Considerations for Furnace Brazing Compared to most other brazing methods, furnace brazing enjoys a distinct advantage in the areas of automation, control, and repeatability. But it also has its limitations, most notably a large initial investment, higher heating costs, and a need for regular equipment maintenance. It follows, then, that volume and speed requirements play a large role in determining whether a brazing furnace can be cost-justified. Furnace brazing also makes unique demands on the design and configuration of the parts being joined, as well as on the metallurgical properties of the base and filler metals used. Physical considerations As mentioned previously, very large assemblies are not good candidates for furnace brazing since the entire structure must be brought to brazing temperature inside the heating chamber. This wastes energy, and there is also the possibility that the large mass of the assembly will put undue stress on the joint and compromise its strength. On the other hand, furnace brazing is ideal for complex subassemblies which can then be brazed or welded to larger assemblies using other joining technologies. Joint and fixture design are also important considerations in furnace brazing. Unlike flame brazing, furnace brazing requires that the filler metal be pre-placed at the joint. This requires care in arranging workpieces and fabricating suitable fixtures, as any inadvertent movement of either the parts or the fixtures during the brazing cycle could have an adverse effect on the quality of the brazement. Equally important to the success of any brazing operation, especially furnace brazing, is a clean unoxidized surface. This is because furnace brazing is considered to be a "fluxless" process. Other brazing methods rely on chemical fluxes to facilitate wettability and optimize braze quality by removing surface oxides and contaminants. While it is true that vacuum and reducing atmospheres act to control oxide formation within the heating chamber, and even act to break down existing oxides, they cannot be solely relied on to remove preexisting oxides and contaminants. Precision cleaning and degreasing prior to brazing are essential. Metallurgical considerations Base metals Many types of ferrous and non-ferrous base metals and their alloys may be brazed with good results in a vacuum or controlled-atmosphere furnace, the most common being cast iron and carbon steels, stainless steels, low-alloy and tool steels (nickelor cobalt-based), copper, aluminum, and the precious metals. Some so-called reactive metals (i.e., those that exhibit the ability to form stable oxides at elevated temperatures) have historically been more suited to vacuum processing, although recent developments in industrial gas-based atmospheres now enable many of these difficult metals to be brazed in a controlled-atmosphere furnace with excellent results. Among these reactive metals are titanium, zirconium, and beryllium. Refractory metals is another class of metals commonly brazed in complete or partial vacuum. These metals include molybdenum, tungsten, niobium, and tantalum. Again, high-purity industrial gas-based atmospheres have now enabled these metals to be brazed successfully in atmosphere furnaces where the process gas flow and composition can be controlled and closely monitored. Superalloys are special metals formulated to display high strength and oxidation resistance at extremely high temperatures. These base metals are usually alloys of cobalt, iron, or nickel. Not surprisingly, superalloys are used extensively in the most demanding service applications, such as aircraft engine components. Cobalt-based superalloys can be furnace brazed with little difficulty; however, iron- and nickel-based superalloys that contain aluminum or titanium require special attention. The presence of surface oxides on these metals interferes with wetting and filler metal flow, requiring that brazing take place in a controlled vacuum or hydrogen atmosphere. Sometimes superalloys are electroplated with nickel before brazing, which helps to minimize the formation of surface aluminum and/or titanium oxides. Filler metals Filler metals suitable for furnace brazing, while similar, often differ from those used in flame brazing in important ways. There are literally hundreds of filler metal formulations, many designed for general brazing applications and many specifically for a particular combination of base metal and service requirements. But in addition to base metal compatibility, the filler metal's compatibility with the relatively high time/temperature characteristics of a brazing furnace is just as important. Generally speaking, silver- or gold-bearing filler metals are more commonly used in flame brazing, while copper- or nickel-bearing filler metals find more