Patent Application
20060090820
1. Field of the Invention
The present invention relates to brazing of metal parts, and more particularly, to a homogeneous, ductile iron-based brazing material useful in brazing stainless steels, and a method for brazing stainless steel components to form articles of manufacture, wherein the brazed stainless steel components reduce the propensity of nickel to leach from such articles in water.
2. Description of the Prior Art
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, a filler metal that has a melting point lower than that of the metal parts to be joined together is interposed between the metal parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the filler metal. Upon cooling, a strong, leak-tight joint is formed. The assembled parts may either constitute a finished article of manufacture or may form a sub-component for use in a further manufacturing operation.
The selection of a particular brazing filler metal for a specific application depends on a variety of factors, including requirements related to the components to be joined and to the conditions under which the assembly ultimately must operate.
One basic consideration is temperature. Brazing filler metals are characterized by their solidus and liquidus temperatures. The term “solidus” refers to the highest temperature at which a metal or alloy is completely solid, and the term “liquidus” refers to the lowest temperature at which the metal or alloy is completely liquid. In any brazing process, the brazing filler metal must possess a solidus temperature that is high enough to provide the brazed assembly with adequate integrity to meet the desired service requirements and yet have a liquidus that is low enough to be compatible with the temperature capabilities of the parts being joined.
Another consideration is corrosion resistance. Many brazed assemblies must operate under environmental conditions that are conducive to corrosion, especially in the vicinity of the brazement. The propensity of a given system to corrode is strongly influenced by the gases or liquids to which the system is exposed and by typical operating temperatures.
One class of devices which are frequently assembled using brazing as a joining technique is heat exchangers. These devices are known in a variety of configurations. Generally stated, heat exchangers allow heat to be transferred across an interface that separates one circulating fluid from another circulating fluid. It is generally essential that the fluids, either of which can be gaseous or liquid, be kept separate. Hence, it is critical that brazed joints which define, at least in part, the interface maintain structural integrity under a full range of operating conditions and for a prolonged service life.
One field of use wherein heat exchangers find utility is in the processing of materials which are ultimately intended for human ingestion and consumption. These include foodstuffs, as well as fluids such as water, beverages, juices, and the like. The metallic materials used for the construction of heat exchangers appointed for such applications are of critical importance. Such metallic materials not only need to provide excellent operative characteristics with regard to heat transfer, but also must be compatible with the substances to which they are exposed. One particular concern is the requirement that there be no undesired leaching or desolution of any elemental or molecular component species of the materials of construction that is harmful or adds undesirable taste to the fluids. If a harmful species or an undesirable taste is present, then it is imperative that any leaching of causative materials be minimized. Frequently, local governmental or regulatory authorities have established maximum amounts of materials, such as metal ions, which may be permitted to leach into fluids passing therethrough. The standard is ordinarily expressed as a maximum amount of leachate that may be present per unit volume of the fluid processed. Ideally, the materials incorporated in heat exchangers (including brazing filler metals) and the associated manufacturing methods result in a device that meets or exceeds applicable regulatory standards under foreseeable operating conditions.
Heat exchangers of the “shell-and-tube,” “plate/plate,” and “plate/fin” types are most usually encountered. In the first configuration, a larger diameter housing typically referred to as a “shell” encompasses one or more small diameter tubes or pipes. According to this configuration, a first fluid (i.e., liquid, gas) passes through the shell and about the exterior of the tubes while simultaneously, a second fluid (liquid, gas) passes through the interior of the tubes. While no physical contact is permitted between the first and second fluids, heat transfer occurs across the walls of the tubes from the hotter fluid to the cooler fluid. In plate/plate and plate/fin type heat exchangers, again a physical member, namely one or more plates separate a first fluid from a second fluid while heat transfer occurs across the plate. In these types of heat exchanger (as well as in other assemblies), metals are most commonly used due to their high strength and effective heat transfer characteristics. Typically, the individual parts, which are used to make up such types of heat exchangers, are joined by brazing. It is imperative that the heat exchanger maintain a physical integrity, and retain the isolation of the fluids from each other and the outside world. In addition, the heat exchanger and the joints that secure the internal components must be resistant to any potential detrimental effects which might result from contact with one or both of the fluids.
