Moisture-free atmosphere brazing of ferrous metals

The present invention discloses a novel, moisture-free atmosphere for brazing carbon steels that provides good braze flow and brazed joint quality with minimum or no formation of soot on brazed joints. According to the present invention, carbon steels are brazed in continuous furnaces using a moisture-free atmosphere containing a mixture of three gases including nitrogen, hydrogen, and carbon dioxide. The key features of the invention involve (1) formation of moisture, which is needed to facilitate braze flow and to minimize formation of soot on brazed joints, in-situ in the heating zone of the furnace by the reaction between hydrogen and carbon dioxide and (2) reduction in the overall amount of a reducing gas required for brazing carbon steels by keeping moisture out of the cooling zone. The use of a moisture-free three gas atmosphere has been unexpectedly found to (1) eliminate the need of an expensive and difficult to control external humidification system, (2) provide flexibility in adjusting moisture content of the atmosphere in the heating zone of the furnace simply by adjusting the flow rate of hydrogen or carbon dioxide or both, and (3) facilitate rapid conditioning of the furnace by keeping moisture out of the cooling zone. The moisture-free atmosphere has also been unexpectedly found to provide good braze flow, fillet formation, and brazed joint quality while minimizing or eliminating formation of soot on brazed joints.

TECHNICAL FIELD OF THE INVENTION 
The present invention pertains to brazing of ferrous metal parts. 
BACKGROUND OF THE INVENTION 
The brazing of ferrous metals (e.g., carbon steel components) involves 
joining surfaces of ferrous metals with brazing pastes or preforms. The 
brazing pastes or preforms generally contains a metal or a mixture of 
metals and an organic (or hydrocarbon) binder. The melting point of metal 
or mixture of metals in brazing pastes or preforms generally is 
substantially lower than that of the base carbon steel components. The 
components are joined by juxtaposing them with the brazing paste or 
preform adjacent to or between them, and heating to a temperature that 
will effect melting of the brazing metal or mixture of metals without 
melting the components. 
The function of organic or hydrocarbon binder is to serve as a vehicle for 
metal or a mixture of metals. It generally consists of pure or mixtures of 
low-boiling point organic or hydrocarbon compounds such as glycols and 
ethers. These compounds are thermally dissociated and removed from joints 
while heating components to be brazed to brazing temperatures. 
For example, carbon steel components are generally brazed in the presence 
of nitrogen-based atmospheres containing controlled amounts of a reducing 
gas such as hydrogen and an oxidant such as moisture. The function of a 
reducing gas is to keep the surface of carbon steel components from 
oxidizing and also to maintain reducing potential in both the heating and 
cooling zones of the furnace. The functions of an oxidant are to help in 
regulating braze flow and in removing the organic binder from the braze 
material and to prevent formation of soot on brazed joints. The use of 
high concentration of a reducing gas in the atmosphere is known to cause 
overflow of brazing material, resulting in poor quality of brazed joints. 
The use of low or insufficient concentration of an oxidant is known to 
result in the formation of soot on brazed joints. Likewise, the use of low 
concentration of a reducing gas or high concentration of an oxidant is 
known to oxidize the braze material and components, resulting in poor 
braze flow and brazed joint quality and unacceptable appearance of brazed 
components. Therefore, it is critical to carefully select concentrations 
of both a reducing gas and an oxidant in the brazing atmosphere to (1) 
minimize overflow and underflow of braze material, (2) maintain reducing 
potential in the furnace, (3) assist in breaking down organic binder, and 
(4) prevent formation of soot on brazed joints. 
The importance of controlling concentrations of hydrogen and moisture in 
the humidified nitrogen-hydrogen atmosphere has been described in detail 
in a paper titled "The Effect of Atmosphere Composition on Braze Flow" 
presented by Air Products and Chemicals Inc. at the 14th Annual AWSANRC 
Brazing and Soldering Conference held in Philadelphia, Pa. on 26-28 Apr., 
1983. The teachings of this paper are incorporated here by reference. 
The humidified nitrogen-hydrogen atmospheres in theory provide brazing 
companies ultimate freedom in terms of selecting concentrations of both 
the reducing gas and moisture. They also provide brazing companies 
ultimate flexibility in changing the overall flow rate and composition of 
the brazing atmosphere. However, in practice, they do not provide brazing 
companies means of precisely changing or controlling the concentration of 
moisture in the atmosphere. Often humidifiers used to add moisture in the 
nitrogen-based atmospheres are either too expensive or sized improperly to 
meet ever-changing atmosphere needs of the brazing companies. Furthermore, 
since a part of these atmospheres travels toward the cooling zone of the 
furnace and exits the furnace through the opening in the discharge 
vestibule, they require high concentration of hydrogen to maintain 
reducing potential in the cooling zone. 
