating temperature of the furnace.
However, even in this instance, due regard must be
paid to any deleterious effects, to either the selected
brazing alloy or the parent metals, which may occur
as a result. (See section 5 under Alloys and Fluxes.)
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5. Parent Metal — Brazing Alloy—
Atmosphere Interaction
Embrittlement. When brazing in a hydrogen
atmosphere, interaction between the hydrogen and
the metals or alloys being joined must also be taken
into account. (For example, dissolution of hydrogen
in the metals with the subsequent possible formation
of embrittling hydrides, as in the case of titanium,
or of the formation of water vapour due to reduction
of entrapped metallic oxides which will lead to the
well known phenomenon of hydrogen embrittlement
(Fig. 5) such as occurs with “tough-pitch” copper,
must be borne in mind.) In practice, however, pure
hydrogen is only occasionally used for brazing
purposes. The reducing atmospheres used for furnace
brazing applications normally contain, in addition to
hydrogen, nitrogen and sometimes carbon monoxide,
carbon dioxide, ammonia and, depending upon the
source of the gas, certain of the hydro-carbon series
(see later). It is, therefore, necessary to consider the
possible interaction between the constituent gases in
the reducing atmosphere and the parent metals when
considering any particular furnace brazing application.
The degree of interaction between the components of
the reducing atmosphere and the metals to be joined,
depends, to a large extent, on the composition of the
metals, the brazing temperature, and the pressure of
the applied atmosphere.
For example, heating some steels for one hour in
a hydrogen atmosphere having a dew point of +30ºC
at a temperature of 1,200ºC causes embrittlement due
to absorption of hydrogen. Yet when heating the same
steel for one hour in a hydrogen atmosphere, having an
identical dew point, at a temperature of only 1,000ºC,
no embrittlement occurs. A similar state of
Fig. 5 Photomicrograph showing the hydrogen embrittlement
of “tough pitch” copper. (Courtesy: The Copper Development
Association).
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affairs can exist with regard to the nitrogen component
of an atmosphere and certain metallic elements.
Decarburisation of Certain Steels. Decarburisation
of high-carbon steels can occur during brazing in
hydrogen-containing atmospheres according to the
general reaction:
Fe3C+2H2=Fe+CH4
Increasing the temperature, and maintaining the
time at the higher temperature constant, favours
decarburisation. It is of fundamental importance
to note that the residual moisture content of the
reducing atmosphere plays a significant part in this
reaction. It is, therefore, axiomatic that to minimise
the decarburisation potential of the atmosphere it is
necessary to use gas atmospheres which possess a
low dew point. However, complete elimination of
the decarburising effect in furnaces where reducing
atmospheres are present cannot be achieved.
Alloys and Fluxes. To ensure that brazing is carried
out satisfactorily, conditions in the furnace must be such
that reducing conditions prevail at the joint interface
for any of the metallic elements present in the furnace
(i.e., the constituents of both the brazing alloy and the
selected parent metals being joined must be considered
when establishing dew point parameters or furnace
temperature conditions). By considering the curves
shown in Fig. 3, as has been explained earlier, it can be
seen that when the furnace conditions are oxidising they
can often be made reducing by the simple expedient of
raising the temperature. It must be stressed, however,
that raising the furnace temperature may result in it
being necessary to change the brazing alloy selection.
This is specifically so if the alloy originally selected
contains an element, or elements, possessing relatively
high vapour pressure at the higher furnace setting.
For example, with a furnace atmosphere of suitable
dew point brazing alloys containing cadmium or
zinc can be effectively used. However, if the furnace
temperature is raised above 750ºC, these elements will
volatilise and difficulty will be experienced in effecting
a joint. A secondary problem will also exist in that the
cadmium and zinc vapours may attack the furnace
lining, whether it be of refractory brick or metal
construction. Many present-day furnaces utilise nickelbase liners (Nimonic or Inconel) and these materials
are particularly prone to attack by cadmium and zinc.
The form of attack frequently results in intercrystalline
failure of the nickel-base alloy.
If conditions in the furnace are such that a flux
has to be used to effect a joint, it is almost always
advisable to modify the conditions to permit fluxless
brazing. This is because many of the fluxes used in
brazing processes contain fluorides and, in
consequence, their vapours are extremely corrosive.
In a relatively short time, these vapours can cause
damage to the heating element sheaths, the furnace
liner, conveyor belt and any other metallic parts within
the furnace chamber. Secondly, and equally important,
one of the prime reasons for selecting a furnace
brazing technique is quite often the need to eliminate
post-braze cleaning operations. If a flux has been used,
its residues have to be removed and the economic
advantages of using a furnace brazing technique are
frequently lost due to the cost involved in carrying out
this flux residue removal process. It often happens that
flux residues on components brazed in a hydrogen/
nitrogen atmosphere are, for some inexplicable reason,
much more difficult to remove than flux residues
formed after brazing identical components, under
identical flux cover, in air.
6. Reducing Atmospheres in
Common Use
As has already been shown, the prime essential of a
reducing atmosphere is that it shall be capable of both
removing the oxides on the workpiece and brazing
alloy and maintain the surfaces of these materials in a
clean condition throughout the duration of the heating
cycle. These conditions facilitate the proper wetting
of the work by the molten brazing material and also
ensure that the work is clean and bright when it is
removed from the furnace.
Since far and away the greatest amount of furnace
brazing involves the joining of mild steel parts with
copper, the commonly used reducing atmospheres will
be considered primarily in relation to this particular
brazing process. In the United Kingdom, reducing
atmospheres are most frequently derived from one of
six sources:
From ammonia: it being catalytically “cracked”
into its two constituent elements, hydrogen and
nitrogen.
Partially burned fuel gas.
Catalytically cracked fuel gas.
Catalytic reformation of hydrocarbon vapours.
“Bottled” hydrogen.
Methanol-water hydrogen generators.
The atmospheres obtained from these various
sources will, therefore, contain some or all of the
following gases unless special precautions are taken
to remove those which might have a deleterious effect
upon the brazing operation:
Carbon dioxide.
Water vapour.
Nitrogen.
Hydrogen.
Undissociated hydrocarbons
compositions.
Carbon monoxide.
Compounds of sulphur.
of
varying
As has already been shown, the control of the level
of water vapour present in a reducing atmosphere is
of fundamental importance if brazing is to occur. By
having too much water vapour present the reaction
set up in the furnace tends to be oxidising. Sulphur
compounds also promote oxidation. It is, therefore,
desirable to control the quantities of both these
products admitted to any furnace if successful brazing
is to be achieved.
Apart from the oxidising properties of sulphur
compounds they also have a deleterious effect upon
furnace heating elements which may contain nickel,
and any other nickel alloys used in the furnace
construction, promoting inter-granular cracking of
those materials.
Hydrogen, the hydrocarbon series, and carbon
monoxide are reducing gases and it is upon these that
the primary effect of reduction of oxides present on the
workpieces and brazing alloy depend.
As already mentioned, an atmosphere constituent
which has a strong affinity for carbon will combine
with carbon from the surface of steel to cause
surface decarburisation, which, in some instances,
is undesirable. The constituents of a protective
atmosphere gas which will produce this phenomenon
are carbon dioxide, water vapour, sulphur compounds
and, under certain conditions, hydrogen itself.
Catalytically Cracked Ammonia. Cracked ammonia
is widely used as a protective atmosphere for brazing.
