Controlled Atmospheres fo...


Fig. 1 - On display are myriad components bright brazed in controlled atmosphere furnaces.
Controlled Atmospheres for Bright Brazing.

Furnace brazing is often the best option for medium and large production runs

BY P. F. STRATTON AND A. McCRACKEN


One way to control the formation of oxides during brazing and also reduce oxides present after precleaning is to surround the braze area with an appropriate controlled atmosphere in a furnace. Controlled atmospheres do not perform any primary cleaning but can reduce surface oxides and promote wetting in the same way as fluxes (Ref. 1). Oxides, coatings, grease, oil, dirt, and other contaminants must be properly removed before brazing.

Kepston Ltd. of Wednesbury, U.K., has more than 25 years' experience in furnace brazing a wide variety of parts (Fig. 1).

Brazing Furnace Atmospheres
The atmospheres used at Wednesbury include pure dry hydrogen (H2), exothermic or endothermic gas in continuous furnaces, and wet or dry H2 in batch furnaces. This flexibility gives Kepston the ability to braze components as small as a few grams to more than 100 kg, in a variety of metals from mild steel to stainless steel, in batches from prototypes to very large production runs.


Fig. 2 - These AISI 304 stainless steel jigged fuel distriburor A-pipes are brazed in pure hydrogen in a humpback furnace.
 
Furnace brazing is particularly applicable for high-production fabrication in continuous conveyor-type furnaces. For medium-production work, batch-type furnaces are preferable. In both types, heating is usually by electrical resistance, although other types of fuel can be used in muffle-type furnaces. The parts should be self-jigging or fixtured and assembled with filler metals preplaced near or in the joint. The preplaced filler metal may be in the form of wire, foil, powder, paste, slugs, or preformed shapes.

A Typical Brazing Application
An example of a jigged part is the fuel distributor A-pipe, manufactured for General Motors by Shalibane Engineering ‹ Fig. 2. These AISI 304 stainless steel assemblies are brazed in a pure H2 atmosphere in a humpback furnace. The assemblies are jigged on small stainless steel brackets to keep the joints away from the belt. In addition, a graphite sheet is sometimes placed between the assembly and the belt to prevent possible contamination of the belt from the copper brazing alloy ‹ in paste form in this example.

The Furnace Temperature Zones
In continuous-type furnaces, several different temperature zones are advisable to provide the proper preheat, brazing, and cooling temperatures. For copper brazing, the hot zone temperatures would typically be set at 1922°, 1967°, and 1994°F, (1050°, 1075°, and 1090°C), although the final zone temperature may be raised to 2012°F (1100°C) for thicker sectioned parts.


Fig. 3 - These self-jigged heavy elbow sections require a slow conveyor belt speed to braze properly.
 
The atmosphere in each zone can be tailored to the function of that zone using BOC's Nitrazone technology (Ref. 2). The speed through a conveyor-type furnace must be controlled to allow the correct time at the brazing temperature. The speeds range from about 6 in./min (150 mm/min) for heavier sections, such as the elbows in Fig. 3, to about 20 in./min (500 mm/min) for thin-wall sections.

Critical Aspects of Furnace Loading
Furnace loading is critical for consistent results. Great care must be taken to ensure the loading is consistent, either by weighing each batch of small parts or specifying separations for larger ones. It is also necessary to support the brazing assembly properly so the part does not fall out of position while traveling through the furnace on the conveyor belt. This may require special fixtures, but most brazements are designed to be self-supporting.

An example of self-jigging parts is the mild steel hydraulic fitting brazed in a flat bed furnace using an endothermic atmosphere shown in Fig. 3. This component is often pressure tested after brazing to verify the joints are sound before shipping them back to the customer. Induction Brazing
Controlled atmospheres are most commonly used in furnace brazing, but may also be appropriate in induction or resistance brazing. Furnace brazing requires an atmosphere to protect the assemblies from oxidation and, in the case of steels, from decarburization during brazing and also during cooling, which takes place in chambers close to, or contiguous with, the brazing furnace.

Benefits of Controlled Atmospheres
Controlled atmospheres have several advantages over the use of flux. If the job can be done without flux, there is no need for a postbraze cleaning to remove residues. A controlled atmosphere will also prevent the formation of oxides and scale on the part. Therefore, in many applications, parts can be machine-finished prior to brazing, and then go directly to a coating or plating operation without an intermediate cleaning step. Finally, the use of protective atmospheres is the only way to prevent damage from flux contamination produced by some techniques.