use in furnace brazing (with the exception of some stainless steels). Cobalt-based filler metals are also used, generally for brazing cobalt-based components. Elements such as boron and silicon are frequently added to filler metals used to braze some refractory metals (Mo, W, Nb, and Ta) as well as base metals containing Ti and Al. Filler metals containing silicon are commonly used in brazing aluminum and refractory metals. The low melting points of gold and silver make these metals inappropriate for applications where service temperatures exceed 700 F (370 C). Also, the higher temperatures encountered in a brazing furnace do not have to rely on the low melting points of gold and silver, making the use of the less-expensive, but higher-melting-point metals possible. In addition, many silver-bearing filler metals contain trace constituents of undesirable elements such as zinc and chlorine which, at brazing temperatures, can outgas and contaminate the brazing atmosphere in the furnace, and form difficult-to-remove deposits on the chamber walls and components. 19
In the exothermic process, fuel gas is mixed with air to form a rich source of combustible gas that generates enough heat to sustain the reaction. In an endothermic reaction, which uses a lower fuelto-air ratio, additional heat and/or a catalyst must be supplied to continue the gas generation process. Dissociated ammonia is produced by a catalytic reaction (cracking) that results in an atmosphere of approximately 75% hydrogen and 25% nitrogen. This atmosphere, while suitable for brazing many metals, can be problematic for use with some stainless steels, which can undergo unintentional nitriding if any raw ammonia survives the dissociation process. In addition, the dissociated atmosphere is hygroscopic and must often pass through a dryer to ensure a low dewpoint in the brazing chamber. ' Approved handling and storage facilities and equipment for explosive, flammable, corrosive and toxic gases with complete Material Safety Data Sheets (MSDS) and other documentation of safe handling procedures (available from most process gas manufacturers). Protective clothing, gloves, goggles, respirators, etc. for workers. Adherence to all relevant code standards for electrical, mechanical, vacuum, and piping/plumbing systems. Approved design and operation of flow regulators. Training in proper handling and operation of compressed and cryogenic cylinder gas. Confined entry space permits For more detailed information on the safe operation and maintenance of brazing equipment, refer to ANSI Standard Z49.1, Safety in Welding and Cutting, which is the standard used by OSHA for evaluating brazing facilities. Detailed procedures for safe use of atmospheres for various types of furnaces and atmosphere systems are published in NFPA 86 C and 86 D: Standard for Industrial Furnaces Using a Special Processing Atmosphere and Vacuum Furnaces. Industrial gases, whether delivered in cylinders, in bulk form, or generated on site, enable precise dewpoint control. They also eliminate undesirable atmospheric constituent elements produced as byproducts of the exothermic, endothermic, and dissociated ammonia chemical reactions. These unwanted elements are typically methane, carbon dioxide, carbon monoxide, and excess water vapor. Safety/environmental considerations In furnace brazing, just as in virtually all manufacturing processes, there are potential risks to personnel and the environment if safety precautions are not followed. While it is beyond the scope of this publication to provide a detailed description of safety and anti-pollution procedures, the main concerns specific to protecting workers, property, and the environment as they relate to furnace brazing can be summarized here. They are: Adequate exhaust ventilation or burning of process and byproduct gases, and conformance to local, state, and federal environmental guidelines and regulations. Confined space oxygen-level monitoring and oxygen monitoring of pits when using all inert gases. Avoidance of cadmium filler metals wherever possible. (See AWS Danger Notice on page 20.) Other suitable, safer filler metals are generally available. Many countries have banned the use of filler metals containing cadmium. Figure 13. Representative vapor pressure curve of various filler metal elements. (From Brazing Handbook, American Welding Society. Used with permission.) 21
