ASSEMBLY AND METHOD FOR MAKING CATALYTIC CONVERTER STRUCTURES

CROSS REFERENCE TO RELATED APPLICATIONS The present application is a continuation-in-part of

U.S. application Ser. No. 08/501,755, filed July 12, 1995 by

David Thomas Sheller and William A. Whittenberger, and related to concurrently filed U.S. applications, entitled

Assembly and Method for Making Catalytic Converter Structures, as follows: Ser. No. (Atty Dkt. 04605.0078) by William A. Whittenberger, John J. Chlebus, Joseph E. Kubsh, and Boris Y. Brodsky; Ser. No. (Atty Dkt. 04605.0079) by David T. Sheller and William A. Whittenberger; Ser. No. (Atty Dkt. 04605.0080) by William A. Whittenberger and Boris Y. Brodsky; Ser. No. (Atty Dkt. 04605.0081) by David T. Sheller, Steven Edson and William A. Whittenberger; Ser. No. (Atty Dkt. 04605.0083) by David T. Sheller, William A. Whittenberger and Joseph E. Kubsh; Ser. No. (Atty Dkt. 04605.0084) by William A. Whittenberger, Gordon W. Brunson, and Boris Y. Brodsky; and Ser. No. (Atty Dkt. 04605.0087) by William A. Whittenberger and Gordon W. Brunson; The complete disclosure of all of these applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to metallic catalytic converters, and, more particularly, to such converters especially adapted for use in vehicular engines to control

exhaust emissions, and to methods for the manufacture of such converters.

Description of the Related Art Catalytic converters containing a corrugated thin metal

(stainless steel) monolith typically have been formed of a plurality of thin metal strips or foil leaves wound about a central pin or about spaced "fixation" points. Such prior catalytic converters bodies, have supported both the outer and inner end of the individual layers by fixing them to the housing for the converter body and a central pin or post. In certain instances, the interior support has been provided by looping the foil leaves about a fixed point or porions whereby the inner ends of the leaves have been supported by other foil leaves. The thin metal strips or leaves forming the multicellular honeycomb body also have been brazed together at points intermediate the ends to form a rigid honeycomb monolith. Various techniques such assoldering, welding, brazing, riveting, clamping, reverse wrapping or folding, or the like, have been used to secure the inner and outer ends, and usually the intermediate portion, of the leaves or strips to the support member. While many techniques have been used to assemble the leaves into the housing and many leaf arrangements have been constructed, many arrangements have been unable to survive severe automotive industry tests known as the Hot Shake Test, the

Hot Cycling Test, combinations of these tests, cold vibratio testing, water quench testing, and impact testing.

The Hot Shake test involves oscillating (50 to 200 Hertz and 28 to 80 G inertial loading) the device in a vertical, radial or angular attitude at a high temperature (between 800 and 1050 degrees C . ; 1472 to 1922 degrees F., respectively) with exhaust gas from a gas burner or a running internal combustion engine simultaneously passing through the device. If the device telescopes, or displays separation or folding over of the leading or upstream edges of the foil leaves, or shows other mechanical deformation or breakage up to a predetermined time, e.g., 5 to 200 hours, the device is said to fail the test.

The Hot Cycling Test is run with exhaust flowing at 800 to 1050 degrees C; (1472 to 1922 degrees F.) and cycled to 120 to 200 degrees C. once every 13 to 20 minutes for up to 300 hours. Telescoping or separation of the leading edges of the thin metal foil strips, or mechanical deformation, cracking or breakage is considered a failure.

Also, the Hot Shake Test and the Hot Cycling Test are sometimes combined, that is, the two tests are conducted simultaneously or superimposed one on the other.

The Hot Shake Test and the Hot Cycling Test are hereinafter called "Hot Tests." While they have proved very difficult to survive, the structures of the present invention are designed to survive these Hot Tests and other tests similar in nature and effect that are known in the industry.

From the foregoing, it will be appreciated that catalytic converter bodies and their method of manufacture have received considerable attention, particularly by the automotive industry, are complex in design and manufacture, and are in need of improvement.