Effects of the Gases on Brazed Parts
All components of a controlled atmosphere make contributions to the brazing process. Hydrogen is an active agent for the reduction of most metal oxides at elevated temperatures. Hydrogen can cause embrittlement in some materials but is not usually a problem in slow-cooled products such as brazements. Carbon monoxide (CO) is an active reducing agent for some metal oxides such as those of iron, nickel, cobalt, and copper at elevated temperatures, but CO is toxic and must be handled with care (Ref. 3). This gas can serve as a source of carbon through cracking in the cooling zone of the furnace to form free carbon on the surface of the brazement. This may be useful in brazing some carbon steels but is undesirable in other applications.


Fig. 4 - The oxidation boundaries of seven metals.
 
Carbon dioxide (CO2) is both an oxidant and a decarburizing agent for steels. It will oxidize iron, and some alloying elements such as chromium, manganese, and vanadium.

Nitrogen used as a diluent in the brazing section of a furnace allows the proportion of H2 in the mixture to be kept below the explosive level. As a straight atmosphere, nitrogen does not react with most metals and will therefore prevent oxidation during cooling. At high temperatures, nitrides may be formed in susceptible materials such as stainless steels.

Water vapor is both an oxidant and a decarburizing agent for steels. The reducing ability of an atmosphere containing H2 depends primarily on the H2 to H2O ratio, which must exceed 10 to 1 if the atmosphere is to be reducing to steels, and must be even higher for other elements such as chromium ‹ Fig. 4. The amount of water in any atmosphere is given by its dew point, i.e., the temperature at which the moisture in the gas will condense.


Fig. 5 - Shown is a copper-brazed mild steel select lever assembly.
 

Oxygen in the brazing atmosphere is always undesirable.

Methane may come from the atmosphere gas as generated or from organic materials left on the part by inadequate cleaning. It can serve as a source of carbon and hydrogen. Sulfur or sulfur compounds may be an unintentional contaminant in the atmosphere and can react with the base metal to inhibit wetting. They usually come from contaminated fuel gases.

Inorganic vapors such as zinc, cadmium, lithium, and fluorine compounds can serve to reduce metal oxides and scavenge oxygen from the atmosphere. They are useful to replace constituents of alloys that are formed during brazing. Such vapors are also toxic, and proper safety precautions should be taken.


Fig. 6 - Copper-brazed piston assemblies
 
Inert gases such as helium and argon form no compounds with metals. They inhibit the evaporation of volatile components and permit the use of weaker retorts than are required for vacuum processes.

Vacuum Brazing Is Most Expensive
A vacuum process removes essentially all gases from the brazing area and therefore eliminates the need for purifying the supplied atmosphere. It is, however, the most expensive option. Certain base metals, such as stainless steels, superalloys, and aluminum alloys, have oxides that will dissociate in the vacuum at brazing temperatures. A vacuum also prevents difficulties sometimes experienced when gases given off by the base metal contaminate the joint interface. The low pressure also removes volatile impurities from the base and filler metals. Vacuum brazing is particularly useful in the aerospace, electronics, and nuclear fields, which may use metals that react chemically with reducing atmospheres or cannot tolerate entrapped fluxes or gases (Ref. 4).

Some Conclusions
The only components of the usual brazing atmosphere that act as fluxes or reducing agents are H2 and CO. All other constituents are either neutral or may even promote reactions potentially harmful to the service life of the brazement.


Fig. 7 - Bright-brazed AISI 304 stainless steel water pressure assemblies
 
Even more significant is the fact that the absolute levels of H2 and CO are not critical. What counts is the ratio of H2 to H2O and the ratio of CO to CO2 (Ref. 5). These two ratios alone determine the reducing capacity of the atmosphere. Of the two gases, H2 is more reactive than CO, and the H2 to H2O vapor ratio is most critical as far as the fluxing capability of an atmosphere is concerned.