The cracking process, endothermic in nature, proceeds
in accordance with the general formula:
2NH3
3H3+N2
Unfortunately, this reaction never reaches
equilibrium, with the result that the output gas
from a “cracker” plant invariably carries with it
trace quantities of uncracked ammonia. Thus, in
cases where stainless steel is being brazed under an
atmosphere of cracked ammonia, there is a risk, albeit
small, of this residual ammonia causing a certain
degree of nitriding of the steel. Nitriding of stainless
steels can result if the protective atmosphere contains
an appreciable amount of molecular nitrogen. Since
nitriding of nickel-base alloys and certain classes of
stainless steels heated under cracked ammonia is an
established fact, the use of nitrogen-free atmospheres
is clearly desirable. Moreover, it generally happens
that the dew point of the gas as it leaves the
“cracker” plant is about —40ºC. Consequently,
9
Fig. 6 Diagrammatic representation of a plant for the production of
an exothermic gas atmosphere.3
cracked ammonia atmospheres can be considered to be
powerfully reducing and can, therefore, be used for not
only mild steel but alloys which contain such readily
oxidisable constituents as chromium, manganese
and silicon. Under these conditions of purity, the
atmosphere can be considered to be virtually nondecarburising. However, it must be stressed that the
important feature of this form of atmosphere with
regard to decarburisation phenomena is that it must be
kept pure and dry while it is inside the furnace, and not
merely when it is ready to leave the “crackerplant”.
The cost of cracked ammonia is high when compared
to that of atmospheres derived from fuel gases. Thus
the use of cracked ammonia for brazing applications
is generally confined to relatively small furnaces, and
particularly those where materials of high intrinsic
value, or where the parts require an exceptionally
reducing atmosphere to effect a brazed joint, are being
dealt with.
Partially Burned Fuel Gas. The reducing atmosphere
most widely used for brazing is undoubtedly that
known as burnt town’s gas.
Coal gas from the ordinary mains supply is
partially burned with controlled quantities of air
in a specially constructed plant which is in general
accordance with that shown in the diagram in Fig. 6.
After combustion, the gases are dried as thoroughly
as is necessary and, since most fuel gases contain
quantities of sulphur it is necessary to use some form
of “scrubber” to remove this element. A simple, yet
effective, scrubber is iron-oxide, and a box containing
this material is usually placed in the outlet line from
the combustion chamber to remove sulphur-bearing
compounds. The composition of the resultant
10
atmosphere is capable of wide variations by adjusting
the gas/air ratio in the combustion chamber.
Within the range of air/gas ratios shown in Fig. 7,
the heat generated during the combustion cycle is more
than sufficient to maintain the temperature needed for
burning. For this reason the word “exothermic” is used
to distinguish these atmospheres from those which,
as described later, require an external heat source for
their production.
Fig. 7 Partially burnt coal gas. Composition ranges which are
useful for brazing.4
In general terms, similar atmospheres to those obtained
by burning coal gas can be derived by the partial
combustion of hydrocarbon gases such as propane,
butane or methane.
A burnt town’s gas atmosphere is relatively cheap and
is satisfactory for brazing those parent metals whose
oxides are not particularly stable and, in consequence,
are easily reduced. It is specifically applicable to the
brazing of mild- and low-carbon steels with copper,
but it should be remembered that it is relatively highly
decarburising. In consequence, if decarburisation of the
part is to be avoided, it is probably better not to select
burnt town’s gas as the protective atmosphere.
Catalytically Cracked Fuel Gas. When brazing
medium- and high-carbon steels, or mild steel
assemblies which have been carburised prior to
brazing so that subsequent case-hardening of these
components can be carried out, the need arises for a
reducing atmosphere which can be produced cheaply
and yet which can be adjusted to be non-carburising
or, in certain instances, slightly carburising. Such an
atmosphere can be derived by passing a mixture of
hydrocarbon gas and a smaller amount of air than would
be required for an exothermic atmosphere over a heated
catalyst. The catalyst temperature and gas/air ratio are
adjusted to promote break-down of the hydrocarbon
present in the mixture without actual combustion of the
hydrocarbon occurring. This prevents the formation of
undesirable products such as water vapour, and carbon
dioxide. The principal reaction may be expressed by the
typical equation:
2CH4+O2
2C0 +4H2
By varying the gas/air ratio, it is possible to retain
a small quantity of methane or carbon dioxide in the
producer gas. By this means the decarburising or
carburising tendency of the gas, its so-called “carbon
potential” can be modified at will. Thus, in work where
the brazing of steels of a known carbon content is to
be carried out, it is possible to adjust the atmosphere to
be either non-decarburising or, in some instances even
slightly carburising, as may be necessary for the type of
steel being brazed.
The chemical reactions necessary to produce this
type of atmosphere do not produce much heat and, in
consequence, it is necessary to apply external heating to
the catalyst chamber so as to maintain the temperature
required for the reaction to proceed. For this reason,
the word “endothermic” is used to distinguish these
atmospheres from the better known burnt town’s gas
atmospheres which, as has been mentioned above are
“exothermic”.
Catalytically
reformed
Hydrocarbons.
The
reforming of hydrocarbons with water vapour is
relatively common since cheap starting materials are
readily available. The reformer plant comprises a
catalyst-filled vessel to which the liquefied hydrocarbon
gas and water vapour is fed in precisely controlled
proportions. Catalytic cracking, at approximately
1,000ºC occurs. The reaction occurring can be expressed
by the general formula:
CnH(2n+2) +nH2O
nCO+(2n+1)H2
The carbon monoxide produced in this reaction is
subsequently converted to carbon dioxide in a further
catalytic reaction which can be represented by:
nCO+nH2O
nCO2+nH2
By using suitable scrubbing procedures, usually with
monoethanolamine, the carbon dioxide component of
this gas mixture can be removed.
However, these chemical reactions rarely proceed to
completion, and the hydrogen gas leaving the reformer
is frequently contaminated with trace quantities of both
CO and CO2. Sometimes this is not too critical, but in
applications where the brazing of stainless steel is to
be carried out, the presence of even small amounts of
carbon in the atmosphere gas is highly undesirable since
rapid carburisation of the steel, with a consequent loss
of corrosion resistance, will inevitably occur.
In such work, it is necessary to reduce the CO
component to a level of about 0.00l and the CO2 level to
about 0.0l volume per cent if the carburisation problem
is to be avoided. Moreover, it should be remembered
that carbon may also have an oxidising effect on the
stainless steel. To reduce the level of these contaminant
gases can be a very expensive operation, requiring
considerable capital expenditure.
“Bottled” Hydrogen. Commercial grade bottled
hydrogen can be used as a protective atmosphere for
furnace brazing applications. There is, however, one
major disadvantage with gas derived from this source.
The dew point of such gas is rarely better than 10ºC
(although the major industrial gas suppliers can provide
drier hydrogen at extra cost), which is, of course, far too
wet for the successful brazing of most of the stainless
steels or nickel-base alloys now in common use. In
addition, due to the relatively high dew point of an
atmosphere derived from this source, decarburisation of
steels will be an ever-present hazard.