To minimize such undesired effects, the materials of construction for heat exchangers, particularly those used for foodstuffs, need to be very carefully selected. Stainless steels, which contain up to about 20% nickel, are very commonly utilized, for stainless steels exhibit desirable properties, including low leaching rates into fluids or gases, and generally effective corrosion resistance. However, brazing manufacturing processes carried out at high temperatures may also adversely affect the propensity of the stainless steels to leach. Previously, elemental copper was used as a brazing filler metal since copper featured low leaching of nickel into fluids, especially water. However, the corrosion resistance of heat exchangers having components brazed using copper as the brazing filler metal is poor. Typically, such heat exchangers required frequent replacement, resulting in significant costs for the replacement device and the associated labor, as well as economic losses resulting from manufacturing downtime. To improve corrosion resistance, it was recently found that brazing filler metals with compositions based primarily on nickel and chromium could be employed to join stainless steel parts used in such assemblies. Unfortunately, it was also found that when such nickel-based brazing filler metals were used, an undesirably high amount of nickel often leached into water or other fluids flowing through such assemblies.
Inasmuch as such nickel-based brazing filler metals include a significant proportion of nickel, nickel-based brazing filler metals are believed to be the source of the undesired nickel leachate. For this reason, use of nickel-based brazing filler metals should be avoided in applications where nickel leaching into a fluid presents a concern, as is the case when materials passing through the heat exchangers are to be used for human ingestion or consumption. Not surprisingly, governmental regulations in some countries have imposed strict limitations on the amount of nickel which may be leached into fluids for human ingestion or consumption. It is to one or more of these technical needs that the present invention is directed.
The present invention provides a method to fabricate heat exchangers and other articles of manufacture by brazing components thereof with an iron-based brazing filler metal composition. Brazed assemblies advantageously exhibit effective general corrosion resistance and low rates of leaching of nickel into fluids passing through either side of the heat exchanger. As a result, the heat exchanger is highly suited for exposure to items intended for ingestion by humans or animals.
In a first aspect, the present invention provides a method to manufacture assemblies, especially assemblies which include parts comprising stainless steels. Such assemblies comprise parts joined using iron-based brazing filler metals. When manufactured, the assemblies are characterized by general corrosion resistance and by low leaching rates of nickel. The method comprises: juxtaposing at least two parts to define one or more joints therebetween; supplying to the one or more joints an iron-based brazing filler metal composition that is a ductile, amorphous brazing foil; heating the juxtaposed parts and the brazing filler metal to cause melting of the iron-based brazing filler metal; and cooling the melted iron-based brazing filler metal to produce a brazed joint that minimizes an amount of nickel leaching into a fluid contacting the brazed joint. The heating and cooling operations are generally carried out either in a protective gas atmosphere or in a vacuum.
In a second aspect, an iron-based brazing filler metal alloy is used. Typically, the iron-based brazing filler metal having essentially a composition with the formula FeaCrbBcSidXe, wherein X is molybdenum or tungsten and incidental impurities, wherein the subscripts “a”, “b”, “c”, “d”, “e” are all in atom percent, and wherein “b” is between about 0 and 5, “c” is between about 10 and about 17, “d” is between about 4 and about 10, “e” is between about 0 and about 5, and a sum “a”+“b”+“c”+“d”+“e” is approximately equal to 100.
The iron-based brazing filler metal is especially suited for fabricating heat exchangers and other assemblies of the invention which require low nickel leaching rates. Generally, the iron-based brazing filler metal is prepared in the form of a homogeneous, ductile ribbon or strip.