U.S. Pat. No. 4,450,017 discloses the use of a moisture-free nitrogen-based 
atmosphere for decarburize annealing carbon steels. It exposes the metal 
to be decarburize to a moisture-free atmosphere containing 1-50% carbon 
dioxide, 1-20% hydrogen, and the balance being nitrogen to a temperature 
close to the ferrite-austenite transition temperature of about 927.degree. 
C. This patent does not teach anything about (1) forming moisture in-situ 
in the heating zone of the furnace by reaction between hydrogen and carbon 
dioxide and (2) using moisture-free atmosphere for brazing carbon steels 
at a temperature above about 1,080.degree. C. 
SUMMARY OF THE INVENTION 
The present invention relates to processes for moisture-free atmosphere 
brazing of ferrous metals, e.g., carbon steels, that provides good braze 
flow and brazed joint quality with minimum or no formation of soot on 
brazed joints. The processes permit brazing of ferrous metals (e.g., 
carbon steel) components in continuous furnaces using a moisture-free 
atmosphere containing a mixture of three gases including nitrogen, 
hydrogen, and carbon dioxide. The moisture needed to provide good braze 
flow and brazed joint quality as well as to minimize or eliminate 
formation of soot on brazed joints is formed in-situ in the heating zone 
of the furnace by the reaction between hydrogen and carbon dioxide. The 
use of a moisture-free three gas atmosphere has been unexpectedly found to 
(1) eliminate need of an expensive and difficult to control external 
humidification system, (2) provide flexibility in adjusting moisture 
content of the atmosphere in the heating zone of the furnace simply by 
adjusting the flow rate of hydrogen or carbon dioxide or both, and (3) 
facilitate rapid conditioning of the furnace by keeping moisture out of 
the cooling zone. The moisture-free atmosphere has also been unexpectedly 
found to provide good braze flow, fillet formation, and brazed joint 
quality while minimizing or eliminating soot formation on brazed joints. 
According to one aspect of the present invention, carbon steel components 
are brazed in a continuous furnace operated above about 1,080.degree. C. 
using a gaseous mixture of nitrogen, hydrogen, and carbon dioxide. The 
concentrations of hydrogen and carbon dioxide in the moisture-free gaseous 
feed gas are controlled in such a way that they facilitate formation of 
the desired amount of moisture in the heating zone of the furnace and 
provide the desired reducing potential both in the heating and cooling 
zones of the furnace.

DETAILED DESCRIPTION OF THE INVENTION 
Nitrogen-based brazing atmospheres needed for brazing of ferrous metals 
(e..g, carbon steel components) are generally supplied either by producing 
them on-site using exothermic generators or by humidifying blends of pure 
nitrogen and hydrogen. The exothermically generated nitrogen-based 
atmospheres generally contain a mixture of nitrogen, hydrogen, carbon 
dioxide, carbon monoxide, and trace amounts of oxygen and hydrocarbons. 
These atmospheres are introduced into the continuous furnace through an 
inlet port in the transition zone, which is located between the heating 
and cooling zones of the furnace. A part of these atmospheres travels 
toward the heating zone and exits the furnace through the opening in the 
feed vestibule. The remaining part travels toward the cooling zone and 
exits the furnace through the opening in the discharge vestibule. Since it 
is difficult to (1) change and precisely control the composition of 
exothermically generated nitrogen-based atmosphere and (2) change overall 
flow rate, brazing companies prefer the use of humidified blends of 
nitrogen and hydrogen. These nitrogen-hydrogen blends in theory provide 
brazing companies ultimate freedom in terms of selecting the 
concentrations of both reducing gas and moisture. They also provide 
ultimate flexibility in changing the overall flow rate and composition of 
the brazing atmosphere. However, in practice, they do not provide brazing 
companies means of precisely changing or controlling the concentration of 
moisture in the atmosphere. Often humidifiers used to add moisture in the 
nitrogen-based atmosphere are either too expensive or sized improperly to 
meet ever-changing atmosphere needs of the brazing companies. Furthermore, 
since a part of these atmospheres travels toward the cooling zone and 
exits the furnace through the opening in the discharge vestibule, they 
require high concentration of hydrogen to maintain reducing potential in 
the cooling zone. Therefore, there is a need to develop a nitrogen-based 
atmosphere that eliminates the use of a humidifier, provides brazing 
companies an economical means of changing and precisely controlling the 
moisture level in the heating zone of a brazing furnace, and makes 
economical use of hydrogen gas. 