It is apparent, therefore, that if the brazing of
stainless steel is to be carried out in conditions that
are free from the risk of decarburising or nitriding,
conventional reducing atmosphere furnace brazing
is seemingly inadmissible. Naturally, one can select
vacuum furnace brazing as an alternative, accepting the
inevitable high cost of installing suitable equipment,
—
11
which a methanol-water mixture is reacted to produce
a mixture of carbon monoxide and hydrogen. The
product of this exothermic reaction is conducted to a
hydrogen diffusion unit contained within the generator
where the hydrogen is separated from the mixture in an
identical manner to that described in section 6 under
Catalytically Cracked Fuel Gas.
Not only does this provide an inexpensive hydrogen
atmosphere, but users of such a system have as a
bonus an atmosphere gas with a dew point of —80ºC,
or better.
7. Types of Furnaces
Fig. 8 A hydrogen diffusion unit. This particular unit will
provide an output of 14,000 litres an hour of hydrogen, the
dew point of which being better than —80ºC.
or one can choose to use the ultra-dry hydrogen derived
from a relatively inexpensive hydrogen diffusion unit
(Fig. 8).
Ultra-pure Hydrogen. Until relatively recently,
there has been no truly economic method of achieving
dew points much below about —60ºC. With the
development of inexpensive hydrogen diffusion units,
which operate on the principle of selective diffusion
of hydrogen atoms through heated membranes of a 23
per cent silver :77 per cent palladium alloy, dew points
which are better than 80ºC can be readily achieved
on a continuous basis. Diffusion units of this type are
designed to operate under all manner of hydrogen-rich
gases, but for brazing and heat-treatment applications
the gas source is usually one of:
—
Catalytically cracked ammonia.
“Bottled” hydrogen.
Reformed hydrocarbons.
Methanol-Water Hydrogen Generators. With the
recent development of methanol-water hydrogen
generators a cheap source of pure hydrogen has
become available.
These units incorporate a heated catalyst bed over
12
Muffle Furnaces. In the United Kingdom, most
muffle furnaces are fired by either coal-gas or
electricity and, providing that heating is uniform over
the whole extent of the chamber and the temperature
can be controlled, satisfactory brazed joints can be
achieved.
A gas-fired muffle furnace will normally incorporate
a refractory chamber so that the flames from the
burner pass round the outside of the chamber and do
not actually enter it. This arrangement is conducive to
an even temperature in all parts of the chamber and,
in addition, ensures that neither unburnt gas, nor the
products of combustion, come into contact with the
workpiece.
With electrically heated muffle furnaces, it is
essential that the heating elements, usually made of
nickel-chromium, are not directly exposed to flux or to
fumes evolved from the flux during the heating cycle.
A refractory muffle insert will satisfy this requirement,
but occasionally the furnace will be designed so that
the elements are protected from contamination by
inserting nickel-chromium plates between them and
the workpiece. While this will protect the elements
from accidental mechanical damage it does not, of
course, protect them from contamination by flux
fumes.
In general terms, any furnace which can operate
in the temperature range 700—1,000ºC can be used
for the brazing of steel, copper, and copper alloys,
utilising copper-zinc brazing materials, or silver
brazing alloys, typified by the 50 per cent silvercopper-cadmium-zinc alloy. While muffle furnaces are
not recommended for large scale production runs, this
does not mean that they cannot be used for this class
of work, and this type of furnace is widely employed
for small scale production runs, or for the brazing of
assemblies which are too large to be introduced into
protective atmosphere conveyor furnaces. They are
also used for the development of furnace brazing
techniques when this method is being investigated for
medium or large scale applications.
It is of paramount importance that intending users of
muffle furnaces should realise that, as a general rule,
brazing is carried out in air with a flux. Consequently,
the same post-braze flux removal operation will have
to be carried out as that required with other heating
operations carried out in air.
In some instances, portable boxes are employed
which contain the assemblies required to be brazed,
and which are so designed that a protective atmosphere
can be admitted to their interiors while they are in the
furnace. In this way, muffle furnaces can be used to
carry out protective atmosphere brazing techniques.
Unfortunately, the process is rather cumbersome and
can really only be recommended for small scale batch
production. The major problem in this instance being
the difficulty in maintaining accurate temperature
control of the parts contained within the box.
Bell Furnaces. This type of furnace can be
considered to be the logical development from the
muffle furnace, but one that is intended to be used
with a protective atmosphere. It is ideally suited for
the brazing of single, bulky, components.
The work to be brazed is placed on a hearth,
covered with a gas-tight metal bell, and the bell
purged with a protective atmosphere to remove all
traces of air. An electrically heated outer furnace is
lowered over the inner bell and remains there until
the work reaches the required brazing temperature.
Once this has been achieved, the outer furnace can
be removed and placed over a second bell which has
been previously prepared for heating. As the cooling
cycle is almost always longer than the heating cycle,
one furnace can be used to heat several bells. In this
way, virtually continuous production can be achieved
by loading, purging, heating and cooling, each bell in
rotation.
Continuous Furnaces. Modern brazing furnaces,
intended for large-scale production are frequently of
the continuous type where the work is fed in at one
end and emerges brazed at the other. The majority of
these furnaces utilise a protective atmosphere of one
kind or another. There are, however, examples of this
type of furnace being used with an atmosphere of
normal air. Naturally, in such conditions, brazing is
carried out in association with the appropriate flux.
Most furnaces of this type are electrically heated to
permit accurate control of temperature in the hot zone.
For the high temperature necessary when brazing
steel components with copper, (1,120—1,150ºC),
nickel-chromium elements are used despite the fact
that the element will be operating near its maximum
recommended temperature.
Fig. 9 A continuous-conveyor reducing atmosphere brazing
furnace. (Courtesy: Birlec Ltd.)
The most popular type of continuous furnace is the
belt conveyor (Fig. 9), which is ideally suited for the
brazing of large numbers of components. The belt is
almost always fabricated from a nickel-chromium alloy
made in the form of a mesh or as a continuous sheet.
In general terms, the work should be placed directly on
the moving furnace belt so as to maintain economical
production rates during the brazing cycle by ensuring
rapid heat transfer to the parts. In some types of work it
is necessary to use trays in which the parts to be brazed
are placed. This is particularly so when there is a danger
of molten brazing alloy falling from the component on
to the belt during the furnace cycle.
The lengths of the heating and cooling chambers are
arranged so that the workpiece cools to a temperature
low enough to avoid oxidation of the part when it
emerges from the furnace. The cooling chamber is
surrounded by a water-jacket to ensure that the length
of the cooling chamber, and hence the furnace, is kept
to a minimum.
To ensure that satisfactorily brazed joints are
produced, it is essential that the protective atmosphere
should fill the entire furnace. This is achieved by
maintaining a positive pressure of gas within the
furnace so that ingress of air into the heating and
cooling chambers is avoided. The excess gas flowing
from either end of the furnace is invariably ignited.
Where the opening of a furnace is large, and where
it is difficult to maintain a gas flow rate sufficient
to prevent the ingress of air, adjustable doors are
fitted. These doors usually have a contoured hole
13
cut into them which will permit the entry of the parts
to be brazed, but which can be efficiently purged
with the protective gas. Again, the gas issuing from
the furnace doors is ignited. Alternatively, where it
is either uneconomic or impossible, because of the
variety of parts to be brazed, to use contoured doors,
a curtain of asbestos string at the entry and exit can
be used.
While most continuous furnaces maintain the belt
in a horizontal plane, there are furnaces in which the
heating zone, and hence brazing zone, is raised a foot
or so above the entry and exit doors of the furnace.