Furnace Brazing Technologies Controlled-atmosphere processing As mentioned previously, the most common atmospheres used in controlled-atmosphere furnace brazing operations are classified as exothermic, endothermic, dissociated ammonia, and industrial gas-based (generated or delivered). What all of these atmosphere types have in common is that they are used for moderate- to highvolume production applications, mostly in a continuous or semi-continuous (retort or bell) furnace. They can also be used in vacuum furnaces, as a source for inerting, purging, or backfill gas. Typically, these controlled-atmosphere furnaces will be of a multi-chamber design, with each chamber (pre-heat, high heat, cooling) separated by either gas curtains or flame curtains at the entry and exit points to protect against air/oxygen infiltration. A schematic representation of a typical controlled-atmosphere brazing furnace is shown in Figure 14. All of the brazing atmosphere types reduce oxide formation after precleaning and control the formation of oxides during brazing. They help to control wettability and braze flow, and assist in optimal microstructure formation. In cases where brazing filler metal pastes containing organic binders are used, the atmosphere dewpoint must be precisely Table V. Common constituents of brazing atmospheres Constituent Composition Function in Brazing Range% Nitrogen 90 to 98% Used as inert gas and to keep out oxygen Hydrogen 2 to 10% Used as reducing gas and to help control filler metal flow Water Vapor 0.1 to 2.0% Used to control sooting (adds oxygen) and aids in fillet formation Natural Gas 0.0 to 1.0% Used to control dewpoint controlled and must contain a sufficient amount of an oxidizer (water or CO2) to react with any carbon residue to form carbon monoxide, thus removing the carbon soot. Perhaps most importantly, controlled-atmosphere brazing eliminates the need for fluxing in most applications, which means lower labor costs since parts can be finish-machined or used immediately without post-braze cleaning. Also, the absence of flux residue is a benefit for parts with complex geometries where flux can become entrapped, or threaded holes where complete removal of flux is difficult or impossible (some parts may still require application of a so-called "stop-off" material to control filler metal flow onto unwanted areas). Composition of furnace gas atmospheres Nitrogen Nitrogen (N2) constitutes 78.03% of the air, has a gaseous specific gravity of 0.967, and a boiling point of -320.5 F (-195.8 C) at atmospheric pressure. It is colorless, odorless, and tasteless. Nitrogen is often used as an "inert" gas due to its nonreactive nature with many materials, notable exceptions being chromium, titanium, niobium, tantalum, zirconium, and beryllium. However, nitrogen can form certain compounds under the influence of chemicals, catalysts, or high temperature. As mentioned previously, it may cause an undesirable nitriding effect in certain stainless steels (although nitride inhibitors are available). Fast cooling may also help to prevent this unwanted nitriding. Commercial nitrogen is produced by a variety of air separation processes, including cryogenic liquefaction and distillation, adsorption separation, and membrane separation. In brazing applications, gaseous nitrogen is often used as a blanketing or purging agent to displace air which contains atmospheric constituents that can interfere with braze flow and wettability. It is often used as a non-reactive carrier gas for other atmosphere components, such as hydrogen and controlled amounts of water vapor. The higher purity of nitrogen used, the less reducing gas (hydrogen) is required. 22
Air is not a suitable feedstock for carbon dioxide production because of carbon dioxide's low concentration in the atmosphere. Rather, carbon dioxide is obtained from byproduct streams from various manufacturing processes. Bulk quantities of carbon dioxide are usually stored and shipped as liquid under elevated pressure and refrigeration. Figure 15. Carbon-steel coupon brazed in a moisture-free brazing atmosphere system showing good braze flow and fillet formation, along with absence of any soot on brazed joint. Cryogenic (liquid) nitrogen has a very low dewpoint and, when mixed with hydrogen, can be easily metered to achieve variable reducing properties. Nitrogen-methanol and nitrogen-carbon dioxide mixes are also available to provide an atmosphere for brazing ferrous metals that is virtually moisturefree, but with enough oxidant properties to minimize sooting and promote proper braze flow. Pure nitrogen is also an excellent brazing atmosphere for copper base metals brazed with silver filler metals. ETP (Electrolytic Tough Pitch) copper can be brazed without the blistering or embrittlement that occurs in a hydrogen-containing atmosphere. Hydrogen Hydrogen (H2), the lightest element, has a gaseous specific gravity of 0.0695 and a boiling point of -423 F (-252.8 C) at atmospheric pressure. It is a colorless, odorless, tasteless, flammable gas found at concentrations of about 0.0001% in air. Hydrogen is produced by several methods, including steam/methane reforming, dissociation of ammonia, and recovery from