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