This design is claimed to minimise the ingress of air
to the heating zone. This type of furnace is referred to
as a “hump-back” type (Fig. 10). With these furnaces,
the angle of the belt track is rarely greater than 15º
from the horizontal on the entry side and, normally,
the angle of the belt track on the exit side of the
furnace is shallower than this. This arrangement is
necessary to ensure that there is sufficient time for the
brazed part to cool to a temperature where, as with
continuous furnaces, it will not oxidise once it leaves
the cooling chamber.
Pusher Furnaces. Pusher furnaces (Fig. 11) are
generally similar to continuous furnaces. While
continuous furnaces have open ends and a moving
belt on which to place the parts, the “pusher” furnace
comprises a purging chamber immediately adjacent to
the entry to the furnace heating chamber, and a second
purging chamber at the exit.
The parts to be brazed are loaded into heat-resisting
metal trays. These trays are placed on a platform at
the side of, and level with, the furnace track and the
furnace cycle is then commenced. On initiation of the
cycle, the outer door of the purging chamber opens,
a hydraulic ram fitted to the loading table pushes
the tray into the purging chamber, the door closes,
and purging is carried out. Once the purging cycle
has been completed, the door between the purging
chamber and the furnace is opened, a second hydraulic
ram pushes the tray into the throat of the furnace, and
the door closes. The furnace cycle then continues
automatically with loaded trays being pushed into
the purging chamber, purging being carried out and
the purged material being admitted to the furnace.
Every time the furnace door opens and a further tray
is pushed into the throat of the furnace, all other trays
already in the furnace are pushed along by a distance
equal to the length of individual trays making up the
furnace load.
At the exit from the furnace, the procedure is
slightly different. In this instance, there is not
usually a door at the exit of the furnace chamber,
only a door at the side of the purging chamber. The
14
Fig. 10 These “hump-back” mesh belt conveyor furnaces
are used for the brazing of electronic components in a
hydrogen atmosphere. (Courtesy: Birlec Ltd.)
trays are removed from the furnace by a hydraulic ram
which is activated by the same electrical signal which
operates the loading ram at the far end of the furnace.
Thus, as one tray containing unbrazed parts is loaded
into the purging chamber, at the exit end of the furnace
a tray containing brazed parts is ejected.
The total length of the furnace is, naturally, equal
to a multiple of the length of individual trays used to
carry the parts through it so that the hydraulic ram
removing the parts from the furnace makes contact
with the centre of the tray.
Fig. 11. A single track “pusher” furnace. (Courtesy: Birlec Ltd.)
By utilising this form of approach to a furnace brazing
problem one is able to control the quality of the furnace
atmosphere with far greater precision than is possible
with open-ended continuous furnaces although the
output rate of brazed parts is usually somewhat lower
than that achieved with the latter type.
Hydrogen Quench Furnaces. In essence, the
hydrogen quench furnace is similar to a conventional
batch furnace in that it comprises a hearth on which
the parts to be brazed are situated and a movable lid
which contains the electrical heating elements and the
protective gas inlet (Fig. 12). In addition, such furnaces
are normally provided with a connection to a vacuum
pumping system.
In operation, the furnace hearth is loaded with the
parts to be brazed, the furnace lid is placed in position,
provision being made to give a completely gas-tight seal
between the hearth and the furnace lid. Once the seal has
been effectively made, the furnace is connected to the
vacuum pumping system and to a source of hydrogen
(which is used as the protective atmosphere in this class
of furnace).
To initiate the furnace cycle, it is first necessary
to evacuate the furnace chamber to a pressure of
approximately 100 torr, and then switch off the vacuum
system. Hydrogen is admitted to the furnace until the
pressure reading within the furnace shows about 400
mm of mercury. The hydrogen source is turned off, and
the vacuum pump reactivated until the pressure again
reads 100 torr. This back filling and evacuation cycle
is carried out three or four times to ensure removal of
the air that was trapped in the furnace when the seal
between the lid and the hearth was made.
On completion of the purging cycles, hydrogen is again
admitted to the furnace until the pressure of hydrogen is
between 400—600 mm of mercury. The heating cycle
is then commenced. Hydrogen quench furnaces are
heated by electricity, the heat being transmitted to the
parts by a combination of radiation from the elements
themselves and the excellent conducting properties of
the hydrogen gas contained within the furnace. The rate
of temperature increase is extremely good and such
furnaces can be controlled with an accuracy of ± 05C at
temperatures up to l,300ºC.
Once the assemblies within the furnace attain brazing
temperature, measured by thermocouples in contact with
the workpieces, the electrical power is disconnected and
the quench cycle is started.
Quenching is carried out by admitting large
quantities of hydrogen into the furnace and, by
using a fan, forcing the hydrogen round the furnace,
over the heated parts, through a water-cooled
heat-exchanger, and back into the furnace. Such a
quenching method is highly efficient, and cooling
Fig. 12 This specialised hydrogen quench furnace is about to
be used for the simultaneous brazing of several hundred mild
steel assemblies. (Courtesy: Kepston Ltd.)
rates of 900ºC/min. have been recorded. The hydrogen
quench furnace is probably the most efficient type of
protective atmosphere furnace currently available,
because not only can it be used for brazing but it also
has many applications for the heat-treatment of special
steels and nickel-base alloys of the Nimonic and
Inconel type where rapid rates of cooling are essential
to maintain the metallurgical properties developed
during the heat-treatment cycle.
8. Vacuum Brazing
Vacuum is defined as a state which exists in a
completely sealed space from which all gases and
vapours have been removed. No method of producing
absolute vacuum has yet been devised and, even
the so-called “vacuum” of outer space (found to be
about 1 x 1027 torr), cannot be considered to be a
true vacuum since even in this environment one finds
concentrations of several hundreds of molecules per
cubic metre of “space”. Thus, progress towards the
goal of a true vacuum must be described in various
degrees of vacuum. Clearly, the pressure attained
is limited by the materials chosen to enclose the
space, the nature of the gases and vapours to be
removed from the space, and, the method of pumping
employed. The degree of vacuum attained in various
systems bears a definite relation to atmospheric
pressure. (At sea level, this pressure is taken to be
15
Table 2
The Relationship between the Degree of Vacuum
and Pressure5
Condition
Low vacuum
Medium vacuum
Fine vacuum
High vacuum
Very high vacuum
Ultra high vacuum
Pressure Range
760 to 25 torr
25 to 1.0 torr
1.0 to 10 -3 torr
1.0 x 10 -3 to 1.0 to 10 -6 torr
1.0 x 10 -6 to 1.0 to 10 -9 torr 1.0
x 10 -9 torr and below
Table 3
Relationship between Pressure and the Magnitude
of the Mean Free Path5
Pressure
(Torr)
No. of Molecules
per cm3
760
1.0 x 10-7
2.7 x 1019
2.7 x 109
Mean Free
Path (mm)
9.5 x 16-5
14 x 103
760 mm of mercury.) Table 2 shows the relationship
existing between the degree of vacuum and the
pressure. In general terms, it can be stated that the
behaviour of gases and vapours become more complex
as conditions of temperature and pressure change.