The amount of hydrogen gas required for brazing can be reduced by using two 
or more feed gas inlet ports, with at least one port each located in the 
heating and cooling zones. This kind of arrangement will provide means of 
introducing humidified nitrogen-hydrogen atmosphere in the heating zone 
and dry nitrogen-hydrogen atmosphere in the cooling zone, thereby keeping 
moisture out of the cooling zone and facilitating economical use of 
hydrogen. This arrangement, however, requires delicate instruments to 
balance the furnace and to prevent air from infiltrating the furnace. 
Furthermore, this arrangement still requires an expensive and difficult to 
control humidifier. 
It is believed that the amount of hydrogen gas required for brazing can be 
reduced and the need of an expensive and difficult to control humidifier 
can be eliminated at the same time provided moisture needed for the 
brazing operation is formed in-situ in the heating zone of the furnace. It 
is also believed that the formation of moisture in-situ in the heating 
zone will provide brazing companies ultimate freedom in terms of selecting 
concentrations of both reducing gas and moisture. 
It has surprisingly been found that the amount of hydrogen gas required for 
brazing is reduced and the need of an expensive and difficult to control 
humidifier is eliminated at the same time by using a moisture-free mixture 
of three gases including nitrogen, hydrogen, and carbon dioxide. The 
gaseous mixture, according to the present invention, is introduced into 
the furnace through an inlet port located in the transition zone. A part 
of the feed gas travels toward the heating zone and exits the furnace 
through the opening in the feed vestibule. While traveling through the 
heating zone, some of the carbon dioxide present in the feed gas reacts 
with hydrogen following the reaction described below, forming moisture 
required for brazing carbon steels. The 
EQU CO.sub.2 +H.sub.2 .fwdarw.CO+H.sub.2 O 
amount of moisture formed in-situ in the heating zone of the furnace 
depends upon the concentration of both carbon dioxide and hydrogen present 
in the feed gas. It can be varied by changing the flow rate of carbon 
dioxide, or hydrogen, or both. It also depends greatly upon the operating 
temperature of the furnace. It is, therefore, critical to maintain a 
certain minimum temperature in the furnace. 
The remaining part of the feed gas travels toward the cooling zone and 
exits the furnace through the opening in the discharge vestibule. Since 
this part of feed gas does not contain moisture and the temperature in the 
cooling zone is not high enough to facilitate reaction between hydrogen 
and carbon dioxide, the cooling zone is isolated from moisture containing 
gases. It helps in reducing both the amount of hydrogen required to 
maintain reducing potential in the cooling zone and the time required to 
condition the furnace. 
The amount of moisture required for brazing ferrous metal components 
depends upon the nature and type of brazing paste or preform used during 
the brazing operation. Some brazing pastes require atmospheres that 
contain high dew point (high moisture content); whereas, others require 
either medium or low dew points (medium or low moisture contents). 
Generally speaking, brazing pastes containing nickel require low dew point 
(low moisture content) atmospheres. Brazing pastes requiring low dew point 
or low moisture content are generally preferred for brazing steel 
components where it is critical to maintain carbon level in the base 
metal. The atmospheres disclosed in the present invention are suitable for 
brazing carbon steel components both with low and high dew point brazing 
pastes. 
The present invention, therefore, discloses a novel, moisture-free 
atmosphere for brazing carbon steels that provides good braze flow and 
brazed joint quality with minimum or no formation of soot on brazed 
joints. According to the present invention, carbon steels are brazed in 
continuous furnaces using a moisture-free atmosphere containing a mixture 
of three gases including nitrogen, hydrogen, and carbon dioxide. The 
concentrations of hydrogen and carbon dioxide in the moisture-free gaseous 
feed gas are controlled in such a way that they facilitate formation of 
the desired amount of moisture in the heating zone of the furnace and 
provide the desired reducing potential both in the heating and cooling 
zones of the furnace. 
Nitrogen required for brazing operation is pure and contains less than 10 
ppm residual oxygen content. It can be supplied by producing it using well 
known cryogenically distillation technique. It can alternatively be 
supplied by purifying non-cryogenically generated nitrogen. Hydrogen gas 
can be supplied by producing it on-site using an ammonia disssociator. It 
can also be supplied in gaseous form in compressed gas cylinders or 
vaporizing liquefied hydrogen. Carbon dioxide can be supplied in gaseous 
form in compressed gas cylinders or vaporized liquid form. 