The alloys of the present invention include substantial amounts of boron and silicon, which are present in the crystalline solid state in the form of hard and brittle borides and silicides. Accordingly, the alloys of the invention are particularly suited for fabrication into flexible thin foil by rapid solidification techniques. Foil produced in such a manner is a metastable material having at least a 50% glassy structure and a thickness ranging from about 18-50 μm (approximately 0.0007 to 0.002 inches). Use of a thin flexible and homogeneous foil as a filler metal is especially beneficial for brazements wherein the mating surfaces have wide areas with narrow clearances and for brazing joints having complex shapes. The alloys of the present invention may also be produced in powder form by gas or water atomization of the alloy or by mechanical comminution of a foil comprised thereof. Other methods, such as rolling, casting, and other powder metallurgical techniques may be also be used to prepare such alloys.
Further aspects and features of the invention will become more apparent from the following description.
The invention will be more fully understood and further advantages will become apparent when reference is had to the following detailed description of the various embodiments of the invention and the accompanying drawings, wherein like reference numerals denote similar elements throughout the several views and in which:
The present invention is directed to methods to manufacture assemblies which include brazed metal components, wherein the manufactured assemblies are advantageously characterized by low leaching rates of nickel into fluids which flow through the manufactured assembly and general corrosion resistance. The invention further provides an iron-based brazing filler metal suitable for such a manufacturing process.
In accordance with the present invention, an iron-based brazing filler metal alloy is used. Generally, the iron-based brazing filler metal has essentially a composition with the formula FeaCrbBcSidXe, wherein X is molybdenum or tungsten, and incidental impurities, wherein the subscripts “a”, “b”, “c”, “d”, “e” are all in atom percent, and wherein “b” is between about 0 and 5, “c” is between about 10 and about 17, “d” is between about 4 and about 10, “e” is between about 0 and about 5, and a sum “a”+“b”+“c”+“d”+“e” is approximately equal to 100.
In any brazing process, the brazing filler metal must have a melting point high enough to provide joint strength meeting service requirements of the brazed metal parts. Too high a melting point may weaken or sensitize the base metal. Additionally, too high a melting point may erode the base metal in the vicinity of the joint region. A filler material must also be compatible, both chemically and metallurgically, with the materials being brazed.
Iron-based brazing filler metals particularly useful in the methods and assemblies of the present invention are metal alloys which may be produced in various forms, including, but not limited to, powders, foils, ribbons and wires, according to well known techniques. Methods commonly used to fabricate alloys in powder form include gas or water atomization, as well as mechanical pulverization. Alloys of the present invention are most generally formed into ductile foils, ribbons or wire by rapid solidification. Production of metal alloys by rapid solidification typically entails quenching a melt of the requisite composition by rapidly cooling at a rate of at least about 103° C./sec, although higher rates are known and more commonly used. Of the rapid solidification processes available today, the most typical process employs a rapidly rotating chill wheel onto which a molten alloy is impinged and cast into wide ribbon. One such process suitable for the manufacture of the brazing filler metal of the present invention as wide, ductile ribbon is disclosed by U.S. Pat. No. 4,221,257.
Ideally, the iron-based brazing filler metal of the present invention is in the form of a ductile foil which may be readily handled. In such a form, the iron-based brazing filler metal of the present invention is conveniently prepared in a variety of shapes that conform to contours used in the assembly of complex part assemblies. Formation into complex shapes may occur by bending or stamping the ductile foil.
Generally, the brazing foil of the invention is essentially homogeneous in composition, that is to say, that it includes no binders, such as organic binders which would provide the potential for void formation or the deposition of contaminating residues during brazing. The homogeneous composition of the foil results in liquidus and solidus temperatures that are uniform throughout, further promoting uniform melting and the formation of a strong, uniform, void-free brazed joint.