The ability of an H2-containing atmosphere to reduce metal oxides depends on the temperature, the oxygen content (measured as dew point), and the pressure of the gas. Since most furnaces operate at atmospheric pressure, only temperature and dew point play a part. For the more reactive metals, the higher the processing temperature, the higher the dew point (or oxygen content) that can be used ‹ Fig. 4. In other words, the higher the brazing temperature, the lower the H2 to H2O ratio can be for any given metal. Or, to put it yet another way, the reducing capacity of a given amount of H2 increases with temperature.

The selection of the H2 to H2O ratio depends on which oxide is to be removed. If copper is to be brazed to stainless steel, for example, then a ratio suitable for reducing chromium oxide must be selected. Also, sufficient time must be allowed for the reducing action to take place. Even when brazing steels, high-H2 exothermic atmospheres are required, particularly when the dew point of the atmosphere is high.

Exothermically Generated Atmospheres
Many brazing atmospheres are produced from the products of combustion of a hydrocarbon fuel. The most common are exothermic atmospheres where the burning of metered mixtures of hydrocarbon fuel gas and air generates sufficient heat to maintain the reaction. An exothermic atmosphere containing small quantities of H2 and CO is the least expensive of the generated atmospheres. It reduces adequately for many applications, has a relatively low sooting potential, and requires a minimum of generator maintenance. Because the gas usually has a high dew point (water vapor content), it reacts with residues from brazing pastes and eliminates the small amounts of soot that might otherwise be formed. The majority of brazing atmospheres are exothermic, and they are generally used to braze mild steel or low-carbon steel.


Fig. 8 - Copper-brazed solenoid guide tubes
 
Exothermically generated atmospheres have certain drawbacks. They are flammable, and they are toxic due to the CO content. They are also decarburizing to medium- and high-carbon steels, and cannot be used where decarburizing must be avoided.

Endothermically Generated Atmospheres
If the ratio of fuel gas to air is high enough, the reaction becomes endothermic and requires the addition of heat and a catalyst for combustion to occur. Endothermically generated atmospheres are more reducing than exothermically generated ones but are more expensive to produce. They have a high CO content and can therefore be carbon neutral to medium- and high-carbon steels although they can also be used for mild steels. Their high reducing capability makes them suitable for a fairly wide range of metals and alloys.

Take, for example, the select lever assembly manufactured by West Bromwich Spring for Ford ‹ Fig. 5. This mild steel assembly is brazed using a copper paste at three different joints in an endothermic atmosphere. Because the metal is mild steel, the carbon content of the atmosphere and the component are in balance and no additions are needed to control carbon potential. There are big variations in cross-sectional area, so this part would be difficult to process in a single-zone furnace. A three-zone furnace, with the first two zones set below the melting point of the copper to act as a preheat can be used. The parts then enter the third zone, which is only 18°­27°F (10°­15°C) above the melting point of copper. This means the time at final temperature is minimized, giving excellent control over the flow of the copper.


Fig. 9 - Nitrogen-hydrogen-brazed power steering subassemblies.
 
Another example is the mild steel piston assembly brazed for Jebron Door Closures. In this assembly, shown in Fig. 6, the rack has a copper shim under it and copper paste is placed on the end plates. Care must be taken when processing components with such wide variations in cross section. The braze alloy can melt, a fillet can form, but capillary flow can be limited because the thicker section has not reached the brazing temperature.

High-H2 Atmospheres
Dissociated ammonia has a high H content and is therefore a very reducing atmosphere. It is used mostly in brazing stainless steel or other nickel alloys. However, ammonia is extremely toxic and its use is discouraged by legislation in many countries. Mixtures of nitrogen and H2 or 100% H2, depending on the reducing power required, have largely supplanted the use of dissociated ammonia.

Hydrogen is relatively expensive, so everything is done to reduce the quantity consumed. A humpback furnace can reduce the total flow of H2 needed (Ref. 6), requiring some 30% less gas flow than a conventional flat bed furnace. BOC Nitrazone technology also reduces the proportion of H2 used, with high H2 used in the hot zone only, and high nitrogen at the ends of the furnace.

One of the components treated in such a furnace at Kepston is a water pressure assembly manufactured by Shalibane Engineering Ltd. for Audi AG. The AISI 304 stainless steel components are brazed in a 100% dry H2 atmosphere using an internal copper ring. It can clearly be seen from Fig. 7 that the result is a bright, well-brazed part. These assemblies are jigged on stands to ensure consistency of throughput and assist with the flow direction of the internal copper ring. The stands also make inspection easier.