According to the present invention, ferrous metals, e.g., carbon steel 
components, are brazed in a continuous furnace operated above about 
1,080.degree. C. using a moisture-free three gas mixture containing 
nitrogen, hydrogen, and carbon dioxide. The flow rates of hydrogen and 
carbon dioxide are controlled to provide a hydrogen to carbon dioxide 
ratio of at least 1.0 in the gaseous feed mixture. Furthermore, they are 
controlled in such a way that they produce in-situ the desired amount of 
moisture and provide a hydrogen to moisture ratio greater than 2.0 in the 
heating zone of the furnace. 
The following examples are provided to illustrate various embodiments of 
the invention and are not intended to restrict the scope thereof. 
A number of experiments were carried out in a Watkins-Johnson continuous 
conveyor belt furnace operated at about 1,100.degree. C. to braze 1010 
carbon steel components. The furnace consisted of an 8.75 in. wide, about 
4.9 in. high, and 86 in. long heating zone and a 90 in. long cooling zone. 
A flexible conveyor belt with a fixed belt speed of 5 in per minute was 
used to feed carbon steel components into the furnace for brazing in all 
the experiments. A total flow rate of about 350 SCFH of mixture of 
nitrogen and carbon dioxide, nitrogen and hydrogen, humidified nitrogen 
and hydrogen, or nitrogen, hydrogen, and carbon dioxide was introduced 
into the transition zone of the furnace to develop moisture-free 
atmosphere for brazing carbon steel components. 
Several gas samples were taken from the heating and cooling zones to 
monitor and regulate the composition of atmosphere present in the heating 
and cooling zones of the furnace. Specifically, the flow rates of both 
hydrogen and carbon dioxide in the feed gas were regulated to provide the 
desired moisture content and reducing potential in the heating zone and 
the desired reducing potential in the cooling zone of the furnace. 
A commercially available brazing paste CNG-1900-750 requiring high dew 
point atmosphere was used in most of the brazing experiments. It was 
supplied by Fusion, Inc. of Willoughby, Ohio. An experimental nickel 
containing brazing paste 21 2D requiring low dew point atmosphere was also 
used in brazing experiments. It was supplied by SCM Metal Products, Inc. 
of Research Park Triangle, N.C. 
The quality of brazed joints were determined either visually or by 
cross-sectioning and analyzing them. 
EXAMPLE 1 
Flat strips of 1010 carbon steels were brazed with commercially available 
(Fusion, Inc.) and experimental (SCM Metal Products, Inc.) brazing pastes 
in the Watkins Johnson continuous belt furnace operated at 1,100.degree. 
C. using pure and dry (dew point less than -55.degree. C.) mixture of 
nitrogen and hydrogen atmosphere containing 4.0 vol. % hydrogen. The use 
of commercially available brazing paste resulted in unacceptable brazed 
joints with heavy sooting and very minimal braze flow. The use of 
experimental paste also resulted in unacceptable brazed joints with 
excessive braze flow and medium to heavy soot formation. It is, therefore, 
clear that a pure and dry mixture of nitrogen and hydrogen cannot be used 
for brazing carbon steel components. 
EXAMPLE 2 
The brazing procedure described in Control Example 1 was repeated using 
similar furnace, brazing temperature, components, and brazing pastes. 
However, pure and dry mixture of nitrogen and carbon dioxide (oxidant) 
atmosphere containing 1.5 vol. % carbon dioxide was used instead of using 
a mixture of pure and dry nitrogen and hydrogen. The results of this 
experiment visually showed brazed joints with good braze flow both with 
commercially available and experimental brazing pastes. However, the use 
of a mixture of pure and dry nitrogen and carbon dioxide resulted in 
medium to heavy soot formation on brazed joints with presence of carbon 
particles in brazed joints. This atmosphere mixture also resulted in 
oxidizing the base material, as evidenced by the presence of bluish film 
on the surface of carbon steel components. This example, therefore, showed 
that a mixture of nitrogen and an oxidant cannot be used to braze carbon 
steel components. 
EXAMPLE 3 
The brazing procedure described in Control Example 1 was repeated several 
times using similar furnace, brazing temperature, components, and brazing 
pastes. A mixture of humidified nitrogen and hydrogen gas with varying 
moisture and hydrogen contents was used in these experiments instead of 
using a mixture of pure and dry nitrogen and hydrogen. 