Rapidly solidified products produced from homogeneous melts of the alloys are usually homogeneous in the solid state. The products may be glassy or crystalline, depending upon the alloy compositions and processing parameters. In addition, products that are at least 50% glassy usually exhibit sufficient ductility to enable foil, ribbon and wire forms of the alloys to be bent to a radius as small as ten times a thickness of the foil, ribbon or wire without fracture. Typically, the iron-based brazing filler metals of the present invention are metal alloys which are formed by rapidly solidifying a melt of the metal alloy at quenching rates of at least about 105° C./sec. Such quenching rates typically produce alloys which are at least about 50% glassy and, as a result, are sufficiently ductile so as to enable the alloys to be stamped into complex shapes. More typically, the alloys of the present invention are at least about 80% glassy. Most typically, the alloys are substantially fully glassy (i.e., at least 90% glassy), and thus exhibit a significantly elevated degree of ductility.
The alloys provided by the present invention are particularly suited for use as brazing filler metals in the methods described herein. Most generally, the alloys are produced in foil form and are useful regardless of whether the foil is glassy or microcrystalline. Alternatively, the alloys may be prepared in the form of a foil with a crystalline solid solution or glassy metal structure, and in both cases, may be heat treated to obtain therein a fine-grained crystalline structure that promotes longer die life when stamping of complex shapes is contemplated. The foils of the present invention typically are between about 18 to 50 micrometers (about 0.0007 inches and about 0.002 inches) thick. In many instances, the foil thickness corresponds approximately to the desired gap between parts to be brazed.
The brazing filler metals of the present invention are particularly useful for the joining of metal parts, and particularly, stainless steel parts. Stainless steels are most frequently used in processing of fluids, including foodstuffs such as juices or other beverages, such as water, which are ultimately intended for human consumption. Exemplary grades of such stainless steels include: steel S31603 according to UNS Classifications, as well as type 316L stainless steel, which is described as typically having approximately 0.03 wt. % carbon, 2.00 wt. % manganese, 1.0 wt. % silicon, 10 to 14 wt. % nickel, 16 to 18 wt. % chromium, 2 to 3 wt. % molybdenum, 0.1 wt. % nitrogen and iron as the balance to 100 wt. %. It is contemplated that other materials benefiting from the teaching herein may also be used in accordance with the invention to afford reduced nickel leaching rates and increased corrosion resistance. By way of non-limiting example, such materials include other grades of stainless steel, as well as other corrosion resistant alloys, such as those including nickel.
Most typical brazing filler metals of the present invention include an iron-based brazing filler metal alloy that comprises essentially a composition with the formula FeaCrbBcSidXe, wherein X is molybdenum or tungsten, and incidental impurities, wherein the subscripts “a”, “b”, “c”, “d”, “e” are all in atom percent and, and wherein “b” is between about 0 and 5, “c” is between about 10 and about 17, “d” is between about 4 and about 10, “e” is between about 0 and about 5, and a sum “a”+“b”+“c”+“d”+“e” is approximately equal to 100.
The typical brazing filler metal is readily quenched into a significantly ductile metal strip and exhibits low liquidus temperatures that are generally below liquidus temperatures of materials to be brazed. Moreover, heat exchangers and other similar assemblies brazed using the typical brazing filler metal are characterized by rates of nickel leaching that are advantageously lower than average leaching rates and by general corrosion resistance.
In another aspect of the invention, a brazing method is used to manufacture devices such as heat exchangers and other equipment comprising brazed parts. The devices are selected to process materials for human ingestion or human consumption and are characterized by reduced leaching rates of nickel into fluids in contact with the devices. The method includes the operations of: juxtaposing at least two parts to define one or more joints therebetween; supplying to the one or more joints an iron-based brazing filler metal in the form of a ductile, amorphous brazing foil; heating the juxtaposed parts and the brazing filler metal to cause melting of the brazing filler metal; and cooling the melted brazing filler metal to produce at least one brazed joint that minimizes an amount of nickel leaching into a fluid contacting the brazed joint.