Ideal gases conform to both Boyle’s and Charles’ Law,
but non-condensable gases and vapours do not, and
in any range of vacuum being used, one is normally
dealing with a combination of a variety of gases and
vapours. Table 3 shows the relationship between the
number of molecules present in one cubic centimetre
of gas at atmospheric pressure and at a pressure of
10-7 torr. Clearly, due to the far fewer numbers of
molecules of gas present at the lower pressure, it
automatically follows that the distance that individual
molecules have to travel before they collide with each
other (known as the mean free path), also increases
by a large factor. The average distances for both one
atmosphere pressure and a pressure of 10-7 torr are
given in Table 3.
Pumping Systems. Several major factors need to be
considered when designing a suitable pumping system
for a vacuum furnace. These are the time required to
attain the required operational pressure, the value of the
operational pressure, and the ability of the system to
maintain the pumping speed at the operational pressure.
Clearly, the time taken to “pump down” the chamber
to the required level of vacuum for any given job is an
important factor in the economics of vacuum brazing
because extended pumping times will increase the cost
of processing parts which may only need, from the
theoretical stand-point, short heating cycles. Thus, the
creation of a vacuum for brazing applications is almost
16
always carried out by a pumping system. When
pumping is applied to a system initially held at
atmospheric pressure, gas flows from the chamber
being evacuated to the pump. The gas extracted
expands into the pumping chamber and is compressed
until it is finally ejected at an outlet valve. The entire
mechanism is usually submerged in a bath of special
oil to prevent back diffusion of air into the chamber
which is being evacuated. However, a point is reached
where the pressure in the chamber is so low that the
molecular movement within the chamber becomes
random and is no longer disposed towards the outlet
orifice of the vacuum pump. It is then only possible to
reduce the pressure remaining in the chamber by using
a diffusion pump.
The principles by which the diffusion pump
operates are simple. When a liquid, oil or mercury, is
boiled, its vapour rises. Convection currents cause the
vapour to move upwards in the vacuum system and
eventually it is forced against the walls of the vacuum
chamber. The walls of the chamber are kept cool by
a coil arrangement so that the vapour condenses to
a liquid. Any molecules in the gas chamber that, by
virtue of their random motion, wander into this liquid
stream are caught and taken back towards the base
of the pump in the liquid. Here, the pressure of gas
so caught builds up to a point where a conventional
mechanical pump can remove it. It is, therefore, clear
that one can consider the diffusion pump to be acting,
in effect, as a gas compressor.
9. Vacuum Brazing Furnaces
Vacuum brazing, as a production technique, has
been in use for a little over twenty years. However, in
the United Kingdom the process is not widely used.
This is almost certainly due to the relatively high
capital cost of equipment coupled with the relative
complexity of operation. Recent developments
attributed to manufacturers of vacuum brazing
equipment have, however, resulted in substantial
reductions in the cost of furnace equipment coupled
with simplification of the operating procedure.
For certain applications, such as the brazing
of super-alloys and metals where the needs are to
eliminate such gases as hydrogen and nitrogen, for
the brazing of certain nickel-base alloys and stainless
steels for aerospace applications, and for the brazing
of some electronic components where all traces of
oxygen must be excluded, vacuum brazing frequently
offers the only solution.
As has been mentioned earlier in this survey,
hydrogen, and other reducing atmospheres, have
the effect of reducing the metallic oxides which are
present on the workpiece and/or the brazing alloy
during the heating cycles. It has also been pointed out
that the amount of water vapour present in a gas is a
critical parameter when considering furnace brazing
applications which are to be carried out without the
use of additional fluxing agents. In general terms,
it has been shown that it is necessary for the more
specialized furnace brazing applications to maintain the
dew point of the gas atmosphere at levels below 40ºC.
Another, and equally important factor, is the presence
of gases known to have a deleterious effect, from the
metallurgical point of view on the materials being
brazed or ones which form films on the surface of the
parent metal and/or brazing alloy, thus inhibiting the
wetting and flowing of brazing alloy when it becomes
molten. Both of these factors, when considered in
relation to vacuum as a furnace atmosphere, are
significantly reduced. For example, the equivalent dew
point of the residual water vapour at a pressure of 1 x
10-4 torr is 90ºC (assuming that the residual atmosphere
contains 70 per cent of water vapour by volume).
Again, the concentration of oxygen at a pressure of
1 x 10-3 torr is approximately l.3 parts per million
of oxygen. This compares most favourably with the
residual oxygen content of ultra-pure argon, though not
with ultra-pure hydrogen derived from the diffusion
units described earlier. With a properly designed leakproof system, it is normal to arrive at the situation in a
vacuum furnace where the oxygen content is somewhat
lower than 0.01 parts per million.
Even in instances where the presence of oxygen and/
or nitrogen is only deleterious because they will produce
film on the workpiece, and hence inhibit wetting and
flowing of the molten brazing alloy, the use of vacuum
as an atmosphere can frequently be supported. This
is specifically so when brazing complex workpieces
since, if pure, dry, hydrogen or other reducing gas, is
used as the furnace atmosphere, it can happen that the
purging action through restricted capillary paths will
not be effective in removing the trapped impurities. On
the other hand, vacuum will effectively remove these
pockets, thus facilitating the subsequent wetting and
flowing by the molten brazing alloy. It is not unusual
to find that modern vacuum furnaces are capable of
removing by pumping over 1,000 1 of gas a second.
Clearly, this is far in excess of the rate of removal of
impurities by any known purging action.
—
Hot-Wall Vacuum Furnaces
The earliest types of vacuum furnace in use were the
“hot-wall” variety. In essence, these were simply metal
retorts, containing the work load, which were sealed,
evacuated, and placed in a conventional furnace
for heating. Obviously this approach has several
major limitations. Among these are the relatively
slow heating and cooling rates which the parts being
treated experience, and the problem of ensuring that
the material from which the retorts are fabricated is
physically strong enough to withstand the applied
external pressure (due to atmospheric pressure), at
its operating temperature. These factors, coupled with
the requirements for higher operating temperatures,
as well as the need for faster heating and cooling
rates, have resulted in this type of furnace becoming
virtually obsolete.
Cold-wall Vacuum Furnaces
With the cold-wall vacuum furnace, the heating
elements, reflector plates, and the work itself are all
enclosed in a water-cooled vacuum chamber. With
more advanced types of furnace, movable baffles,
internal heat-exchangers, and high velocity cooling
gas circulating fans are also provided within the
vacuum chamber.
The reflector plates, designed to reflect the heat
generated by the furnace elements back into the
furnace, normally comprise multiple layers of
polished molybdenum. The effectiveness of these
plates depends almost entirely on their reflectivity,
and in consequence their tarnishing will lead to a
substantial fall in the furnace heating efficiency.
In many instances, and where tarnishing is only
slight, cleaning of the plates can be accomplished by
pumping the furnace down to its lowest rated pressure,
and operating it at its maximum rated temperature.
Sometimes cleaning can be achieved more readily by
admitting a partial pressure of pure, dry, hydrogen into
the furnace during the heating cycle; this has the effect
of reducing oxides which are difficult to remove by
dissociation - dissolution phenomena.
Radiation shield furnaces are nearly always
employed where operating temperatures in excess of
1,400ºC are required. For temperatures below 1,400ºC,
the insulated furnace, where thermal insulation is
achieved by employing a refractory blanket faced with
molybdenum, is often used.
Since thermal insulating furnaces are substantially
cheaper than the radiation shield type, the majority
of the cold-wall furnaces built in recent years have
been of this type. Moreover, this type of furnace is
more suitable for run-of-the-mill vacuum brazing
operations, where it is rarely necessary to employ
furnace temperatures in excess of about 1,250ºC
because they are less liable to damage than radiation
shield types and require considerably less power
to maintain any given temperature. However, for
specialised high temperature applications, such as
the brazing of tungsten with molybdenum-vanadium
alloys, radiation shield furnaces are recommended.