EXAMPLE 3A 
The brazing experiment carried out with 0.2% moisture and 3.8% hydrogen in 
the humidified nitrogen and hydrogen feed atmosphere showed brazed joints 
with excessive and unacceptable braze flow both with commercially 
available and experimental brazing pastes. The use of commercially 
available brazing paste resulted in medium to heavy soot formation on 
brazed joints, indicating that the amount of moisture present in the 
atmosphere was not high enough to eliminate soot formation. The use of 
experimental paste, on the other hand, did not show formation of any soot 
on brazed joints, indicating that the amount of moisture present in the 
atmosphere was high enough to eliminate soot formation. These experiments, 
therefore, showed that the use of 0.2% moisture in the nitrogen-hydrogen 
atmosphere was not high enough to braze carbon steel components with good 
brazed joints quality with commercially available brazing paste. They also 
showed that the use of a hydrogen to moisture ratio of 19.0 in the feed 
gas was too high to braze carbon steel components with good brazed joints 
quality with experimental brazing paste requiring low dew point 
atmosphere. 
EXAMPLE 3B 
The brazing experiment carried out with 0.4% moisture and 3.6% hydrogen in 
the humidified nitrogen and hydrogen feed atmosphere showed brazed joints 
with acceptable braze flow and brazed joints quality both with 
commercially available and experimental brazing pastes. The use of this 
atmosphere composition also resulted in little or no soot formation on 
brazed joints. These experiments, therefore, showed that a moisture level 
greater than 0.2% and a hydrogen to moisture ratio less than 19.0 were 
required in the humidified nitrogen-hydrogen atmosphere to braze carbon 
steel components with acceptable braze flow and brazed joints quality. 
EXAMPLE 3C. 
The brazing experiment carried out with 0.6% moisture and 3.4% hydrogen in 
the humidified nitrogen and hydrogen feed atmosphere showed brazed joints 
with acceptable braze flow and brazed joints quality both with 
commercially available and experimental brazing pastes. The use of this 
atmosphere composition resulted in very little or no soot formation on 
brazed joints. These experiments, therefore, confirmed that a moisture 
level greater than 0.2% and a hydrogen to moisture ratio less than 19.0 
were required in the humidified nitrogen-hydrogen atmosphere to braze 
carbon steel components with acceptable braze flow and brazed joints 
quality. 
EXAMPLE 3D 
The brazing experiment carried out with 0.8% moisture and 3.2% hydrogen in 
the humidified nitrogen and hydrogen feed atmosphere showed brazed joints 
with acceptable braze flow and brazed joints quality both with 
commercially available and experimental brazing pastes. The use of this 
atmosphere composition resulted in no soot formation on brazed joints. 
These experiments, therefore, confirmed that a moisture level greater than 
0.2% and a hydrogen to moisture ratio less than 19.0 were required in the 
humidified nitrogen-hydrogen atmosphere to braze carbon steel components 
with acceptable braze flow and brazed joints quality. 
EXAMPLE 3E 
The brazing experiment carried out with 1.0% moisture and 3.0% hydrogen in 
the humidified nitrogen and hydrogen feed atmosphere showed brazed joints 
with acceptable braze flow and brazed joints quality both with 
commercially available and experimental brazing pastes. The use of this 
atmosphere composition resulted in no soot formation on brazed joints. The 
ratio of hydrogen to moisture present in the humidified nitrogen-hydrogen 
atmosphere (hydrogen to moisture ratio of 3.0) was high enough to yield 
brazed components with bright, unoxidized surface finish. These 
experiments showed that a moisture level greater than 0.2% and a hydrogen 
to moisture ratio less than 19.0 were required in the humidified 
nitrogen-hydrogen atmosphere to braze carbon steel components with 
acceptable braze flow and brazed joints quality. 
EXAMPLE 3F 
The brazing experiment carried out with 1.0% moisture and 2.0% hydrogen in 
the humidified nitrogen and hydrogen feed atmosphere showed brazed joints 
with acceptable braze flow and brazed joints quality both with 
commercially available and experimental brazing pastes. The use of this 
atmosphere composition resulted in no soot formation on brazed joints. 
However, the use of this atmosphere resulted in brazed components with 
oxidized surface finish, indicating that a ratio of hydrogen to moisture 
of 2.0 was not high enough to yield brazed components with bright, 
unoxidized surface finish. These experiments showed that a moisture level 
greater than 0.2% and a hydrogen to moisture ratio less than 19.0 but more 
than 2.0 were required in the humidified nitrogen-hydrogen atmosphere to 
braze carbon steel components with acceptable braze flow, brazed joints 
quality, and surface finish. 
The foregoing examples showed that carbon steel components cannot be brazed 
in pure and dry mixture of nitrogen and hydrogen or nitrogen and carbon 
dioxide. They also showed that, for the pastes used, a moisture level 
greater than 0.2% and a hydrogen to moisture level less than 19.0 but 
greater that 2.0 are required in humidified nitrogen-hydrogen atmospheres 
for brazing carbon steel components. 