Referring now to
In one aspect, the assembly of heat exchanger 80 comprises the operations of positioning brazing foil preform 10 against a primary face 94 of plate 90; folding tabs 14 to contact or at least to extend alongside the tapered edge 92; and orienting preform 10 such that perforations 16 correspond suitably with positioned tube ends 86 and the plate 90. Thereafter, the assemblage is inserted into the shell 82; and the assemblage is brazed in accordance with specific requirements necessary for the materials of construction of the assembly, and with regard to the iron-based brazing filler metal of which the brazing foil preform 10 is composed.
In a typical embodiment of the method described above, the iron-based brazing filler metal comprises essentially a composition with the formula FeaCrbBcSidXe, wherein X is molybdenum or tungsten, and incidental impurities, wherein the subscripts “a”, “b”, “c”, “d”, “e” are all in atom percent, and wherein “b” is between about 0 and 5, “c” is between about 10 and about 17, “d” is between about 4 and about 10, “e” is between about 0 and about 5, and a sum “a”+“b”+“c”+“d”+“e” is approximately equal to 100.
Typically, in the process described above, the heating and cooling of the juxtaposed parts to cause the brazing thereof occurs in a closed oven in a presence of a protective gas such as argon, helium, or nitrogen. Alternately, heating and cooling may occur in a closed oven under vacuum conditions as well, and in certain instances, such conditions are typical. The brazing conditions are typically used in industry to achieve a high joint strength and integrity when using filler metals containing oxygen-active elements such as boron, silicon, and phosphorus.
Manufactured assemblies, and especially heat exchangers manufactured according to the methods described herein are characterized by reduced leaching rates of nickel into water-based fluids passed therethrough when compared to assemblies manufactured in accordance with known methods comprising brazing using nickel and nickel-chromium based filler metals. While it is to be understood that any reduction in nickel leaching, particularly into a liquid such as water is to be considered to fall within the scope of the present invention, a reduction on the order of at least 50%, typically at least about 70%, and most typically a reduction of at least about 85%, is attained. Such percentages are based upon a comparison of nickel leaching rates under identical test conditions of two identical heat exchangers (or other manufactured assembly) which have been similarly manufactured, but wherein one is manufactured in a process which includes the use of an iron-based braze filler metal and an optional, post-brazing conditioning operation described herein, and the other is manufactured conventionally, such as using a nickel or nickel/chromium-based braze filler metal. For example, optional post-brazing conditioning may include annealing by heating to a temperature below solidus for a predetermined period of time to improve microstructure and associated ductivity of a joint.
It is widely understood that corrosion of metallic parts is a consequence of galvanic action. Corrosion is manifested in a variety of forms of degradation. Corrosion may occur over a large part of a given surface, or may be localized in a region, such as a region in and around a brazement. Advantageously, the manufactured assemblies and heat exchangers of the present invention exhibit effective general corrosion resistance. That is, such assemblies are resistant to a wide variety of localized and generalized manifestations of corrosion, including generalized or localized removal of material, surface oxidation, rusting, pitting, and the like. The particular mechanism that operates in a given situation depends on the materials used in constructing an assembly, the materials to which the assembly is exposed, and a time, temperature, and duration of that exposure. In particular, manufactured assemblies, and especially heat exchangers constructed according to the methods described herein are characterized by superior resistance to corrosion in water-based fluids when compared to assemblies constructed using copper-based filler metals. While it is to be understood that any improvement in resistance to corrosion is to be considered to fall within the scope of the present invention, a reduction on the order of at least 20%, typically at least about 40%, and most typically a reduction of at least about 60%, is attained. Such percentages are based upon a comparison of the corrosion rates under identical test conditions of two identical heat exchangers (or other manufactured assembly) which have been similarly manufactured, but wherein one is manufactured in a process which includes the use of an iron-based brazing filler metal. The general corrosion resistance of the heat exchangers and other assemblies produced according to the process described herein advantageously leads to a substantially longer expected service life of the assembly. The increased service life not only lessens the risk of failure during operation, but also reduces the expected frequency of replacement or maintenance of such heat exchangers and other assemblies and the attendant disruption of service.
