The largest number of applications in modern production brazing are still satisfied by flame brazing. However, there is no doubt that today the largest numbers of brazed joints are made in protective atmosphere braz-ing furnaces.
The use of reducing atmosphere furnace brazing is a preferred joining process for mild steel and stainless steel assemblies, copper and certain copper-base alloys and is increasing yearly. This is undoubtedly due to the fact that as a result of economic pressures, combined with the impact of environmental issues, industry has become increasingly aware of the attrac-tions of the process. These include:
1. Attractive per-hour equipment operating costs
2. Parts that emerge from the furnace cleaner and brighter than when they enter
3. The high output rates, leading to low per-joint labor costs that are readily achievable
4. The ability to braze without the need to use a flux, eliminating the costly and environmentally unfriendly flux residue removal and subsequent disposal problems
5. The ability to braze a range of components that are geometrically different without the need to alter the process settings
6. The ease with which the brazing temperature and component tem-perature can be very closely controlled
7. The ability to undertake brazing and heat treatment in a single process cycle by using furnaces of special design
8. In the majority of cases, the comparatively low cost of the filler materials that will be used to make the joints
Set against these undoubted advantages is the fact that the capital cost of a furnace, particularly one designed to braze stainless steel, can be high when compared, for example, with mechanized brazing systems employing flame heating. In addition, and to derive maximum benefit from furnace brazing,
it is advisable to operate the equipment on a round-the-clock basis. Once continuous conveyor furnaces are in operation they should be switched off only when it is required to undertake repairs or for maintenance to their interiors. Maintaining them at a moderately elevated temperature, even when no work is being processed, will significantly extend the life of the belt and the refractory or metal-muffle lining. In short, frequent thermal cycling of a furnace is harmful and, in general, should be avoided.
Whether a continuous conveyor furnace will provide the best-practice solution to a particular production brazing problem requires that the person charged with making the decision has an understanding of the basic theory and practice of reducing atmosphere furnace brazing. This chapter provides the necessary background information.
7.1 Furnace Atmospheres
When one thinks of protective atmosphere furnace brazing, situations that generally come to mind are where heating of the parts to brazing temperature is accomplished in a furnace under vacuum, or a reducing or neutral atmo-sphere. In these situations, the necessity to use a fusible chemical flux to remove the oxide films from the work can be avoided. The notable exception to this norm is where heat exchangers and condensers, fabricated in alumi-num and its alloys, are furnace-brazed under nitrogen in association with a specialized flux. This procedure is universally known as the NOCOLOK® process.1 Several hundred furnaces are making billions of joints by this procedure each year worldwide, and more installations are planned. This procedure is a specialized and important process, and it is certainly the most automated of any of the continuous furnace brazing processes. A discussion of the NOCOLOK® process can be found in Chapter 9.
7.1.1 Atmosphere Considerations
One of the fundamental requirements of all brazing operations is that the surfaces of the parent metals to be joined must be chemically clean (i.e., free from oxide films) at the point in the process where the joint surfaces have attained a temperature that is sufficient to ensure that the filler metal will melt and flow. It is also a fundamental requirement that those oxides present on both the workpieces and brazing filler materials when the components are assembled, or those formed during the heating process, are removed prior to the brazing filler material’s becoming molten. When brazing is undertaken in air, oxide removal is generally achieved by the use of a fusible
1 *NOCOLOK® is a registered trademark of Solvay Fluor und Derivate GmbH & Co. K.G., Hanover, Germany.
2112_book.fm Page 178 Tuesday, November 4, 2003 1:07 PM
chemical flux. In protective atmosphere furnace brazing, one of the funda-mental advantages of the process is the ability to produce a brazed joint without recourse to the use of flux. It therefore follows that an essential part of the study of reducing atmosphere furnace brazing must include a discus-sion of the mechanism by which these flow-inhibiting oxide films are removed and prevented from reforming.
In furnace brazing technology, two atmosphere categories have to be con-sidered:
1. Chemically inert atmospheres that protect the parts being brazed from coming into contact with other gaseous elements: These atmo-spheres might react with the metals being joined to produce surface films, which may inhibit flowing and wetting by the molten filler material.