byproduct streams from chemical manufacturing and petroleum reforming. Water vapor is produced as a byproduct of the oxide reduction process, requiring the addition of more dry hydrogen as needed to control the dewpoint, which varies with the type of metal oxide present. Hydrogen as a reducing agent may not be suitable for reducing the surface oxides of some heat-resistant metals, especially those alloys containing significant amounts of aluminum or titanium. Plating these metals with nickel, copper, or similar metals with surface oxides readily reactive with hydrogen can often solve the problem. Also, a high-temperature chemical flux can be used or the oxides may be removed in a chemical bath before being brazed. Sometimes carbon monoxide is used as a reducing agent. Methane Methane (CH4) in the brazing chamber is often present as a constituent byproduct of generated exothermic or endothermic atmospheres, or may be outgassed from brazements containing residual oil. It is also sometimes added intentionally (for example, as a source of carbon to counter the decarburizing effect of carbon dioxide and water vapor). Although not truly inert, carbon dioxide is nonreactive with most metals and often used for inerting purposes, such as gas blanketing and purging. However, it can decompose to carbon monoxide (CO) at brazing temperatures, becoming a flammable and reactive compound that can cause carburization of some steels and carbon alloys. It has also been used successfully in blended, moisture-free atmospheres (0.5 to 0.8% carbon dioxide to nitrogen and 4% hydrogen) for brazing carbon steel with excellent results, in terms of braze flow, fillet formation, and preventing soot (Figure 15). Hydrogen can be stored and transported as either a gas or a cryogenic liquid. In brazing applications, hydrogen is commonly used as a reducing (fluxing) agent to break down surface oxides and prevent them from reforming during the brazing cycle. Carbon dioxide Carbon dioxide (CO2) is a nonflammable, colorless, odorless gas. It is found in air at concentrations of about 0.03%. Carbon dioxide may exist simultaneously as a solid, liquid, and gas at a temperature of -69.9 F (-56.6 C) and a pressure of 60.4 psig (416 kpa). 23
Carbon monoxide Carbon monoxide (CO) is a colorless, tasteless, and odorless gas that is sometimes intentionally added during high-temperature brazing of non-ferrous metals (e.g., nickel, cobalt, and copper) because of its ability to reduce difficult metal oxides at elevated temperatures. It can also serve as a source of molecular carbon where desirable, such as in some carbon steels. While stable at high temperatures, carbon monoxide decomposed from carbon dioxide at low temperatures may release undesirable amounts of carbon and oxygen into the brazing atmosphere. Carbon monoxide is a toxic gas and requires adequate venting or scrubbing. Water vapor Water vapor (H2O) is present as intrinsic moisture in the brazing gases and/or as a byproduct of the chemical reactions and high temperatures found in the brazing environment. In addition to the moisture in the gases themselves, water vapor is liberated from filler metals or even from furnace walls (especially in refractory materials). temperature to which a given parcel of air must be cooled at constant pressure and constant watervapor content in order for saturation to occur." Furnace atmosphere dewpoints are determined at room temperature. The volumetric concentration of water vapor (measured in parts per million) in the brazing chamber is directly correlatable to the furnace atmosphere's dewpoint. While excess moisture (humidity) in the brazing chamber is undesirable (causing voids and inadequate filler metal flow or promoting oxidation and decarburization), some brazing processes, such as carbon steels benefit greatly from controlled amounts of water vapor. In these applications (e.g., low-carbon steel with copper filler metal), intentional humidification added to a high-purity, dry hydrogen-nitrogen atmosphere mixed in a blending panel (Figure 18, page 25), results in improved wettability and filler metal flow. The amount of hydrogen introduced and, thus, its reducing effect, can be precisely adjusted to balance its wetting effects against the anti-wetting effects of the water vapor present and arrive at an optimal gas composition. In a brazing furnace, precise dewpoint control is essential (Figures 16 and 17). The CRC Handbook of Chemistry and Physics defines dewpoint as "the Figure 16. The effect of varying dewpoints on braze flow using lap joints of 0.097 g copper. Hydrogen level is constant. Figure 17. The relationship between dewpoint at a given temperature to water content (in parts per million). (From Brazing Handbook, American Welding Society. Used with permission.) 24