Significant fields of application, wherein the inventive heat exchangers and other assemblies manufactured according to the methods described herein, include the cooling of drinking water or other beverages. Of course, the methods described herein may be used in manufacture of other devices or articles useful both within the technical area related to in food and beverage processing, as well as outside of such a technical area.
It will be understood that the present invention has utility not only in the manufacture of heat exchangers, but also in any application where it is desired to reduce an amount of nickel which may leach from an assembly comprising brazed metal parts and to maintain an advantageous effective general corrosion resistance. More generally, the invention additionally relates to a process for joining two or more metal parts, and particularly two or more stainless steel parts, comprising the operations of: juxtaposing the at least two parts to define one or more joints therebetween; supplying to the one or more joints an iron-based brazing filler metal having a melting temperature less than that of any of the parts and having a composition that is a ductile, amorphous brazing foil; heating the juxtaposed parts and the brazing filler metal to cause melting of the brazing filler metal; and cooling the melted brazing filler metal to produce at least one brazed joint that minimizes an amount of nickel leaching into a fluid contacting the brazed joint.
In a typical embodiment of the method of joining parts, the iron-based brazing filler comprises essentially a composition with the formula FeaCrbBcSidXe, wherein X is molybdenum or tungsten, and incidental impurities, wherein the subscripts “a”, “b”, “c”, “d”, “e” are all in atom percent, and wherein “b” is between about 0 and 5, “c” is between about 10 and about 17, “d” is between about 4 and about 10, “e” is between about 0 and about 5, and a sum “a”+“b”+“c”+“d”+“e” is approximately equal to 100.
The following examples are presented to provide a more complete understanding of the invention. The specific techniques, conditions, materials, proportions and reported data set forth to illustrate the principles and practice of the invention are exemplary and should not be construed as limiting the scope of the invention.
Preparation of Iron-based Brazing Filler Metal Strip
Strips of about 2.5 to 25 mm (about 0.10 to 1.00 inch) width and about 18 to 50 μm (about 0.0007 to 0.002 inch) thick are formed by squirting a melt of a preselected composition, such as the composition with the formula FeaCrbBcSidXe noted above, by overpressure of argon onto a rapidly rotating copper chill wheel (surface speed about 3000 to 6000 ft/min.). Metastable, ductile, homogeneous ribbons of substantially glassy alloys comprising essentially the compositions (atom percent) set forth in Table I are produced, wherein “bal.” for iron indicates a balance (100% minus the values stated for other components of the composition).
Characterization of Iron-based Brazing Filler Metal Strip
The liquidus and solidus temperatures of selected ribbons having compositions set forth in Table I are determined by a Differential Thermal Analysis (DTA) Technique. The individual samples are heated side by side with an inert reference material at a uniform rate, and a temperature difference between the individual sample and the inert reference material is measured as a function of temperature. A resulting curve, conventionally known as a thermogram, is a plot of relative changes in the temperatures of the sample and the reference materials during simultaneous heating vs. temperature, from which the beginning of melting and end of melting, which represent the solidus and liquidus temperatures, respectively, are determined. Values thus determined are set forth in Table II below.
Alloys of the present invention, with solidus and liquidus temperatures listed in Table II, may be used as filler metals to braze stainless steels. Such alloys will melt and flow at temperatures that will not damage the stainless-steel-based metal parts and, at the same time, are convenient for industrial brazing processing. As may be seen from Table II, the amounts of boron and silicon may be varied to adjust the liquidus and solidus temperatures of the brazing filler metal material to desired liquidus and solidus temperatures.
Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.