17
Fig. 13 Relationship between vapour pressure and temperature of elements
commonly employed in brazing filler materials.6
18
From what has already been said, it is clear that
contamination of vacuum furnaces can result in a
marked decrease in efficiency. In consequence, great
care should be taken to ensure that contamination
is kept to a minimum. For example, foreign matter
such as lubricants used in deep drawing operations,
notably silicone oils, volatile metals, or even such
unexpected materials as industrial gloves have been
inadvertently carried into vacuum chambers with
resultant gross damage to the radiation shields. This
has inevitably resulted in a very expensive repair
being necessary in order to clean and/or repair the
shields. As far as insulated furnaces are concerned,
the heating effect in which heating rate does not
depend on reflectivity, a simple outgassing process
is usually able to return the furnace to a usable state
within a few hours.
The heating elements used in radiation-type
vacuum furnaces are usually fabricated from
molybdenum, tungsten, or less frequently, tantalum.
With insulated furnaces, graphite as well as refractory
metal elements can be employed. One minor
disadvantage with graphite is that there is a certain
amount of outgassing, mainly water vapour, which
occurs during the initial runs. However, once this has
completely outgassed no further problems should be
experienced from this source.
10. Vapour Pressure
Vapour pressure is the pressure exerted when a
substance is in equilibrium with its own vapour,
and is a function of the temperature and pressure to
which it is exposed. Fig. 13 shows vapour pressure
versus temperature for a number of pure metals.
It will be noted, for example, that if pure gold is
heated to l,100ºC at an applied pressure of 1 x 10-5
torr, the material will start to vaporise. Clearly, if
either the temperature were raised, or the applied
pressure reduced, an appreciable amount of gold
would be vaporised and this could tend to condense
on cool parts of the furnace. It is therefore necessary
to pay due regard to vapour pressure phenomena
when considering vacuum brazing operations.
Consequently, should the selected temperature and
pressure be such that appreciable volatilisation can
be expected, provision should be made to introduce
a partial pressure of a pure, dry, gas into the furnace
chamber to minimise the effect (Fig. 14). Naturally,
the presence of either oxygen or water vapour in a gas
intended for either fast cooling of treated parts, or for
partial pressures, could result in the work becoming
discoloured and the radiation shields, if used, badly
oxidised, thus negating the advantages of processing
the work in a vacuum furnace.
Fig. 14 This vacuum furnace incorporates a device for
the automatic introduction of a partial pressure of an inert
atmosphere so as to minimise the evaporation of such
elements as silver from the parent metal or the brazing alloy
during the brazing cycle.
(Courtesy: Wentgate Engineers Ltd.)
11.
Recent Developments
Until comparatively recently, vacuum furnace
operation was relatively unsophisticated because it
was necessary to load the furnace, close the door,
pump down, heat, cool, open the door, and remove
the parts. Recently, however, vacuum furnace
manufacturers have made great strides, improving
vacuum technology almost beyond recognition when
compared to the early models. Now, for example,
it is possible to purchase vacuum equipment which
is virtually continuous in operation and, in certain
instances, contains facilities for oil quenching of
the finished parts. Such a furnace comprises three
separate chambers. The outer chambers can be loaded
independently of each other and, more importantly, of
the central (heating) chamber which can be maintained
permanently under vacuum conditions. As will be
realised, parts can be loaded, treated, and removed on
a semi-continuous basis, thus effectively halving the
cycle time of the more usual, single chamber furnace.
If oil quenching of the parts is necessary, this can be
done by moving the treated parts from the central to
the right-hand chamber, introducing a partial pressure
of a pure dry gas so as to minimise the vaporisation of
the oil, and then, by using conventional drop-hearth
techniques, depositing the parts in the oil.
Even more advanced types of furnaces are on the
drawing board and it is confidently anticipated that
fully continuous vacuum brazing furnaces will be
available within the next few years.
19
Table 4
A Representative List of the More Important Industrial Brazing Alloys
No.
Specification or
Description
1
Nominal
Melting
Range ºC
83 Mg-12 Al-5 Zn-(Be)
410 - 550
Brazing magnesium alloys.
Typical Uses
2
BS 1942/2
88 AI-12 Si
550 - 570
Brazing aluminium alloys.
3
4
5
BS 1845 Type AG1
—
—
50 Ag-Cu-Zn-Cd
34 Ag-Cu-Zn-Cd
23 Ag-Cu-Zn-Cd
620 - 630
612 - 668
616 - 735
General purpose low melting-point alloys
for brazing ferrous and non ferrous
engineering materials other than aluminium
and magnesium.
6
7
BS 1845 Type CP2
BS 1845 Type CP1
92 Cu-2 Ag-6 P
80 Cu-15 Sg-S P
644 - 740
644 - 700
Fluxless brazing of copper in air.
8
9
10
11
—
—
—
—
50 Ag-Cu-Zn-Cd-Ni
50 Ag-Cu-Zn-Cd-Ni-Mn
96 Cu-Ni-Si
85 Cu-Ni-Mn
634 - 656
639 - 668
1090 - 1100
965 - 995
12
—
56 Ag-In-Cu-Ni
600 - 711
Brazing of stainless steel to produce joints
resistant to interface corrosion.
13
14
BS1845 Type AG5
DIN 8513 L-Ag 85
43 Ag-Cu-Zn
85 Ag-15 Mn
689 - 788
960
Brazing of steam turbine blading and parts
for marine service (No. 13 only).
15
—
80 Ag-20 Zn
725 - 785
Brazing of parts operating in contact with
ammonia.
16
DIN 1735 L-Ag 67
67 Ag-Cu-Zn
70S - 723
Brazing of silver.
17
18
19
9 carat gold solder
14 carat gold solder
18 carat gold solder
37.5 Au-Ag-Cu-Zn-Cd
58.3 Au-Ag-Cu-Zn-Cd
75.0 Au-Ag-Cu-Zn-Cd
695 - 715
703 - 730
633 - 705
Brazing of hallmarked jewellery articles.
20
Pure copper
100 Cu
1083
21
—
97 Cu-Ni-B
1081 - 1101
Furnace brazing of steel parts with joint
gaps too wide for brazing with copper.
22
23
BS 1845 Type N16
BS 1845 Type N18
73 Ni-Cr-Fe-Si-B-C
81 Ni-Si-Cr-C
1060
1100
Brazing for service at elevated temperatures and for nuclear energy applications
(No. 23 only).
24
25
BS1845 Type AG7(v)
BS1845 Type PD4(v)
71 Ag-29 Cu
15 Pd-Ag-Cu
778
856 - 880
Brazing of vacuum tubes.
26
27
—
BS 1845 Type AU5(v)
68 Au-Cu-Ni-Cr-B
82.5 Au-Ni
9S0 - 980
950
Brazing for service in ultra-high vacuum
and for strength and oxidation resistance at
elevated temperatures.
28
85 184S Type PD14 (v)
60 Pd -40 Ni
1237
Brazing for high strength and oxidation
resistance at elevated temperatures.
29
Platinum
100 Pt
1769
Brazing molybdenum and tungsten for
service at ultra-high temperatures.
