Metal fusion bonding – Process – Critical work component – temperature – or pressure
Reexamination Certificate
1998-10-09
2001-07-10
Jones, Deborah (Department: 1775)
Metal fusion bonding
Process
Critical work component, temperature, or pressure
C228S248100, C228S248500, C228S183000, C075S255000
Reexamination Certificate
active
06257483
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a nickel-based brazing material and a method of brazing with the brazing material. More particularly, this invention relates to a nickel-based brazing material which has good flowability and gives highly tough brazed joints, and to a brazing method with this brazing material.
The present invention further relates to a nickel-based brazing material for use in EGR cooler fabrication, a process for producing an EGR cooler using the brazing material, and an EGR cooler. More particularly, this invention relates to a nickel-based brazing material for EGR cooler fabrication which can have an improved structure after brazing, a process for producing an EGR cooler using this brazing material, and an EGR cooler.
BACKGROUND OF THE INVENTION
In producing heat exchangers, e.g., radiators, there have been cases where a joint between two members comprising stainless steel is brazed with a brazing nickel.
As prescribed in JIS Z3265-1986, this brazing nickel is an alloy comprising nickel as the main component and additives such as boron, silicon, chromium, iron, and phosphorus.
These additives serve to regulate the melting point of the brazing material, to improve the flowability of the molten brazing material and the ability thereof to wet base materials, and to improve the toughness of the hardened brazing material.
When such a nickel-based brazing material solidifies, an &agr;-phase of nickel crystallizes out as highly tough primary crystals and this crystallization begins at the interface with the base material. Hence, brittle phases comprising intermetallic compounds of other metals, e.g., silicon, boron, and phosphorus, are apt to crystallize out at the center of the bonding part and in inner parts and surfaces of the fillets.
If such brittle phases generate continuously in brazing parts, cracking is apt to occur in the brazing parts, in particular in the fillet parts (see FIG.
1
). Since continuous brittle phases frequently concentrate in the finally solidified areas, cracks also concentrate therein. In particular, the finally solidified areas tend to develop long cracks extending to the surfaces of the fillets.
The brazing fillet parts which have developed long cracks extending to the fillet surfaces in products in which a liquid is in contact with the brazing fillet parts, e.g., in radiators, suffer penetration of the liquid into the long cracks extending to the fillet surfaces, resulting in crevice corrosion. It is hence desired to prevent the generation of such cracks extending to fillet surfaces.
A prior art technique which has been regarded as effective in eliminating brittle phases for coping with long cracks in brazing fillet parts is to reduce the fillet size and employ a higher brazing temperature and a longer brazing time.
However, it is difficult to prevent the crack generation with the above technique.
Furthermore, especially in the case of EGR coolers, there is a problem that since corrosive fluids such as high-temperature exhaust gas, cooling water, and exhaust gas condensate water come into contact with brazing fillet parts, these corrosive fluids penetrate into cracks in the brazing parts to cause crevice corrosion. In addition, the brittle phases themselves exposed on the fillet surfaces are susceptible to corrosion by corrosive fluids, and the corrosion of the brittle phases accelerates the crevice corrosion.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a nickel-based brazing material which does not generate continuous brittle phases, is hence prevented from developing cracks, and can give a highly tough bonding part structure, and to provide a method of brazing with this brazing material.
A second object of the present invention is to provide: a nickel-based brazing material for EGR cooler fabrication which attains an improved brazing part structure, has been regulated so as not to yield continuous brittle-phase crystals in fillet parts and as to yield many crystals of an &agr;-phase of nickel in the fillet part surfaces, and is hence prevented from yielding brittle-phase crystals in the fillet surfaces, and which therefore can be prevented from undergoing cracking or crevice corrosion attributable to the partial corrosion of surface brittle phases, has satisfactory corrosion resistance, and is excellent in high-temperature oxidation resistance and resistance to repeated thermal stress; a process for producing an EGR cooler using the brazing material; and an EGR cooler.
In order to accomplish the first object, the inventor has found that fillet parts can be prevented from yielding continuous brittle phases and developing long cracks extending to the fillet surfaces by mixing powdery brazing nickel with a given amount of a powder of at least one specific metal.
In order to accomplish the second object, the inventor made various experiments and, as a result, has found that the formation of continuous crystals of brittle phases within fillet parts can be prevented and many crystals of an &agr;-phase of nickel can be formed in the fillet surfaces while diminishing the crystallization of brittle phases in the fillet surfaces, by mixing powdery brazing nickel for EGR cooler fabrication with a given amount of a powder of at least one specific metal.
Thus, fillet parts can be prevented from developing cracks in the brittle phases present therein, and the brittle phases present in the fillet surfaces can be prevented from corroding to thereby effectively prevent the crevice corrosion caused by the penetration of a corrosive fluid. Furthermore, since densely dispersed crystals of an &agr;-phase of nickel which is excellent in toughness and corrosion resistance are formed in the fillet parts at joints in an EGR cooler, the fillet parts can be improved in high-temperature oxidation resistance, resistance to repeated thermal stress, and the toughness of the joints.
Namely, the means for accomplishing the above objects are as follows.
(1) A nickel-based brazing material obtained by adding to powdery brazing nickel a powder of at least one metal selected from the group consisting of nickel, chromium, nickel-chromium alloys, and SUS (stainless steel) in an amount of from 2 to 10% by weight, excluding 10% by weight, based on the nickel-based brazing material, followed by mixing.
(2) The nickel-based brazing material as described in (1) above, wherein the content of the metal powder is from 4 to 7% by weight based on the nickel-based brazing material.
(3) The nickel-based brazing material as described in (1) above, wherein the metal powder has a particle diameter of 20 &mgr;m or smaller.
(4) A method of brazing which comprises bonding two base materials each made of a stainless steel with the nickel-based brazing material as described in (1) above.
(5) A method of brazing which comprises bringing two base materials each made of a stainless steel into contact with each other at the respective bonding areas and causing a melt of the nickel-based brazing material as described in (1) above to penetrate into the crevice between the contact surfaces of the base materials by the capillary phenomenon to bond the base materials.
(6) A nickel-based brazing material for use in bonding stainless-steel members for constituting an EGR cooler, said nickel-based brazing material being obtained by adding to powdery brazing nickel a powder of at least one metal selected from the group consisting of nickel, chromium, nickel-chromium alloys, and SUS (stainless steel) in an amount of from 2 to 22% by weight, followed by mixing.
(7) The nickel-based brazing material as described in (6) above, wherein the content of the metal powder is from 5 to 20% by weight based on the nickel-based brazing material.
(8) The nickel-based brazing material as described in (6) above, wherein the metal powder has a particle diameter of 20 &mgr;m or smaller.
(9) A process for producing an EGR cooler which comprises bonding stainless-steel members for the EGR cooler with the nickel-based brazing material as described in (6) above.
(10) A
Calsonic Corporation
Jones Deborah
Savage Jason
Sughrue Mion Zinn Macpeak & Seas, PLLC
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