SUMMARY OF THE INVENTION

The advantages and purpose of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purpose of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. One aspect of the present invention is based on a finding by the inventors that the structure of a metallic catalytic converter body can be improved, particularly in high cell density converter bodies, by allowing at least some of the inner ends of the metal sheets, also referred to as foil leaf core elements or foil leaves, to move relative to others of the inner foil leaf ends. In this manner, flexure or compliance of the foil leaf core elements in response to thermal and fluid flow variations, as well as mechanical vibration can a desirable attribute of the converter bodies. On the other hand, the rigidity needed to prevent failure in the "Hot Tests" for low cell density converter bodies may require a central body structure with a greater measure of foil leaf interconnection because of the reduced amount of

metal foil in such converter bodies. Correspondingly, the central body structure may be required to provide varying degrees of rigidity, or flexure for cell densities between the high and low cell densities. One aspect of the present invention, therefore, is to provide a catalytic converter body design in which a central region of corrugated foil leaf connection may be varied without major variation in the construction of the converter body.

To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the catalytic converter body of the present invention comprises a jacket tube having opposite open ends, an annular leaf section including a plurality of foil leaves extending in adjacent curved paths from outer leaf ends joined to the jacket tube to inner leaf ends, and a central core of metal foil. The foil leaves and the metal foil of the central core are shaped to form generally axial fluid passage cells between opposite ends of the jacket. A multi-piece tube connects the inner leaf ends to the central core to enable the foil leaves to yield to mechanical and thermal stresses.

According to one aspect of the invention, the multi- piece tube section includes at least two tube-like parts, such as an inner tube part connected to the central core, and at least one outer tube part connected to the inner ends of at least some of the leaves in the annular leaf section. The configuration of the tube parts and manner in which they are interconnected, both of which are variably embodied,

determine stiffness of the converter body. That stiffness may be characterized by a natural resonant frequency in the converter body of from 10 to 100 hertz.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings, Fig. 1 is an end view of a catalytic converter body embodiment of the present invention;

Fig. 2 is an end elevation of a foil leaf subassembly from which the annular foil leaf section of the embodiment in Fig. 1 is formed; Fig. 3 is an end elevation illustrating the assembly of

Fig. 2 in relation to a forming fixture;

Fig. 4 is an end elevation illustrating a progression of forming of the foil leaf subassembly;

Fig. 5 is an end elevation illustrating the final step of forming the foil leaf subassembly in the illustrated fixture;

Fig. 6 is an end elevation illustrating the first step in an alternative method for forming the foil leaf subassembly into an annular configuration;

Fig. 7 is the schematic representation of an involute funnel fixture for the alternative forming method;

Fig. 8 is a cross-section on line 8-8 of Fig 7;

Fig. 9 is a cross-section on Fig. 9-9 of Fig. 7;

Fig. 10 is a cross-section on line 10-10 of Fig. 7; and

Fig. 11 is a cross-section on line 11-11 of Fig. 7; Fig. 12 is an isometric view illustrating a foil assembly for forming the central core of the converter body of the present invention;

Fig. 13 is a side elevation illustrating the assembly shown in Fig. 12 and depicting the winding thereof into a double spiral coil;

Fig. 14 is an alternative foil subassembly for the central core of the converter body of the present invention and depicting the winding thereof into a single spiral coil;

Fig. 15 is still another embodiment of a foil assembly for the central core;

Fig. 16 is a still further alternative foil of subassembly embodiment for forming the central core;

Fig. 17 is a still further foil assembly for forming the central core; Fig. 18 is a side elevation of the embodiment shown in

Fig. 14 but in more detail;

Fig. 19 is a side elevation showing the foil subassembly of Fig. 18 being wound into a coil;

Fig. 20 is an isometric view showing a central core unit depicting the insertion of a diametric pin; Fig. 21 is an isometric view showing an alternative form of the central core unit shown in Fig. 20;

Fig. 22 is an isometric view showing still another embodiment of a central core unit of the invention;

Fig. 23 is a perspective view of a multi-piece central tube structure for connection to the inner ends of leaves in the annular section of the converter body of the present invention;

Fig. 24 is a perspective view of an alternative form of the tube structure shown in Fig. 23; Fig. 25 is a perspective view of a further embodiment of the tube structure illustrated in Fig. 23;