2. Chemically active atmospheres that will react with any surface films present on either the parts to be brazed or the filler-metal performs during the brazing cycle. This normally results in the removal of these films, but, in some circumstances, the conditions in the furnace can result in the generation of additional layers of film.
In both cases, when the parts are placed in the furnace it is the partial pressure of any oxygen present in the protective atmosphere that determines whether there will be removal or generation of contaminating films on the surface of the components or filler material preform.
During the brazing cycle the chemical activity of the atmosphere employed can promote removal of any continuous surface films (particularly oxides) from the surface of the parts to be brazed; this is accomplished either by decomposition or reacting with them. Such films may comprise either simple or complex compounds of sulphides, borides, phosphides, oxides, and organic products. In this book we shall consider only the formation and decomposition of oxides in any detail.
7.1.1.1 Oxide Films
One of the fundamental requirements for any successful furnace brazing operation is to ensure that the surfaces of the metals being brazed are free from oxide or other films that may inhibit wetting when the filler material melts. The ease with which surface oxides can be removed from any given material is a function of the ease with which the oxygen ions can be detached from the metallic ions present in the oxide. The degree of difficulty experi-enced depends on the strength of the chemical bond existing between the oxygen ions and the metal involved.
The strength of such a bond can be assessed in several different ways:
1. The heat of formation (DH) of the particular oxide in question (this will provide only an approximate guide).
2. The change in free energy (DF) in the system during the reaction 3. The maximum energy obtainable from the general chemical reaction:
where Me = metal and m = 1 mol of oxygen
Table 7.1 presents some data on the heat of formation of a number of differing oxides. As shown in this table, metals like gold, silver and palla-dium possess low heat of formation values for their oxides; they are consid-ered to be relatively unstable and can be readily decomposed. The oxides of metals such as copper, cobalt, nickel, iron and cadmium are higher on the stability scale and are more difficult to reduce. The oxides of chromium, manganese, titanium, aluminum and beryllium have even higher stability.
In fact, the various oxides of beryllium have a far higher degree of stability than any other element that will be encountered when furnace brazing.
From this it follows that metals and their alloys may be classified in groups according to the difficulty that is experienced in separating the oxygen ions from the respective metallic ions. This is directly related to the degree of difficulty that one might expect to experience when undertaking a furnace
TABLE 7.1
The Heat of Formation of a Series of Oxides That Might Be Encountered when Furnace Brazing
Oxide
Heat of Formation (kJ) related to 1 mol of Oxygen
Au2O3 –30.6
Ag2O 61.1
PdO 175.8
CuO 314.0
Cu2O 343.3
Co3O4 411.1
CoO 481.4
NiO 489.0
CdO 520.8
FeO 540.1
ZnO 698.3
Cr2O3 751.9
MnO 774.6
Ta2O5 835.7
TiO2 916.9
ZrO2 1082.3
Al2O3 1116.2
BeO 1233.4
nMe + m / 2. 0 2 nÆ Me 0m
¨
2112_book.fm Page 180 Tuesday, November 4, 2003 1:07 PM
brazing operation when these elements are present in the brazing environ-ment. For example, the brazing of the noble metals presents no difficulty, while copper, cobalt, nickel and iron are slightly more difficult; chromium and manganese are even more troublesome. If refractory elements such as titanium, tantalum, aluminum, or beryllium are present in the parent mate-rial or brazing filler matemate-rial at levels above about 0.75%, it is reasonable to consider that reducing atmosphere furnace brazing techniques will not be successful and so are best avoided. This explains why unstabilized stainless steels are always preferred to stabilized steels if reducing atmosphere furnace brazing is to be the joining method of choice.
To complicate matters, it must also be remembered that that some metals form more than one oxide, and that these different oxides have different levels of stability. When assessing the degree of difficulty likely to be encoun-tered in a reducing atmosphere brazing process, it is necessary to consider which particular oxide or groups of oxides are present.
Oxides that are formed on the surfaces of alloys are usually solid solutions of the oxides of the metals that compose the alloy, and not just a single oxide.