30
Ti-Cored Ag/Cu
S per cent Ti-Ag-Cu
900*
Direct brazing of metals to alumina
ceramics.
* Working temperature.
20
Nominal Composition
Fabrication of cemented carbide-tipped
tools and rock drills.
Furnace brazing of closely fitting steel
parts.
12.
Brazing Alloy Selection
Anyone familiar with brazing practice will be
aware that there are many hundreds of different
brazing alloys commercially available. While it is not
possible, in an survey of this type, to offer specific
recommendations regarding the selection of an alloy
for any particular purpose, one can certainly delineate
the fundamental considerations affecting the selection
of a brazing alloy.
The service conditions of the finished joint are
always the prime consideration, as it is this feature
which will dictate the properties that the brazed joint
must possess. As a result of this, the choice of brazing
alloy is almost always determined by one or more of
the following factors:
The strength and ductility of the finished joint
under the specific conditions of temperature at
which it will operate.
Some physical property dictated by the specific
service environment (for example resistance
of the joint to oxidation or corrosion, or its
ability to operate at elevated temperatures in an
evacuated environment).
Some specific physical properties of the
brazing alloy, such as its electrical or thermal
conductivity.
Statutory requirements (for example, the
restriction on the use of lead- and cadmiumcontaining brazing alloys for use in foodhandling equipment).
Aesthetic considerations (for example colour
matching of the brazing alloy with the parent
materials for jewellery and other decorative
applications).
Table 4 lists thirty different brazing alloys which are
widely used in modern industry. However, it should be
noted that while all of the alloys listed can be used for
either reducing or neutral atmosphere furnace brazing
not all of them are suitable for use in vacuum brazing
applications.
In certain instances, where the development of
a furnace brazing technique is being undertaken,
difficulties can arise. These almost always occur
because the user fails to appreciate the fact that not
only must the brazing alloy selected meet the service
requirements but it must also be compatible with the
base materials being brazed as well as with the specific
joint design which has been selected.
For a brazing alloy to be compatible with any
particular parent metal combination it must meet the
following requirements:
The melting point must be lower than that of
either parent metal.
It must be capable of wetting the parent metal at
brazing temperature.
It must not produce excessive erosion of the
parent metal during the brazing cycle by interalloying effects.
It must be capable of producing a joint at a
temperature which will not create undesirable
effects in the metallurgical structure of the
parent materials.
It must not contain either major proportions, or
trace quantities, of elements which might alloy
with the parent materials to produce brittle
joints.
When considering the compatibility of brazing
alloys with brazing methods the following points
should be remembered:
As has already been mentioned, while both
zinc- and cadmium-bearing alloys are widely
used for furnace brazing both with and without
the application of brazing fluxes due care will
have to be taken to prevent any harmful effects
on the furnace equipment of the cadmium, zinc,
and flux fumes.
When the joint design employed demands the
use of a preplaced brazing alloy, it is desirable to
use alloys that do not have an extended melting
range. This is particularly true if the rate of
heating through the melting range of the brazing
alloy is relatively slow. Under these conditions,
liquation, the gradual separation of the molten
portion of the brazing alloy from its solid
fraction, can be a hazard which may prevent the
formation of a completely filled joint.
Close control of the furnace temperature will
be necessary when the difference between the
liquidus of the brazing alloy and the solidus
of one or other of the parent materials is less
than 100ºC. If close control is not exercised the
situation can arise where melting of the parent
metals may occur.
Due to the relatively high vapour pressure of
both cadmium and zinc, alloys containing either
of these elements must not be used for vacuum
brazing applications (Fig. 13).
For certain vacuum brazing applications,
alloys containing silver are employed. While
some loss of silver from the molten brazing
alloy can take place due to volatilisation if too
high a brazing temperature is obtained, this
problem can be avoided by using a vacuum
technique during the early stages of heating,
and subsequently introducing into the furnace
chamber a partial pressure of dry argon, or
21
Table 5
Suitability of Various Brazing Alloy/Parent Metal Combinations for Furnace Brazing in Reducing
or Neutral Atmospheres7
Composition
Aluminium Alloys
Copper
No.
Magnesium Alloys
Copper Alloys
Silver Alloys
Gold Alloys
Carbon Steels
Stainless Steels
Cemented Carbides
Nickel Alloys
Cobalt Alloys
Titanium Alloys
Tungsten
Molybdenum
Tantalum
Niobium
Parent Metals
1
Mg-Al-Zn
R
P
N
N
N
N
N
N
N
N
N
N
N
N
N
N
2
Al-Si
O
R
N
N
N
N
N
N
N
N
N
P
N
N
N
N
3,4,5,8,9
Ag-Cu-Zn-Cd-(Ni,Mn)
O
O
R
R
R
P
R
R
R
R
P
N
P
P
N
N
13,16
Ag-Cu-Zn
O
O
R
R
R
P
R
R
R
R
P
N
P
P
N
N
6,7
Ag-Cu-P
O
O
R
R
R
P
N
N
N
N
N
N
P
P
N
N
12
Ag-Cu-In-Ni
O
O
P
P
P
P
P
R
P
P
P
N
P
P
P
P
10,20,21
Cu(Ni-Si,B)
O
O
O
O
O
O
R
R
R
R
P
N
R
P
P
P
14
Ag-Mn
O
O
N
N
O
N
P
R
R
R
P
N
P
P
P
P
15
Ag-Zn
O
O
P
P
P
P
P
R
P
R
P
N
P
P
N
N
17,18,19
Au-Ag-Cu-Zn-Cd
O
O
P
P
P
R
P
P
P
P
P
N
P
P
N
N
22,23
Ni-Cr-Si-B-C
O
O
O
O
O
O
P
R
P
R
P
N
R
R
P
P
24
Ag-Cu
O
O
R
R
P
P
P
R
P
R
P
P
P
P
P
P
25
Ag-Cu-Pd
O
O
R
R
P
P
P
R
P
R
P
R
R
R
P
P
28
Pd-Ni
O
O
O
O
O
O
P
R
P
R
P
N
R
R
R
P
26,27
Au-Ni(Cu-Cr-B)
O
O
P
P
O
P
P
R
P
R
P
P
R
R
R
P
29
Pt
O
O
O
O
O
O
O
O
O
O
O
O
R
R
P
P
Brazing Alloys
Notation:
O
N
P
R
=
=
=
=
Impossible
Not recommended
Possible but seldom used
Recommended and known to produce satsifactory results.
Notes:
1. Brazing fluxes may have to be used in addition to a protective atmosphere when the parent metals brazed contain
alloy constituents forming refractory oxides (e.g. Cr, Ti, Al, Zn. etc.).
2. It is not recommended to furnace braze tantalum, niobium, and tough pitch or oxidised copper in hydrogen-bearing
atmospheres.
helium, to minimise the rate of evaporation. In
some work, particularly with vacuum furnace
brazing, the brazing alloy may be maintained
in the molten state for a considerable period of
time. Under these circumstances the possibility
of erosion of parent materials by the molten
brazing alloy is an ever-present hazard. Due
22
regard must be paid to this possibility when
vacuum brazing is the selected heating method.
The data presented in Tables 5 and 6 illustrate
how the characteristics discussed above affect the
suitability of the alloys listed in Table 4 for the brazing
of a wide variety of parent metals by either reducing
or neutral atmosphere, or vacuum furnace brazing.