Pure H2 atmospheres are equally suitable for brazing steels other than stainless steels. In the example shown in Fig. 8, the solenoid guide tube assembly, destined for an off-highway application, consists of both AISI 304 stainless steel and mild steel. The surfaces are so bright that is difficult to tell the steels apart.

Nitrogen-based Atmospheres
A nitrogen-based atmosphere is applicable in furnace brazing whenever exothermic gas, endothermic gas, or dissociated ammonia can be used as the reducing agent. There are several advantages to a nitrogen-based atmosphere. First, cryogenic nitrogen has a very low dew point. In other words, it is a very dry gas, and when H2 (from a liquid or compressed gas supply) is added, the resulting H2 to H2O ratio is relatively high. That makes for a high reducing capacity or good fluxing. Furthermore, if a nitrogen-based atmosphere is used, the amount of H2 required is usually below the explosive limit of the mixture (4.9%).

One example of the use of such an atmosphere, in this case nitrogen-5% H2, is the brazing of power steering subassemblies, shown in Fig. 9. These mild steel assemblies, manufactured by Roulunds Codan for Renault, are brazed using a copper paste. In this case, the valve is assembled onto the tube using a dedicated assembly tool just prior to brazing, thus avoiding any problems associated with the transport of preassembled brazements.

Another advantage of a nitrogen-based atmosphere is that no chemical flux is needed if its main purpose would have been to reduce oxides in atmospheres with low reducing power such as exothermically generated gas. The use of fluxes requires larger joint clearance, to allow flux to escape and be displaced by the filler metal. This may produce a weaker joint. There is also the additional task of removing flux residue after brazing.

A particular advantage of a nitrogen-based atmosphere is that it can be tailored to provide just the right level of reduction, depending on the metal being processed or the stage within the brazing cycle. For example, it may be desirable to have a slightly oxidizing atmosphere in the preheating section of a furnace to help burn off organic compounds in paste-type filler metals. The brazing section of the furnace may need a strong reducing atmosphere; one may want the CO to CO2 ratio to change with temperature changes at different points of the furnace to maintain a neutral atmosphere. Also, the type of atmosphere can have a detrimental effect on furnace components by, for example, carburizing the metal belts. Accordingly, BOC provides for adjustments in furnace atmosphere composition either by introducing different compositions at different points in the cycle or, in the case of continuous furnaces, in the different zones.

Summary
Controlled atmosphere furnace brazing is the optimum technique to process large volumes of assemblies. The atmosphere performs many functions, and the choice of atmosphere requires a knowledge of the entire brazing process. The use of pure gases, particularly where atmosphere zoning is applied, often gives the best results, as can be seen from the excellent results obtained by Kepston in partnership with BOC.

References
1. Metals Handbook, Desk Edition. 1995. Eds. H. E. Boyer and T. L. Gall. Materials Park, Ohio: ASM International, 30­61.
2. Morris, J. 1987. Optimised sintering in nitrogen-based atmospheres ‹ the Nitrazone approach. Metallurgia 54(11): S26­S28.
3. Stratton, P. F. 1992. An introduction to furnace atmospheres safety. Heat Treatment of Metals Vol.19, No.1, pp. 10­13.
4. Stratton, P. F., and Wilson, R. C. 2001. Brazing in high power electronics. 21st ASM International Heat Treating Society Conference. CD-ROM, ASM.
5. Stratton, P. F. 1995. Protective atmospheres for brazing and soldering. Welding and Metal Fabrication 63(5):191­194.
6. Stratton, P. F. 2000. Atmosphere brazing of stainless steel. Proceedings of the International Brazing & Soldering Conference, Eds. P. T. Vianco and M. Singh, 10­18, AWS/ASM.

P. F. STRATTON paul.stratton@uk.gases.boc.com is Process Specialist ‹ Controlled Atmospheres, Global Technical Solutions, BOC, Holbrook, Sheffield, U. K.

A. McCRACKEN is with Kepston Ltd., Wednesbury, West Midlands, U.K.

Based on a paper presented at the International Brazing & Soldering Conference, February 16 - ­19, 2003, San Diego, Calif.

POSTED 2004-09-22 kt