EXAMPLE 4 
The brazing procedure described in Example 1 was repeated several times 
using similar furnace, brazing temperature, components, and brazing pastes 
to demonstrate the present invention. A mixture of moisture-free nitrogen, 
hydrogen, and carbon dioxide was used in these experiments instead of 
using mixture of dry nitrogen and hydrogen or nitrogen and carbon dioxide 
or humidified nitrogen and hydrogen. 
EXAMPLE 4A 
This example describes results obtained by carrying out a brazing 
experiment using a moisture-free nitrogen-based atmosphere containing 0.2% 
carbon dioxide and 4% hydrogen. Gas sample taken from the cooling zone 
with the introduction of 0.2% carbon dioxide and 4.0% hydrogen along with 
nitrogen into the furnace through transition zone showed only a marginal 
change in the composition of the part of the atmosphere flowing through 
the cooling zone. Gas sample taken from the heating zone, on the other 
hand, showed a dramatic change in the composition of the part of the 
atmosphere flowing through it. Specifically, a part of carbon dioxide 
present in the atmosphere reacted with hydrogen and produced in-situ 
moisture following the reaction described earlier. More specifically, the 
heating zone atmosphere was found to contain close to 0.12% moisture and a 
hydrogen to moisture ratio of approximately 33. This in-situ produced 
atmosphere resulted in brazed joints with poor and unacceptable braze flow 
and the formation of medium to heavy soot on brazed joints with 
commercially available brazing paste. It also resulted in unacceptable 
brazed joints with excessive braze flow and slight soot formation with 
experimental brazing paste. The above information indicated that the 
amount of carbon dioxide present in the feed moisture-free gaseous mixture 
was too low to produce enough moisture in the heating zone of the furnace 
to provide acceptable brazed joints and to eliminate soot formation with 
commercially available brazing paste. Furthermore the ratio of hydrogen to 
carbon dioxide in the feed gas (.about.20) was too high and too reducing 
to provide acceptable braze flow with experimental brazing paste requiring 
low dew point atmosphere. 
This example, therefore, showed that the use of hydrogen to carbon dioxide 
ratio close to 20 in the moisture-free nitrogen, hydrogen, and carbon 
dioxide atmosphere was not desirable to braze carbon steel components with 
good braze flow and brazed joints quality with both commercially available 
and experimental brazing pastes. 
EXAMPLE 4B 
This example describes results obtained by carrying out a brazing 
experiment using a moisture-free nitrogen-based atmosphere containing 0.4% 
carbon dioxide and hydrogen. Gas sample taken from the cooling zone with 
the introduction of 0.4% carbon dioxide and 4.0% hydrogen along with 
nitrogen into the furnace through transition zone once again showed only a 
marginal change in the composition of the part of the atmosphere flowing 
through the cooling zone. Gas sample taken from the heating zone showed 
the presence of 0.25% moisture and a hydrogen to moisture ratio close to 
15. This in-situ produced atmosphere resulted in brazed joints with 
unacceptable braze flow and the formation of medium to heavy soot with 
commercially available brazing paste. It resulted in brazed joints with no 
soot formation and marginally acceptable braze flow with experimental 
brazing paste. The above information indicated that the amount of carbon 
dioxide present in the feed moisture-free gaseous mixture was still too 
low to produce enough moisture in the heating zone of the furnace and 
eliminate soot formation with commercially available brazing paste. 
However, both the amount of carbon dioxide and ratio of hydrogen to carbon 
dioxide in the feed gas of 10 were good enough to provide acceptable braze 
flow and brazed joints quality with experimental brazing paste. 
This example showed that the use of hydrogen to carbon dioxide ratio close 
to 10 in the moisture-free nitrogen, hydrogen, and carbon dioxide 
atmosphere was not desirable to braze carbon steel components with good 
braze flow and brazed joints quality with commercially available brazing 
paste. It was, however, good enough to provide acceptable braze flow and 
brazed joints quality with experimental brazing paste requiring low-dew 
point atmosphere. 