Fig. 26 is a perspective view illustrating a variation of the tube structure shown in Fig. 23;

Fig. 27 is a perspective view illustrating a still further variation of the tube structure shown in Fig. 25; Fig. 28 is a partially schematic cross-section illustrating the brazing of the central tube structure to the inner ends of the leaves of the annular converter body section; Fig. 29 is a fragmentary cross-section illustrating an alternative brazing arrangement;

Fig. 30 is a partially schematic cross-section illustrating the assembly of the central core in accordance with one embodiment; and

Fig. 31 is an exploded partially schematic cross-section showing the assembly of an alternative embodiment of a central core in the annular leaf section of the converter body.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the present preferred embodiments of the invention, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. The foil leaf arrangement may be constructed from

"ferritic" stainless steel such as that described in U.S. patent 4,414,023 to Aggen. One usable ferritic stainless steel alloy contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture of two or more of such rare earth metals, balance iron and trace steel making impurities. A ferritic stainless steel is commercially available from Allegheny Ludlum Steel Co . under the trademark "Alfa IV." Another usable commercially available stainless steel metal alloy is identified as Haynes 214 alloy. This alloy and other useful nickeliferous alloys are described in U.S. patent 4,671,931 dated 9 June 1987 to Herchenroeder et al .

These alloys are characterized by high resistance to oxidation and high temperatures. A specific example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, optionally trace amounts of one or more rare earth metals except yttrium, 0.05% carbon, and steel making impurities. Haynes 230 alloy, also useful herein has a composition containing 22% chromium, 14% tungsten, 2% molybdenum, 0.10% carbon, a trace amount of lanthanum, balance nickel.

The ferritic stainless steels, and the Haynes alloys 214 and 230, all of which are considered to be stainless steels, are examples of high temperature resistive, oxidation resistant (or corrosion resistant) metal alloys that are useful for use in making the foil leaf core elements or leaves of the present invention, as well as the multicellular honeycomb converter bodies thereof. Suitable metal alloys must be able to withstand "high" temperature, e.g., from 900 degrees C. to 1200 degrees C. (1652 degrees F. to 2012 degrees F.) over prolonged periods.

Other high temperature resistive, oxidation resistant metal alloys are known and may be used herein. For most applications, and particularly automotive applications, these alloys are used as "thin" metal or foil, that is, having a thickness of from about 0.001" to about 0.005", and preferably from 0.0015" to about 0.0037". The housings, or jacket tubes, hereof are of stainless steel and have a thickness of from about 0.03" to about 0.08", preferably, 0.04" to 0.06" .

The multicellular converter bodies of the present invention preferably are formed from foil leaves precoated before assembly, such as described in U.S. Patent 4,711,009 Cornelison et al . The converter bodies of the invention may be made solely of corrugated foil core elements which are non-nesting, or of alternating corrugated and flat foil core elements, or of other arrangements providing cells, flow passages, or a honeycomb structure when assembled. In the preferred embodiments, the foil leaves, which will be used as core elements, are precoated before assembly. The ends are masked or cleansed to maintain them free of any coating so as to facilitate brazing or welding to the housing or to an intermediate sleeve.

As indicated in U.S. Patent 4,911,007, supra, the coating is desirably a refractory metal oxide, e.g. , alumina, alumina/ceria, titania, titania/alumina, silica, zirconia, etc. , and if desired, a catalyst may be supported on the refractory metal oxide coating. For use in catalytic converters, the catalyst is normally a noble metal, e.g. , platinum, palladium, rhodium, ruthenium, indium, or a mixture of two or more of such metals, e.g. , platinum/rhodium. The refractory metal oxide coating is generally applied in an amount ranging from about 10 rags/square inch to about 80 mgs/square inch. In some applications, corrugations preferably have an amplitude of from about 0.01 inch to about 0.15 inch, and a pitch of from about 0.02 inch to about 0.25 inch. The

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amplitude and pitch of the corrugations determine cell density, that is, the number of cells per unit of cross- sectional area in the converter body. Typically, the cell density is expressed in cells per square inch (cpsi) and may vary from about 50 cpsi to 2000 cpsi.