Soot formation caused by incomplete volatilization of organic binders is also prevented due to the controlled oxidizing effect of the water vapor present. Sometimes, water vapor is intentionally added to limit filler metal flow, such as in applications where a wide joint clearance is present. Sources of unintentional water vapor include: Air leakage Air carried into furnace Reduction of metal oxides Leakage from water jackets Contaminated gas lines Ineffective flame curtains Oxygen Oxygen (O2) constitutes approximately 21% of the air, has a gaseous specific gravity of 1.1, and a boiling point of -297.3 F (-183 C). In brazing operations, it is generated as a byproduct or outgassed from furnace surfaces. Oxygen is always an undesirable element in brazing because it forms metal oxides which interfere with wettability and braze integrity. Sulfur and sulfur compounds Sulfur (S) and its compounds are found as constituents of some generated atmospheres and may react with base metals and adversely affect wettability. They can also enter the brazing atmosphere as artifacts of residual oils or from furnace components, such as brick muffles. Hydrogen sulfide (H2S) is detrimental to furnace materials, especially those containing nickel (a low melting-point eutectic forms). Inorganic vapors In certain applications, inorganic vapors are used to reduce metal oxides and scavenge the atmosphere of oxygen. Typically, these are compounds based on zinc, lithium, and fluorine. Argon Argon (Ar) is a chemically inert, colorless, odorless, and tasteless gas composing slightly less than 1% of the air. Its gaseous specific gravity is 1.38 and its boiling point is -302.6 F (-185 C). In brazing, argon is used to inhibit volatilization and to prevent hydrogen embrittlement in sensitive materials, such as titanium, zirconium, niobium, and tantalum alloys. Helium Helium (He), the second lightest element, is a colorless, odorless, and tasteless gas that is inert at room temperature and atmospheric pressure. Like argon, it is used for inerting purposes, but less frequently since it is somewhat more expensive. Figure 18. Schematic of humidified hydrogen-nitrogen atmosphere system. 25
Fuel Flowmeters Mixture ^ Pump Burner J Combustion L r Cooling Water Gas Cooler Trap Mill Drain Refrigerant Dryer Prepared Exothermic Atmosphere C M Drain Desiccant Dryer Comparison of atmosphere types Exothermic atmospheres A common type of atmosphere used in furnace brazing applications is the exothermically generated atmosphere. It is a relatively low-cost process suitable for mild steels and some non-ferrous metals, and is typically used where quality and reliability are not major concerns. Exothermic atmospheres can be formulated to be either "lean" or "rich" in hydrogen. However, because they are a byproduct of hydrocarbon combustion, their composition cannot be relied on to be pure or consistent. Their high carbon monoxide component and dewpoint make them unsuitable for most stainless and high carbon steels. A schematic representation of an exothermic generator is shown in Figure 19. Figure 19. Schematic diagram of a typical exothermic generator system. Catalytic Retort Water Jacket Flue Pressure Equalizer L i n e Gas Cooler Air Inlet C l M A i W f t A V A P A To Furnace Endothermic atmospheres Not commonly used in brazing because of a propensity for sooting, as well as their relatively higher cost and equipment maintenance requirements, endothermic atmospheres are sometimes diluted with nitrogen and used to braze high-carbon parts and prevent decarburization. Similar to exothermic atmospheres in composition, they are also not recommended for stainless steels. A schematic representation of an endothermic generator is shown in Figure 20. Automatic ^ J Fire Check Burner for Heating Retort ' Figure 20. Schematic diagram of a typical endothermic generator system. Dissociated ammonia Used in about 15% of all furnace brazing applications, the dissociated ammonia process is a system that results in a 75% H2-25% N2 atmosphere that may be used with some stainless steels. However, unavoidable traces of raw ammonia that survive dissociation can cause a nitriding, or case hardening, effect in certain alloy steels and stainless steels (especially undesirable where secondary annealing is planned). Only metallurgically or chemically pure (CP) grade ammonia should be used. Agricultural-grade ammonia must not be used due to residuals and high amounts of water vapor. Additionally, a dryer is usually required for dewpoint control. Currently, local, state, and federal agencies are imposing stricter regulations and controls surrounding the installation, storage, and use of anhydrous ammonia. A schematic representation of a typical dissociated ammonia system is shown in Figure 21 (page 27). 26