EXAMPLE 4C 
This example describes results obtained by carrying out a brazing 
experiment using a moisture-free nitrogen-based atmosphere containing 0.6% 
carbon dioxide and 4% hydrogen. Gas sample taken from the cooling zone 
with the introduction of 0.6% carbon dioxide and 4.0% hydrogen along with 
nitrogen into the furnace through transition zone once again showed only a 
marginal change in the composition of the part of the atmosphere flowing 
through the cooling zone. Gas sample taken from the heating zone showed 
the presence of 0.30% moisture and a hydrogen to moisture ratio close to 
10. This in-situ produced atmosphere resulted in brazed joints with 
acceptable braze flow and brazed joint quality with marginal soot 
formation with commercially available brazing paste. It resulted in good 
braze flow and brazed joint quality with no soot formation with 
experimental brazing paste. The above information indicated that the 
amount of carbon dioxide present in the feed moisture-free gaseous mixture 
was still too low to produce enough moisture in the heating zone of the 
furnace and eliminate soot formation with commercially available brazing 
paste. However, both the amount of carbon dioxide and ratio of hydrogen to 
carbon dioxide in the feed gas of 6.67 were good enough to provide good 
braze flow and brazed joints quality with experimental brazing paste. 
This example, therefore, showed that the use of hydrogen to carbon dioxide 
ratio close to 6.67 in the moisture-free nitrogen, hydrogen, and carbon 
dioxide atmosphere was not desirable to braze carbon steel components with 
good braze flow and brazed joints quality with commercially available 
brazing paste. It was, however, good enough to provide good braze flow and 
brazed joints quality with experimental brazing paste. 
EXAMPLE 4D 
This example describes results obtained by carrying out a brazing 
experiment using a moisture-free nitrogen-based atmosphere containing 0.8% 
carbon dioxide and 4% hydrogen. Gas sample taken from the cooling zone 
with the introduction of 0.8% carbon dioxide and 4.0% hydrogen along with 
nitrogen into the furnace through transition zone once again showed only a 
marginal change in the composition of the part of the atmosphere flowing 
through the cooling zone. Gas sample taken from the heating zone showed 
the presence of 0.43% moisture and a hydrogen to moisture ratio close to 
7. This in-situ produced atmosphere resulted in brazed joints with good 
braze flow and brazed joint quality and with no soot formation both with 
commercially available and experimental brazing pastes. 
This example showed that the use of hydrogen to carbon dioxide ratio close 
to 5.0 in the moisture-free nitrogen, hydrogen, and carbon dioxide 
atmosphere was desirable to braze carbon steel components with good braze 
flow and brazed joints quality with commercially available and 
experimental brazing pastes. 
EXAMPLE 4E 
This example describes results obtained by carrying out a brazing 
experiment using a moisture-free nitrogen-based atmosphere containing 1.0% 
carbon dioxide and hydrogen. Gas sample taken from the cooling zone with 
the introduction of 1.0% carbon dioxide and 4.0% hydrogen along with 
nitrogen into the furnace through transition zone once again showed only a 
marginal change in the composition of the part of the atmosphere flowing 
through the cooling zone. Gas sample taken from the heating zone showed 
the presence of 0.50% moisture and a hydrogen to moisture ratio close to 
6. This in-situ produced atmosphere resulted in brazed joints with good 
braze flow and brazed joint quality and with no soot formation both with 
commercially available and experimental brazing pastes. 
This example showed that the use of hydrogen to carbon dioxide ratio close 
to 4.0 in the moisture-free nitrogen, hydrogen, and carbon dioxide 
atmosphere was desirable to braze carbon steel components with good braze 
flow and brazed joints quality with commercially available and 
experimental brazing pastes. 
EXAMPLE 4F 
This example describes results obtained by carrying out a brazing 
experiment using a moisture-free nitrogen-based atmosphere containing 1.0% 
carbon dioxide and 3% hydrogen. Gas sample taken from the cooling zone 
with the introduction of 1.0% carbon dioxide and 3.0% hydrogen along with 
nitrogen into the furnace through transition zone once again showed only a 
marginal change in the composition of the part of the atmosphere flowing 
through the cooling zone. Gas sample taken from the heating zone showed 
the presence of 0.50% moisture and a hydrogen to moisture ratio close to 
5. This in-situ produced atmosphere resulted in brazed joints with good 
braze flow and brazed joint quality and with no soot formation both with 
commercially available and experimental brazing pastes. 
This example showed that the use of hydrogen to carbon dioxide ratio close 
to 3.0 in the moisture-free nitrogen, hydrogen, and carbon dioxide 
atmosphere was high enough to braze carbon steel components with good 
braze flow and brazed joints quality with commercially available and 
experimental brazing pastes. 
EXAMPLE 4G 
This example describes results obtained by carrying out a brazing 
experiment using a moisture-free nitrogen-based atmosphere containing 1.0% 
carbon dioxide and 2% hydrogen. Gas sample taken from the cooling zone 
with the introduction of 1.0% carbon dioxide and 2.0% hydrogen along with 
nitrogen into the furnace through transition zone once again showed only a 
marginal change in the composition of the part of the atmosphere flowing 
through the cooling zone. Gas sample taken from the heating zone showed 
the presence of 0.43% moisture and a hydrogen to moisture ratio close to 
4. This in-situ produced atmosphere resulted in brazed joints with good 
braze flow and brazed joints quality and with no soot formation both with 
commercially available and experimental brazing pastes. 