Where a non-nesting corrugated foil leaf core element is used, the corrugations are generally patterned, e.g., a herringbone pattern or a chevron pattern, or skewed pattern. In a "skewed pattern", the corrugations are straight, but at an angle of from 3 degrees to about 10 degrees to the parallel marginal edges of the strips. The latter foil leaf core elements may be layered without nesting.

Where alternating corrugated and flat foil leaf core elements are used in a non-nesting arrangement to form the multicellular bodies, straight-through corrugations may be conveniently used, these exhibiting the lowest pressure drop at high flow in fluid flowing through the converter body. The straight-through corrugations are usually oriented along a line normal to the longitudinal marginal edges of the foil leaves, although, as indicated above, the corrugations may be oriented along a line oblique to the longitudinal marginal edges of the leaves .

To reduce stress, the "flat" foil leaf core elements preferably are lightly corrugated to have corrugations with an amplitude of from about 0.002" to about 0.01", e.g.,

0.005" and a pitch of from about 0.02" to about 0.2", e.g., 0.1".

The coated corrugated and flat foil leaves that form the working gas flow passageways in the converter body of the invention constitute the major metal foil content thereof and are preferably formed of the lower cost ferritic stainless steel alloys. Because of its greater strength, albeit higher cost, the nickeliferous stainless steel alloys may be used in the converter of the invention particularly in the center area and other areas where the requirement for foil strength justifies the higher cost of these alloys. In the ensuing description and in the appended claims, the latter foil alloys may be referred to generically as "high strength" foil and may be uncoated to facilitate joining by spot welding, for example.

In accordance with the present invention, a catalytic converter body is structured to provide a central core, a jacket tube spaced from the central core, an annular leaf section between the central core and the jacket tube, and a multi-piece tube section between the annular leaf section and the central core. The annular leaf section includes a plurality of non-nestable corrugated foil leaves having inner and outer ends and extending in adjacent curved paths from the outer periphery of the multi-piece tube section to the jacket tube. The outer ends of the leaves are joined to the jacket tube by localized brazing. Although the multi-piece tube section may vary in construction for reasons to be described in more detail below, it includes at least an inner tube member joined to the central core and an outer tube

structure to which the inner ends of the corrugated foil leaves are joined preferably by localized brazing in a manner to be described. Although the following preferred embodiments provide a resulting body or assembly that can be inserted into a cylindrical jacket, bodies of other shapes may also be constructed according to the teachings of the present invention.

In Fig. 1 of the drawings, an embodiment of the catalytic converter body of the present invention is designated generally by the reference numeral 10 and shown to include a central core 12 formed by a coil of non-nesting foil, preferably alternating corrugated and flat foil layers 14 and 16. A multi-piece tube section, generally designated by the reference numeral 18 circumscribes the central core 12 and includes an inner tube member 20 and an outer tube structure 22. A diametric pin 23, in the illustrated embodiment, retains the coiled foil layers of the central core 12 against telescopic displacement.

An annular leaf section 24, in the illustrated embodiment, lies between the multi-piece tube structure 18 and a jacket tube 26. The leaf section 24 is also preferably formed of alternating corrugated and flat leaves 28 and 30 which radiate outwardly from the multi-piece tube section 18 to the inner surface of a jacket tube 32 in adjacent curved, preferably involute paths. As will be described in more detail below, the outer ends 34 of the leaves 28 and 30 are joined to each other, preferably by welding, and joined to

the jacket tube by brazing. The interconnection of the inner ends 36 of the leaves 28 and 30 to the multi-piece tube section, and of the multi-piece tube section to the central core, may be effected in various ways to be described. A more complete understanding of the structure of the annular leaf section 24 may be had by reference to Figs. 2-11 of the drawings in which alternative methods for its formation are depicted. In Fig. 2, the leaves 28 and 30 of the annular section 24 are shown to be interconnected at their outer ends 34 by welding overlapping folded end portions from which coating material has been removed or left uncoated by masking during the foil coating process. The overlapped outer end portions 34 are offset, in practice and for practical reasons, by a distance approximately 1 to 2 times the cell height or corrugation amplitude. When the leaves are so assembled, they represent a subassembly 24a in the form of a continuous interconnected strip of the alternating corrugated and flat leaves 28 and 30. The inner ends of the leaves 28 and 30 extend freely in the strip. One technique for shaping the strip subassembly 24a of leaves into the annular configuration shown in Fig. 1, for example, may involve use of a fixture 38 having a curved end

