Moisture-free atmosphere brazing of ferrous metals

Abstract: Ferrous metals are brazed under a humidified atmosphere formed in situ from a moisture-free gaseous mixture of nitrogen, hydrogen, and carbon dioxide. At temperatures above 1080°C, moisture, which is needed to facilitate braze flow and to minimize formation of soot on brazed joints, is formed by reaction between hydrogen and carbon dioxide. In continuous furnaces, the moisture-free gaseous mixture is introduced at a transition zone between the heating and cooling zones thereby permitting a reduction in the overall amount of a reducing gas required for brazing carbon steels by keeping moisture out of the cooling zone.

Description: 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. 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 contain 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 AWS/WRC Brazing and Soldering Conference held in Philadelphia, PA on 26-28 April 1983. 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. US-A-2,085,587 discloses the use of an exothermically generated atmosphere containing carbon dioxide, carbon monoxide, hydrogen, nitrogen and water vapor as an enveloping gaseous reducing atmosphere for brazing ferrous metal parts at a temperature of about 1,100°C. The gas is derived by purifying the gaseous products of combustion of a hydrocarbon source, such as city or natural gas or butane, in air. The purification includes removal of oxygen and any unsaturated hydrocarbons and the control of moisture level to avoid condensation in the brazing furnace. Typically, the gas introduced into the brazing furnace contains 61% nitrogen, 15% hydrogen, 9% carbon monoxide, 5% carbon dioxide, and 10% water vapor. US-A-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°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 1,080°C. 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 containing nitrogen, hydrogen, and carbon dioxide. The gaseous mixture, can be 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. CO 2 + H 2 → CO + H 2 O The 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 provides 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, there is provided a process for brazing ferrous metal assembled for brazing with a brazing paste or preform comprising heating the assembly in a furnace under an atmosphere of a humidified gaseous mixture containing nitrogen and hydrogen for a period sufficient to effect an acceptable brazed joint; characterised in that the furnace temperature is at least 1080°C and said gaseous mixture is formed in situ from a moisture-free gaseous mixture of nitrogen, hydrogen and carbon dioxide in which the amounts of hydrogen and carbon dioxide are such as to react to form sufficient moisture to prevent both soot formation and surface oxidation. The present invention also provides the use of carbon dioxide in a moisture-free gaseous mixture of nitrogen and hydrogen to form in situ in a furnace at at least 1,080°C a humidified atmosphere under which ferrous metals can be brazed without soot formation or surface oxidation. 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 dissociator. 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 1,080°C using a moisture-free three gas mixture containing nitrogen, hydrogen, and carbon dioxide. The flow rates of hydrogen and carbon dioxide preferably are controlled to provide a hydrogen to carbon dioxide ratio of at least 1.0 in the gaseous feed mixture. Furthermore, they preferably 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 Experiments and 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°C to braze 1010 carbon steel components. The furnace consisted of an 8.75 in. (22.25 cm) wide, about 4.9 in. (12.5 cm) high, and 86 in. (218 cm) long heating zone and a 90 in. (229 cm) long cooling zone. A flexible conveyor belt with a fixed belt speed of 5 in (13 cm) 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 (standard cubic feet per hour; 9900 standard litres per hour) of a 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 the atmosphere for brazing carbon steel components. 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 212D requiring low dew point atmosphere was also used in brazing experiments. It was supplied by SCM Metal Products, Inc. of Research Park Triangle, North Carolina. The quality of brazed joints were determined either visually or by cross-sectioning and analyzing them. EXPERIMENT 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°C using a pure and dry (dew point less than -55°C) mixture of nitrogen and hydrogen containing 4.0 vol.% hydrogen. The use of the commercially available brazing paste resulted in unacceptable brazed joints with heavy sooting and very minimal braze flow. The use of the experimental paste also resulted in unacceptable brazed joints with excessive braze flow and medium to heavy soot formation. EXPERIMENT 2 Experiment 1 was repeated using a pure and dry mixture of nitrogen and carbon dioxide (oxidant) containing 1.5 vol.