Parts brazed using these blended atmosphere systems exhibit excellent braze flow and fillet formation (Figures 22a and 22b, page 28), while providing thermal processors with the added benefits of liquid nitrogen systems at a cost competitive with exothermically and endothermically generated Ammonia atmospheres (Figure 23, page 28). Flowmeter There are various supply options available for using industrial gases in brazing applications, driven mostly by production volume. In brazing processes, the typical gases generated on site (from the air or other sources) are nitrogen and hydrogen; however, other common gases supplied are argon, helium, carbon dioxide, and methane. Industrial gas-based atmospheres The chief advantages of using an industrial gasbased brazing atmosphere are consistency, safety, and the ability to precisely control the composition of the furnace atmosphere. Because its high-purity gases can be precisionblended and tailored to specific brazing requirements with regard to reducing action, dewpoint, wettability, soot prevention, etc., industrial gas atmospheres offer greatly improved quality and throughput. Commonly used industrial gas atmospheres are dry nitrogen, hydrogen, N2-H2 mixtures, nitrogenmethanol mixtures, argon, and Ar-H2 mixtures. Using industrial gas-based brazing atmospheres, one can achieve improved economics by concentrating the hydrogen where required and optimizing the hydrogen-to-moisture ratio to the material being brazed. Air Products has developed several low-cost, nitrogen-based atmosphere systems to produce brazing atmospheres from on-site, non-cryogenically generated nitrogen. These systems, marketed under the tradename, PURIFIRE -BR Atmosphere Systems, produce atmospheres equal in quality and performance to liquid nitrogen-based systems for brazing carbon steel components. Table VI. Constituents of furnace brazing atmospheres by atmosphere source Type N2% H2% CO% CO 2 CXHV% O2 ppm Dewpoint Deg.F Endothermic 40 39 20 0.2 0.5 0 to 150 +40 to +50 Exothermic 70 to 98 2 to 20 2 to 20 1 to 6 <.05 10 to 200 +50 to +70 Dissociated Ammonia Nitrogen Based 25 75 0 0 0 10 to 30-40 to -50 0 to 98 100 to 2 0 0 0 0<10-40 to -70 27
Air Products offers the following supply options: Gas cylinder delivery (compressed and cryogenic). For low-volume users. The advantage of cryogenic (liquid gas) cylinders is that they hold a much larger volume of gas relative to compressed gas cylinders. Delivery to on-site cryogenic storage vessel/control systems (bulk storage). For flow rates up to 43,0000 scfh, with tanks from 500 to 20,000 gallons (Figure 24, page 29). Membrane-type generators (non-cryogenic). Air Products' membrane system relies on membrane diffusion technology to separate atmospheric air into its constituent gases, eliminating oxygen, carbon dioxide, and water vapor, resulting in a nitrogen-rich product stream. Figure 25 (page 29) shows a typical membrane system installation. Pressure Swing Adsorption (PSA) generators (non-cryogenic). Air Products' PSA systems utilize a molecular sieve with the ability to adsorb specific gases. Precision control of nitrogen purity and flow rates is easily achieved. Standard models offer flow rates up to 100,000 scfh and N2 purities comparable to cryogenic (liquid) nitrogen. Figure 26 (page 30) shows a typical PSA generator. Figure 23. Relative cost of brazing atmospheres. On-site High-Purity Nitrogen (non-cryogenic) systems. Air Products' Nitrogen HPN systems use a proprietary air separation process to provide very high purity gaseous nitrogen at flow rates up to 45,000 scfh. Figure 27 (page 30) shows a typical HPN installation. On-site cryogenic nitrogen generation plants. These large systems are used to produce tonnage quantities of liquid nitrogen, in volumes up to 1.5 million scfh. Smaller plants are available to generate gaseous nitrogen for applications with flow rate requirements of 15,000 to 400,000 scfh. 28
Vacuum brazing is also a suitable process for joining reactive and refractory metals because of the propensity of some metal oxides to dissociate in vacuum at brazing temperatures. This characteristic makes vacuum brazing popular for brazing superalloys, many aluminum alloys, and with special techniques, a wide range of ceramics and refractory materials. Stringent precautions must be taken to ensure cleanliness in the vacuum chamber as residual oils, moisture, etc. that survive the pumpdown process can contaminate the brazing atmosphere, degrade vacuum pressure, and condense on furnace walls and components. Figure 24. Schematic diagram of a typical on-site cryogenic tank system. Other advantageous characteristics of vacuum brazing include its ability to vaporize chemical flux to eliminate or minimize post-braze cleaning in those rare applications where a chemical fluxing agent is required for oxide