Metal fusion bonding – Process – Plural joints
Reexamination Certificate
2001-09-06
2004-06-15
Stoner, Kiley (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S199000, C228S249000, C148S528000
Reexamination Certificate
active
06749104
ABSTRACT:
FIELD OF THE INVENTION
The invention is generally a method for the manufacture of assemblies which included brazed metal components, wherein the manufactured assemblies are characterized by low leaching rates of nickel into a fluid contacting the brazed metal components.
The invention is particularly directed to methods for the manufacture of assemblies which included brazed metal components, such as heat exchangers as well wherein the manufactured assemblies are characterized by low leaching rates of nickel into fluids subsequently passing therethrough. These methods providing reduced nickel leaching rates are particularly useful for the production of various manufactured assemblies. These methods are particularly advantageous in the manufacture of heat exchangers and food processing equipment, including water treatment equipment, as well as other devices where low nickel leaching rates into a treated fluid would be desired.
BACKGROUND OF THE INVENTION
Brazing is a process for joining parts, often of dissimilar compositions, to each other. Typically, a brazing filler metal having a melting point lower than that of the parts to be joined together is interposed between the parts to form an assembly. This assembly of the parts to be brazed and the filler metal is then heated to a temperature sufficient to melt the filler metal but generally lower than the melting point of the parts. Upon cooling, ideally a strong, void-free joint is formed.
Brazing in widely used in the manufacture of various assemblies, which themselves may be finished articles which may be used or sold, or which assemblies may be components of such articles. One class of products produced by brazing processes are heat exchangers and food processing apparatuses. These may take a wide variety of configurations. For examples, heat exchangers such as “shell-and-tube” types, “plate/plate” and “plate/fin” type heat exchangers 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. 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 good heat transfer characteristics. Typically, the individual parts which are used to make up these type of heat exchangers are joined by brazing. Thus, it is imperative that joints exhibit high strength, and be resistant to any potential detrimental effects which might result from contact with one or both of the fluids.
One field of use wherein heat exchangers find utility are in the processing of materials which were ultimately used for human ingestion and consumption. These include foodstuffs as well as fluids such as water and beverages, juices, etc. Materials of construction used for heat exchangers to be used in such applications are of critical importance in that not only do they need to provide excellent operative characteristics with regard to heat transfer, but also at the same time these materials need to be compatible with the fluids being treated. One particular concern is to ensure that undesired leaching of any elemental or molecular components of the materials of construction used to produce the heat exchanger do not elute into the fluids. If they do, then it is imperative that any leaching of any such undesired 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 per unit volume (liter, gallon) into fluids passing therethrough. Ideally, the materials of construction, including brazing filler metals, are optimized so to meet or preferably not to exceed such governmental regulations.
To minimize this undesired technical effect, the materials of construction for heat exchangers, particularly those used for foodstuffs, need to be very carefully selected. Stainless steels are very commonly encountered for they exhibit desirable properties including low leaching rates into fluids or gases, and generally good corrosion resistance. However, the brazing manufacturing process carried out at high temperatures may also affect stainless steels leaching adversely. Previously, elemental copper was used as a brazing filler metal as such featured low leaching of nickel into fluids, especially water However, the corrosion resistance of heat exchangers whose components were brazed using a copper based brazing filler metal is poor. Typically these heat exchangers required frequent replacement, which incurs cost not only for new heat exchanger assemblies, but also the costs associated with the labor and replacement of such failed heat exchanger assemblies. To improve corrosion resistance it was recently found that brazing filler metals with compositions based primarily on nickel and chromium (“Ni/Cr”) can be used to join stainless steel parts used in such assemblies. Unfortunately, it was also found that when such Ni/Cr-based brazing filler metals are used, often an undesirable high amount of nickel was leached into water or other fluids flowing through these assemblies. As such Ni/Cr-based brazing, filler metals include a significant proportion of nickel, they are believed to be the source of the undesired nickel found to leach into fluids. Thus, such Ni/Cr-based brazing filler metals are usually avoided from use in applications wherein the amount of nickel leaching into a fluid is a concern, as is the case wherein materials are ultimately to be used for human ingestion or consumption. Additionally, governmental regulations in some countries have sometime imposed strict limitations in the amount of nickel which may be leached into said fluids. Accordingly, it is to one or more of these technical needs that the present invention is directed.
SUMMARY OF THE INVENTION
In a first aspect, the present invention provides methods for the manufacture of assemblies especially those which include parts made of stainless steels wherein these assemblies include parts joined using nickel/chromium-based brazing filler metals, wherein the manufactured assemblies ale characterized by low leaching rates of nickel.
In a second aspect, the present invention relates to a method for the manufacture of a heat exchanger or other manufactured assembly which method includes a brazing step, as well as a post-brazing conditioning step for treating the heat exchanger. The manufactured heat exchangers are characterized by reduced leaching rates of nickel into fluids passed through the heat exchanger.
In a third aspect of the invention, there are provided manufactured assemblies which comprise parts joined by nickel/chromium-based brazing filler metals, wherein the assemblies are characterized bid reduced nickel leaching rates.
Further aspects and features of the invention will become more apparent from the following description.
DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS
As noted, the present invention provides methods for the manufacture of assemblies which included brazed metal components, wherein the manufactured assemblies are characterized by low leaching rates of nickel in the manufactured assembly.
As is understood in the art, in any brazing process, the brazing filler metal must have a melting point that will be sufficiently high to provide strength to meet service requirements of the metal parts brazed together. However, the melting point must not be so high as to make the brazing operation difficult. Further the filler material must be compatible both chemically
Buff Ernest D.
Ernest D. Buff and Associates, LLC
Fish Gordon E.
Stoner Kiley
Tran Len
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