% carbon dioxide. The results of this experiment visually showed brazed joints with good braze flow with both the commercially available and experimental brazing pastes. However, there was 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. EXPERIMENT 3 Experiment 1 was repeated several times using mixtures of humidified nitrogen and hydrogen gas with varying moisture and hydrogen contents. (A) Use of a humidified nitrogen and hydrogen mixture containing 0.2% moisture and 3.8% hydrogen provided brazed joints with excessive and unacceptable braze flow with both the 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 the 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 mixture was high enough to eliminate soot formation. However, the quality of the brazed joints was not good. (B) Using a humidified nitrogen and hydrogen mixture containing 0.4% moisture and 3.6% hydrogen provided brazed joints with acceptable braze flow and brazed joints quality with both the commercially available and experimental brazing pastes. The use of this atmosphere composition also resulted in little or no soot formation on brazed joints. (C) Using a humidified nitrogen and hydrogen mixture containing 0.6% moisture and 3.4% hydrogen provided brazed joints with acceptable braze flow and brazed joints quality with both the commercially available and experimental brazing pastes. The use of this atmosphere composition resulted in very little or no soot formation on brazed joints. (D) Using a humidified nitrogen and hydrogen mixture containing 0.8% moisture and 3.2% hydrogen provided brazed joints with acceptable braze flow and brazed joints quality with both the commercially available and experimental brazing pastes. The use of this atmosphere composition resulted in no soot formation on brazed joints. (E) Use of a humidified nitrogen and hydrogen mixture containing 1.0% moisture and 3.0% hydrogen provided brazed joints with acceptable braze flow and brazed joints quality with both the commercially available and experimental brazing pastes. The use of this atmosphere composition resulted in no soot formation on brazed joints and the brazed components had a bright, unoxidized surface finish. (F) Use of a humidified nitrogen and hydrogen mixture containing 1.0% moisture and 2.0% hydrogen provided brazed joints with acceptable braze flow and brazed joints quality with both the commercially available and experimental brazing pastes. The use of this atmosphere composition resulted in no soot formation on brazed joints. However, the brazed components had an oxidized surface finish. Experiments 1 to 3 indicate that carbon steel components cannot be brazed in a pure and dry mixture of nitrogen and hydrogen or nitrogen and carbon dioxide and that 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 1 Experiment 1 was repeated several times using mixtures of moisture-free nitrogen, hydrogen, and carbon dioxide. (A) Use of a moisture-free nitrogen-based atmosphere containing 0.2% carbon dioxide and 4% hydrogen resulted in brazed joints with poor and unacceptable braze flow and the formation of medium to heavy soot on brazed joints with the commercially available brazing paste. It also resulted in unacceptable brazed joints with excessive braze flow and slight soot formation with the experimental brazing paste. Gas sample taken from the cooling 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. More specifically, the heating zone atmosphere was found to contain close to 0.12% moisture and a hydrogen to moisture ratio of approximately 33. (B) Use of a moisture-free nitrogen-based atmosphere containing 0.4% carbon dioxide and 4% hydrogen resulted in brazed joints with unacceptable braze flow and the formation of medium to heavy soot with the commercially available brazing paste. It resulted in brazed joints with no soot formation and marginally acceptable braze flow with the experimental brazing paste. Gas sample taken from the cooling zone 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. (C) Use of a moisture-free nitrogen-based atmosphere containing 0.6% carbon dioxide and 4% hydrogen resulted in brazed joints with acceptable braze flow and brazed joint quality with marginal soot formation with the commercially available brazing paste. It resulted in good braze flow and brazed joint quality with no soot formation with the experimental brazing paste. Gas sample taken from the cooling 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. (D) Use of a moisture-free nitrogen-based atmosphere containing 0.8% carbon dioxide and 4% hydrogen resulted in brazed joints with good braze flow and brazed joint quality and with no soot formation with both the commercially available and experimental brazing pastes. Gas sample taken from the cooling 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. (E) Use of a moisture-free nitrogen-based atmosphere containing 1.0% carbon dioxide and 4% hydrogen resulted in brazed joints with good braze flow and brazed joint quality and with no soot formation with