removal. (Note: Chemical fluxes should generally not be used in a vacuum brazing furnace because of their hygroscopic characteristic, which can make obtaining a proper vacuum difficult.) Similarly, the high vacuum draws out occluded gases from within close-fitting brazement joints that could otherwise remain trapped Vacuum furnace brazing The applications for vacuum furnace brazing have grown considerably as improvements in equipment design were developed to overcome the problems experienced in early efforts. Frequently, vacuum processing and atmosphere processing are used to complement each other. For example, vacuum is sometimes used as a purging atmosphere before brazing with dry hydrogen, and inert gas or dry hydrogen is sometimes used as a purging agent before brazing in a vacuum furnace or as a partial pressure during brazing. Brazing in partial vacuum Vacuum furnaces may be equipped to allow the introduction of a gas (generally an inert gas or sometimes hydrogen) to increase the pressure to create a so-called partial vacuum atmosphere. This environment is useful for minimizing or preventing the volatilization of base metals or filler metals that tend to outgas at brazing temperatures. Brazing in total vacuum When brazing in total vacuum, essentially all gases are removed from the brazing environment and a negative pressure ranging from 0.013 to 0.00013 Pa (10-4 to 10-6 torr) is maintained. Gases are used, however, for quenching the heated parts after brazing. Generally, nitrogen or argon is used for quenching, but sometimes helium or hydrogen gas is used based on production and metallurgical considerations. Vacuum brazing is an ideal application for base metals such as heat-resistant nickel- and iron-based alloys containing aluminum and/or titanium. Good results may also be obtained with metals such as zirconium, niobium, titanium, and tantalum that could become brittle when brazed in a low-purity hydrogen (dissociated) atmosphere. Figure 25. Typical membrane-type atmosphere generator. 29
Figure 26. Typical PSA (pressure swing adsorption) type atmosphere generator. To aid further in "capturing" and neutralizing outgassed contaminants in the vacuum atmosphere, elements that have a high affinity for these gases, such as fluorine, zirconium, and titanium, are sometimes placed in the vacuum chamber next to (but not touching) the part being brazed. These socalled "getters" rapidly absorb the occluded gases, improving the quality of the brazing atmosphere. Sometimes elements such as lithium, magnesium, sodium, potassium, calcium, titanium, and barium are intentionally vaporized in the chamber to reduce the volume of oxides and nitrides. The disadvantage of this "getting" technique is that the vaporized materials may condense on furnace walls or react with the brazement if atmospheric moisture is present. Vacuum brazing relies on so-called "promoters" to chemically reduce oxide films and scavenge any oxygen and moisture remaining in the brazing atmosphere. These materials may be contained in the filler metal (e.g., magnesium) or in a reactive halide gas, such as the bromides or iodides of phosphorus and boron. Other brazing technologies Apart from furnace brazing, there are many other types of equipment used for carrying out brazing processes. However, since the focus of this publication is on furnace brazing, they will be mentioned only briefly here: Induction brazing relies on the electrical energy generated by induction coils to selectively heat the joint area of an assembly to brazing temperature. Resistance brazing generates heat from passing electrical current through the workpieces, which causes the filler metal to flow and complete the brazement. Dip brazing uses a salt bath or pot furnace containing molten flux, or filler metal and a layer of flux, into which the parts to be brazed are immersed, cooled, and cleaned. Figure 27. Typical On-site High-Purity Nitrogen (HPN) installation. Diffusion brazing is an extension of conventional brazing in which the filler metal completely diffuses at the base metal interface to the point where the physical and mechanical properties of the joint become the same as those of the base metal. In many cases, the joint "disappears" completely. Exothermic brazing is a process whereby a chemical reaction provides the heat required to complete the brazing operation. The exothermic reaction may be used with conventional filler metals, or it may create a molten filler metal as a byproduct of the reaction itself. Infrared brazing is similar to furnace brazing; however, heat is supplied by quartz heat lamps rather than electrical heating elements or combusted gas. Electron beam and laser brazing are two relatively new technologies which use a focused beam of energy to deliver heat to the joint being brazed. 30