Moreover, the heat of formation of oxides on the surface of pure metals will not necessarily be the same as that of oxides produced on alloys of those metals. As a consequence, it does not follow that if it is relatively easy to braze a particular alloy, the brazing of the individual metals that compose that alloy will also be relatively easy. For example, stainless steel, which is an alloy of iron, nickel and chromium, is much more difficult to braze under reducing atmosphere than a nickel-iron alloy. This is because the oxides of iron and nickel both possess a heat of formation that is substantially lower than that of chromium oxide. In these circumstances it is the presence of the chromium in the stainless steel that accounts for the difficulty.
During any oxidation cycle the surface of the alloy becomes covered with a heterogeneous oxide film that quite often consists of layers of oxides that have different compositions. The type and composition of oxides present in such layers is dependent on the temperature and time for which the com-ponent has been exposed to the oxidizing environment. A typical example may be found in the range of chromium-bearing steels that are generally considered to be difficult materials to braze in reducing atmosphere furnaces.
The oxide of chromium (Cr2O3) forms a strong bond with surface of the steel on which it is standing and is not readily reduced to metallic ions. More complex oxides, such as FeCr2O3 and FeOCr2O3, will be formed on the surface of the steel during the oxidation cycle, but the ionic bonds of these oxides are weaker than those of Cr2O3, and are easier to reduce.
It is a well-known fact that the free energy associated with the formation of oxides decreases as the temperature of the environment in which the material is located increases. As a result, the tendency of an oxide to disso-ciate increases as the temperature within the furnace rises. The temperature at which dissociation will occur depends directly on the partial pressure of oxygen in the environment. If the partial pressure of oxygen in the surround-ing atmosphere is above about 200 mbar, the dissociation pressure of the
oxide will, for practically all metals and their alloys, exceed their respective melting points. As a general rule, it is not realistic to expect that oxide dissociation will be the method of first choice as the means of their removal from a metallic surface. There are some exceptions to this rule —the oxides of the six platinum group metals (platinum, palladium, iridium, rhodium, ruthenium and osmium) and those of gold and silver. With all eight of these metals oxide dissociation takes place at a temperature that is lower than their respective melting point. A decrease in the partial pressure of oxygen in the atmosphere that surrounds them tends to favor the decomposition of their oxides, increasing the likelihood that brazing them will be completed satisfactorily.
Reduction of the partial pressure of oxygen contained in the gas atmo-sphere may be achieved in two ways:
1. The formation of a vacuum in the vicinity of the parts that are to be brazed
2. The filling of the space surrounding the part to be brazed with an oxygen-free inert or reducing gas
In the first instance, the partial pressure of oxygen is reduced without altering the composition of the atmosphere, while in the second case the composition of the gas atmosphere is altered.
The second method of the reduction of partial pressure of oxygen men-tioned above is being increasingly used in brazing procedures. The joining of titanium and its alloys with silver-containing materials is a typical exam-ple. Difficulties with the brazing of titanium and its alloys arise not only because of the stability of the oxide coatings, but because these metals tend to absorb nitrogen and hydrogen from any atmosphere in their vicinity. Both titanium hydride and titanium nitride will embrittle titanium, and such gases must not be constituents in the atmosphere in which brazing is to be under-taken. In consequence, if a successful outcome is to be achieved, it is impor-tant that only vacuum or an inert-gas atmosphere of high purity is employed when brazing these materials.
7.1.1.2 Brazing in Inert Gas
Brazing in an atmosphere of inert gas is normally carried out in special containers. In some cases a heat-resistant glass tube that is continuously purged with an inert gas, such as argon or helium, and surrounded by an inductor that is connected to the output terminals of a solid-state induction heater is used as the brazing furnace. The more popular inert gases are available in cylinders, but such gases are not pure, but contain some level of trace impurities. Two of these impurities, even in the most highly purified gas, are likely to be parts-per-million quantities of oxygen and water vapor.
When an inert gas such as argon is introduced into a glass-tube furnace of the type described above that is filled with air, a certain amount of it is
2112_book.fm Page 182 Tuesday, November 4, 2003 1:07 PM