both the commercially available and experimental brazing pastes. Gas sample taken from the cooling 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. (F) Use of a moisture-free nitrogen-based atmosphere containing 1.0% carbon dioxide and 3% hydrogen resulted in brazed joints with good braze flow and brazed joint quality and with no soot formation with both the commercially available and experimental brazing pastes. Gas sample taken from the cooling 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. (G) Use of a moisture-free nitrogen-based atmosphere containing 1.0% carbon dioxide and 2% hydrogen resulted in brazed joints with good braze flow and brazed joints quality and with no soot formation with both the commercially available and experimental brazing pastes. Gas sample taken from the cooling 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. (H) Use of a moisture-free nitrogen-based atmosphere containing 1.0% carbon dioxide and 1.5% hydrogen resulted in brazed joints with good braze flow and brazed joints quality and with no soot formation with both the commercially available and experimental brazing pastes. Gas sample taken from the cooling 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. (I) Use of a moisture-free nitrogen-based atmosphere containing 1.0% carbon dioxide and 1.0% hydrogen resulted in brazed joints with good braze flow and brazed joint quality and with slight soot formation with the commercially available paste and no soot formation with the experimental brazing pastes. However, the brazed components were slightly oxidized. Gas sample taken from the cooling 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. Examples 1D to 1H clearly showed that carbon steel components can be brazed with good braze flow and brazed joint quality and unoxidized surface finish by using the 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 1C to 1H showed that carbon steel components can be brazed with good braze flow and brazed joint quality and unoxidized surface finish by using the 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 1A to 1I 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.

Claims: A process for brazing ferrous metal assembled for brazing with a brazing paste or preform comprising heating the assembly in a furnace under an atmosphere of a humidified gaseous mixture containing nitrogen and hydrogen for a period sufficient to effect an acceptable brazed joint; characterised in that the furnace temperature is at least 1080°C and said gaseous mixture is formed in situ from a moisture-free gaseous mixture of nitrogen, hydrogen and carbon dioxide in which the amounts of hydrogen and carbon dioxide are such as to react to form sufficient moisture to prevent both soot formation and surface oxidation. A process according to Claim 1, wherein the furnace is a continuous furnace having heating and cooling zones and said moisture-free gaseous mixture is introduced into a transition zone therebetween. A process according to Claim 1 or Claim 2, wherein the hydrogen content of said moisture-free gaseous mixture is at least 1% by volume. A process according to any one of the preceding claims, wherein the ratio of hydrogen to carbon dioxide is no greater than 15. A process according to Claim 4, wherein the ratio of hydrogen to carbon dioxide is maintained between 1 and 7. A process according to Claim 5, wherein the ratio of hydrogen to carbon dioxide is maintained between 1 and 5. A process according to any one of the preceding claims, wherein said ratio of hydrogen to carbon dioxide is controlled to produce in situ a moisture content in the heating zone of the furnace of at least 0.25% by volume. A process according to Claim 7, wherein said ratio of hydrogen to carbon dioxide is controlled to produce in situ a moisture content of from 0.25% to 0.5% by volume. A process according to any one of the preceding claims, wherein the hydrogen and carbon dioxide contents are controlled to provide a hydrogen to moisture ratio in the heating zone of the furnace of at least 3.0. A process according to Claim 9, wherein the hydrogen and carbon dioxide contents are controlled so the hydrogen to moisture ratio in the heating zone of the furnace is between 3.0 and 14. The use of carbon dioxide in a moisture-free gaseous mixture of nitrogen and hydrogen to form in situ in a furnace at at least 1080°C a humidified atmosphere under which ferrous metals can be brazed without soot formation or surface oxidation. A use according to Claim 11, wherein the hydrogen or carbon dioxide content are as defined in any one of Claims 3 to 10. A use according to Claim 11 or Claim 12, wherein the moisture-free gaseous mixture is introduced into a transition zone between heating and cooling zones of a continuous furnace.

Inventors: BERGER KERRY RENARD, BONNER BRIAN BERNARD, GARG DIWAKAR, BERGER, KERRY RENARD, BONNER, BRIAN BERNARD, GARG, DIWAKAR
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1995
Main classification: B23K35
IPC codes: B23K1, B23K31, B23K35