Table 6
Suitability of Various Brazing Alloy / Parent Metal Combinations for Vacuum Brazing7
Composition
Aluminium Alloys
Copper
No.
Magnesium Alloys
Copper Alloys
Silver Alloys
Gold Alloys
Carbon Steels
Stainless Steels
Cemented Carbides
Nickel Alloys
Cobalt Alloys
Titanium Alloys
Tungsten
Molybdenum
Tantalum
Niobium
Parent Metals
1
Mg-Al-Zn
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
2
Al-Si
O
P
N
N
N
N
N
N
N
N
N
P
N
N
N
N
3,4,5,8,9
Ag-Cu-Zn-Cd-(Ni,Mn)
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
N
13,16
Ag-Cu-Zn
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
N
6,7
Cu-Ag-P
O
O
P
P
P
P
N
N
N
N
N
N
P
P
N
N
12
Ag-Cu-In-Ni
O
O
P
P
P
P
P
R
P
P
P
N
P
P
P
P
10,20,21
Cu(Ni-Si,B)
O
O
O
O
O
O
R
R
R
R
P
N
P
P
R
P
14
Ag-Mn
O
O
N
N
O
N
P
P
P
P
P
N
P
P
P
P
15
Ag-Zn
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
N
17,18,19
Au-Ag-Cu-Zn-Cd
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
N
22,23
Ni-Cr-Si-B-C
O
O
O
O
O
O
P
R
P
R
P
N
R
R
P
P
24
Ag-Cu
O
O
P
P
P
P
P
P
P
P
P
P
P
P
P
P
25
Ag-Cu-Pd
O
O
P
P
P
P
P
P
P
P
P
P
P
P
P
P
28
Pd-Ni
O
O
O
O
O
O
P
R
P
R
P
N
R
R
R
P
26,27
Au-Ni(Cu-Cr-B)
O
O
P
P
O
P
P
R
P
R
P
P
R
R
R
P
29
Pt
O
O
O
O
O
O
O
O
O
O
O
O
R
R
P
P
Brazing Alloys
Notation:
O
N
P
R
=
=
=
=
Impossible
Not recommended
Possible but seldom used
Recommended and known to produce satisfactory results
So far as the compatibility of brazing alloys with the
joint design is concerned, there is a fundamental rule
that should be observed. Where joint gaps presented
are small, the brazing alloy selected should have as
low a viscosity as possible and, moreover, a short
melting range. Ideally, a single melting point is to
be preferred. In addition, the alloy should readily
wet the particular parent metal, and its free-flowing
characteristics should not be affected by inter-alloying
with the parent metals during the brazing cycle.
Materials possessing these characteristics cannot, of
course, be used when the joint gap presented is large,
since this configuration requires alloys that are either
sluggish or have a relatively extended melting range.
It is difficult to give more specific recommendations
because the properties of any particular brazing alloy
which determine the optimum joint design may vary
from workpiece to workpiece, depending on the parent
metals and brazing technique employed.
For example, when furnace brazing mild steel with
pure copper in a reducing atmosphere, interference
fits or joint clearances in the order of 0025 mm will
23
give the best results. However, if brazing of mild
steel components with an alloy typified by that shown
as number 3 in Table 4, and where a flux will be
required to be used, the joint gap presented should
lie in the range 0.025—0.1 mm. This is because the
joint gap presented must be made sufficiently large
to permit access to the brazing flux without which
the capillary flow of the molten brazing alloy would
bound to be inhibited by the presence of the surface
oxide films. With stainless steel, where the surface
oxides are difficult to remove unless very dry reducing
atmospheres are used, it is necessary to employ fairly
wide joint gaps. Typical joint gaps for the furnace
brazing of stainless steel, utilising copper as the filler
material are 0.025—0.075 mm. It will be noted that the
upper limit of this gap size range is really too high for
the successful use of copper.
When nickel-base alloys are to be brazed with
copper, joint clearances in the range 0.050—0.130mm
may need to be employed. This is because molten
copper has a marked tendency to dissolve nickel from
the base material resulting in the formation of coppernickel alloys. With such alloys, increasing dissolution
of nickel results in the solidus value of the alloy also
increasing. Thus, to ensure good capillary flow of this
molten copper-nickel alloy through the joint at any
given brazing temperature, the nickel concentration
in the molten alloy must be kept as low as possible.
The longer the joint that the copper has to fill by the
mechanism of capillary flow, the wider must be the
joint clearance so that the effects of inter-alloying
between the copper and the base material are kept to
a minimum.
Further information regarding the selection of
brazing alloys for a variety of applications such as
brazing for service in vacuum (Table 7), brazing
nonmetallic materials, brazing for strength and
oxidation resistance at elevated temperatures, will be
found in an article by M. H. Sloboda in the October
1966 issue of this journal.
Table 7
Composition of Several Brazing Filler Materials Suitable for the Step
Brazing of Vacuum Equipment8
Composition
Melting
Range
Usual
Joint Gaps
mm
Conforms
to BS 1845
Ag-Cu
Ag
Au
Pd
Pt
5 Pd-Ag-Cu
10 Pd-Ag-Cu
10 Pd-Ag-Cu
15 Pd-Ag-Cu
20 Pd-Ag-Cu
25 Pd-Ag-Cu
5 Pd-Ag
18 Pd-Cu
30 Pd-Ag
60 Pd-Ni
80 Au-Cu-Fe
62.5 Au-Cu
37.5 Au-Cu
30 Au-Cu
70 Au-Cu
82.5 Au-Ni
68 Au-Cu-Ni-Cr-B
75 Au-Ni
778
962
1064
1552
1768
807—810
830—840
824—850
856—880
876—900
901—950
970—1010
1080—1090
1150—1225
1237
908—910
930—940
980—998
996—1018
1030—1040
950
950—980
950—990
0.025—0.1
0.025—0.1
0.025—0.1
0.025—0.1
0.025—0.1
0.025—0.1
0.025—0.1
0.075—0.2
0.075—0.2
0.075—0.2
0.075—0.2
0.075—0.2
0.075—0.2
0.075—0.2
0.025—0.1
0.025—0.1
0.025—0.1
0.025—0.1
0.025—0.1
0.025—0.1
0.025—0.1
0.025—0.1
0.075—0.2
AG 7V
AG 8
—
—
—
PD 1v
PD 3V
PD 2V
PD 4V
PD 5V
PD 6V
PD 7V
PD 8V
—
PD 14V
AU 1V
AU 2V
AU 3V
AU 4V
—
AU 5V
—
AU 6V
71
REFERENCES
1 Kubashevskii, 0., Evans, E.: “Metallurgical Thermochemistry”
IL. 1954. (Translation from the Russian.)
2 Lustman, B.: “Resistance of Metals to Scaling”, Metal
Progress, 50, No. 5, 850, 1946.
3 Brooker, H. R. and Beatson, E. V.: “Industrial Brazing”, p. 58.
4 Hancock, P. F.: Metal Industry, February 11th and February
18th, 1949.
5 Schwartz, M. M.: “Modern Metal Joining Techniques”. WileyInterscience, 1967.
6 AWS Brazing Manual, 1963.
7 Sloboda, M. H.: “The Selection of Brazing Alloys”, Welding
and Metal Fabrication, October 1966.
8 Johnson Matthey Metals Ltd. Data Sheet 1100:200.
24
25
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81 Hatton Garden, London EC1P 1AE, England
26
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