Stock material or miscellaneous articles – All metal or with adjacent metals – Honeycomb – or with grain orientation or elongated elements...
Reexamination Certificate
2002-11-20
2003-09-09
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Honeycomb, or with grain orientation or elongated elements...
C428S594000, C428S599000, C228S181000
Reexamination Certificate
active
06617045
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates a metallic carrier for a catalytic converter used to purify automobile exhaust gas and a method of producing the same and, particularly, to a metallic carrier made of thin metal foil and a method of producing the same.
2. Description of the Related Art
Restrictions on automobile exhaust emissions have been made increasingly stringent in recent years owing to the progressive severity of global air pollution. The most effective of the various technologies developed to comply with the strict regulations have been those that rapidly ramp up the purification performance of the exhaust gas purification catalyst at the time of cold engine start. Considerable effort has, for instance, gone into the development of EHC (electrically heated catalyst) systems that elevate catalyst temperature using electric energy, ways for lowering reaction start temperature by boosting catalyst activity, and techniques for bringing the catalyst up to its active temperature in the shortest time possible by using a metal carrier of relatively small heat capacity and reducing the thickness of the metal foil forming the honeycomb. Among these, EHC has not yet been put into practical use owing to structural complexity that gives rise to problems regarding durability under rapid temperature increases/decreases. On the other hand, increasing the activity of a catalyst at low temperature encounters the drawback of accelerating metal particle sintering at higher temperatures.
In the case of reducing metal foil thickness to lower the heat capacity of the metallic carrier, the solder material component at the solder joints increases relative to the foil material. This lowers the melting point of the solder joints and makes them susceptible to fusing damage under abnormally high temperature owing to engine misfires. Moreover, when the thickness of the metal foil is reduced, the oxidation resistance of the honeycomb is degraded because the absolute amount of Al retained by the foil decreases and also because of the relative increase in the solder material component at the solder joints. In the case of the conventionally used 20Cr-5Al foil, this drawback appears when the foil thickness becomes 25 &mgr;m or less.
The metal foil honeycomb component of the metallic carrier is most often constituted of a corrugated foil and a flat foil joined by Ni solder. In order to ensure the heat resistance of the solder joints, B-Ni5 (center composition: 20Cr-10Si-bal. Ni) is generally used. The solidus of this solder is 1,060° C., while that of the 20 Cr-5 Al foil generally used as the metal foil is 1,510° C. The solder joints are formed by fusion or mutual diffusion of the metal components of the solder and foil. The fusion of the solder joints starts between the solidus of the solder and the foil. However, when the solder material component of the solder joints increases relative to the foil material component thereof, the fusion starts at a lower temperature closer to that of the solder (1,060° C.).
As explained earlier, an effort has been made to reduce the thickness of the metal foil of the metallic carrier to the very minimum. When the foil thickness is made thin, however, the solder material component of the solder joints increases relative to the foil material component thereof to lower the fusion temperature. Moreover, the Al content of the solder joints decreases to degrade its oxidation resistance. The highest temperature of a metallic carrier experiences is ordinarily around 1,000° C. It may, however, be increased to a temperature of around 1,200° C. on rare occasions when one cylinder of the engine misfires to allow raw gas to flow to and burn at the metallic carrier. Although the metallic carrier would be heated to a still higher temperature if two cylinders should misfire, the probability of this happening is very low. From a practical viewpoint, therefore, the metallic carrier is required to be capable of withstanding fusing damage at its solder joints up to a temperature of 1,200° C.
When a metal honeycomb of a foil thickness of 25 &mgr;m or less is soldered by a conventional method, however, the proportion of the solder joints accounted for by the solder material component is large and many of the solder joints incur fusing damage and become unusable at 1,180° C. In addition, the oxidation tests of 1,050° C.×200 hr in air showed that the oxidation resistance of the solder joints declined to the point that abnormal oxidation occurred centering on foil portions adjacent to some of the solder joints.
U.S. Patent Application Publication No. US 2002/0022145 A1 discloses a technology where the ratio of the mass of the foils contacting a brazing joint to that of the brazing medium should be limited between 4 and 8 in the case of using a foil thinner than 40 &mgr;m. However, the total amount of brazing powder applied to a honeycomb has to be limited to a very small amount in order to meet the requirements of this technology. This results in not a few joints which fail to be brazed, even if a small size of brazing powder defined by the above U.S. Patent Application Publication is used. Thus, the present inventors found that these honeycombs do not have sufficient strength to endure an engine bench test.
The present invention was accomplished in light of the foregoing points and has as its object to provide a metallic carrier for automobile exhaust gas purification made of thin metal foil that is improved in fusing damage resistance and oxidation resistance and exhibits excellent durability.
SUMMARY OF THE INVENTION
In order to achieve this object, the present invention provides a metallic carrier for automobile exhaust gas purification made of a thin metal foil comprising a honeycomb unit composed of a 8-25 &mgr;m thick corrugated metal foil and a similar flat metal foil joined by solder joints, wherein solder joints having a thickness of at least 1.5 times and not greater than 4 times the foil thickness account for not fewer than 70% of all solder joints between the two foils. The metal foils are preferably stainless steel foils containing, in percentage by weight, greater than 6.5% to 10% of Al. The invention further provides a method of producing a metallic carrier for automobile exhaust gas purification made of thin metal foil, which method comprises a step of forming a honeycomb unit in which a 8-25 &mgr;m thick corrugated metal foil and a similar flat metal foil are joined by solder joints using a solder powder, the solder powder having a particle diameter not greater than 4.5 times the thickness of the metal foil constituting the honeycomb unit. The metal foils used in this method can be preferably produced by rolling stainless steel enriched in Al by cladding or plating, winding the rolled stainless steel into a coil, and vacuum-annealing the stainless steel in the as-coiled condition to diffuse Al therein. Otherwise the metal honeycomb soldered using the solder powder are preferably enriched in Al by cementation to have an Al content, in percentage by weight, of greater than 6.5% to 10%.
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Kako Takuzo
Kasuya Masayuki
Okazaki Yuichi
Yamanaka Mikio
Kenyon & Kenyon
Nippon Steel Corporation
Zimmerman John J.
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