This example showed that the use of hydrogen to carbon dioxide ratio close 
to 2.0 in the moisture-free nitrogen, hydrogen, and carbon dioxide 
atmosphere was high enough to braze carbon steel components with good 
braze flow and brazed joints quality with commercially available and 
experimental brazing pastes. 
EXAMPLE 4H 
This example describes results obtained by carrying out a brazing 
experiment using a moisture-free nitrogen-based atmosphere containing 1.0% 
carbon dioxide and 1.5% hydrogen. Gas sample taken from the cooling zone 
with the introduction of 1.0% carbon dioxide and 1.5% hydrogen along with 
nitrogen into the furnace through transition zone once again showed only a 
marginal change in the composition of the part of the atmosphere flowing 
through the cooling zone. Gas sample taken from the heating zone showed 
the presence of 0.30% moisture and a hydrogen to moisture ratio close to 
4. This in-situ produced atmosphere resulted in brazed joints with good 
braze flow and brazed joints quality and with no soot formation both with 
commercially available and experimental brazing pastes. 
This example showed that the use of hydrogen to carbon dioxide ratio close 
to 1.5 in the moisture-free nitrogen, hydrogen, and carbon dioxide 
atmosphere was high enough to braze carbon steel components with good 
braze flow and brazed joints quality with commercially available and 
experimental brazing pastes. 
EXAMPLE 4I 
This example describes results obtained by carrying out a brazing 
experiment using a moisture-free nitrogen-based atmosphere containing 1.0% 
carbon dioxide and 1.0% hydrogen. Gas sample taken from the cooling zone 
with the introduction of 1.0% carbon dioxide and 1.0% hydrogen along with 
nitrogen into the furnace through transition zone once again showed only a 
marginal change in the composition of the part of the atmosphere flowing 
through the cooling zone. Gas sample taken from the heating zone showed 
the presence of 0.25% moisture and a hydrogen to moisture ratio close to 
2.7. This in-situ produced atmosphere resulted in brazed joints with good 
braze flow and brazed joint quality and with slight soot formation with 
commercially available paste and no soot formation with experimental 
brazing pastes. However, the brazed components were slightly oxidized. 
This example showed that the use of hydrogen to carbon dioxide ratio of 1.0 
in the moisture-free nitrogen, hydrogen, and carbon dioxide atmosphere was 
high enough to braze carbon steel components with good braze flow and 
brazed joints quality with experimental paste. However, it was not high 
enough to prevent surface oxidation of brazed components. Therefore, it 
would not be desirable to use a hydrogen to carbon dioxide ratio of 1.0 in 
moisture-free nitrogen, hydrogen, and carbon dioxide atmosphere for 
brazing carbon steel components. 
Examples 4D to 4H clearly showed that carbon steel components can be brazed 
with good braze flow and brazed joint quality and unoxidized surface 
finish by using a commercially available brazing paste (requiring high dew 
point atmosphere) and a moisture-free nitrogen-based atmosphere containing 
a mixture of nitrogen, hydrogen, and carbon dioxide. Examples 4C to 4H 
showed that carbon steel components can be brazed with good braze flow and 
brazed joint quality and unoxidized surface finish by using an 
experimental brazing paste (requiring low dew point atmosphere) and a 
moisture-free nitrogen-based atmosphere containing a mixture of nitrogen, 
hydrogen and carbon dioxide. Examples 4A to 4I also showed that both the 
amount of carbon dioxide and hydrogen must be carefully controlled to 
provide (1) the desired moisture content in the heating zone of the 
furnace and (2) the required reducing potential both in the heating and 
cooling zones of the furnace for brazing carbon steel components with 
acceptable braze flow, braze joints quality, and unoxidized surface 
finish. It is, however, important to note that the desired moisture 
content in the furnace atmosphere will generally depend on the composition 
and type of brazing paste used for brazing carbon steel components. 
Although the present invention discloses the use of moisture-free 
nitrogen-based atmosphere for brazing carbon steels, similar atmospheres 
can be used for high temperature (&gt;1,000.degree. C.) glass-to-metal 
sealing and low temperature (700 to 900.degree. C.) brazing of non-ferrous 
metals and alloys such as brazing of copper and copper alloys using 
silver-based brazing materials. 
Having thus described our invention, what is desired to be secured by 
Letters Patent of the United States is set forth in the appended claims.