40 which approximates the involute path conformation of the individual leaves 28 and 30 in the finished annular section 24. Thus, the subassembly 24a is advanced along the fixture

38 in the direction shown in Fig. 3 until the ends of the leading leaves in the subassembly 24a engage and deform as a

result of contact with the arcuate end portion 40. When it is no longer possible to advance the subassembly 24a toward the curve portion 40, the trailing end of the subassembly 24a is curved around the arcuate end portion of the fixture 38, as shown in Fig. 5. This configuration is maintained during insertion of the subassembly 24a into a slightly convergent funnel device (not shown) so that the outer periphery of the assembly 24a may be inserted directly into the jacket tube 32 lined with brazing foil or brazing paste and this assembly inductively heated to fuse the brazing foil and secure the strips to the jacket 32.

To do this, the assembly preferably is put in a chamber. The air is evacuated and preferably backfilled with argon. A vacuum can be used as well so long as the oxygen is removed. Also in that chamber is an induction coil which goes around the housing with about an eighth to a quarter inch clearance between the coil and the housing. When the induction coil is energized, it heats the housing and the outermost tips of the foil by induction with a very localized heating effect, melting the brazing metal at the outside diameter. The outside portion of the leaves do not have the coating on them so they braise nicely at the outside diameter.

In Figs. 12 and 13, a core foil subassembly is generally designated by the reference numeral 50 and includes a central section 52 connected at opposite ends to a pair of tail pieces 54. As shown in Figs. 12 and 13, a length of flat foil or a flat foil segment 56, having a width Wf equal to the axial dimension of the finished central core 12, extends

between the tail pieces 54 and is connected at opposite ends to the tail pieces preferably by spot welding. To facilitate the end connections, the foil 56 is coated with catalytic material throughout its length except for the ends thereof to be attached to the tail pieces 54. A pair of coated corrugated foil leaf segments 58 are then placed on opposite sides of the flat foil segment 56 and extend from opposite ends of the flat segment for slightly more than one half of the length of the flat foil segment 56, as shown. The ends of the corrugated foil pieces 58 are flat, devoid of coating material and welded to the tail pieces 54.

The tail pieces 54 are preferably formed of uncoated stainless steel, preferably high strength foil, and have a width Wt exceeding the width Wf of the foil layers 56 and 58 by an amount on opposite sides to establish projecting side margins 60. As depicted in Fig. 13, a split mandrel 62 is placed over the center of the foil section 52 and wound in the direction of the arrows to form the foil subassembly 50 into a double spiral coil, in this instance, with the tail pieces 54 overlapping on the outer surface of such a coil.

Alternative embodiments of the central foil subassembly shown in Figs. 12 and 13, are shown in Figs. 14, 15, 16 and 17 as subassemblies 50a, 50b, 50c, and 50d, respectively. Thus, in Fig. 14, the core foil subassembly 50a is formed by flat and corrugated foil segments 56a and 58a connected at one end only to a tail piece 54a. In Fig. 15, the core foil subassembly 50b is similar to that of Figs. 12 and 13 except

that the corrugated foil segment 56b extends fully between the tail pieces 54b, whereas the flat foil segments 56b overlie opposite sides of the corrugated foil strip 58b. In Fig. 16, the tail pieces 54a and 54c are superimposed on corrugated foil segments 58c which overlie opposite sides of a central flat foil segment 56c extending for the same length as the corrugated foil segments 58c. Finally, in Fig. 17, a core foil subassembly 50d is illustrated in which continuous foil strips having alternating corrugated and flat segments are superimposed in longitudinal offset relationship and secured at opposite ends to a pair of tail pieces 54d.

In Figs. 18-19, the core foil subassembly 50a is shown first in its initial flat configuration in Fig. 18, in a partially coiled configuration in Fig. 19, and in a completely coiled configuration to form a central core 12 in Fig. 20. In Fig. 20, the tail piece 54a is shown wrapped in an overlapped tube configuration on the exterior of the coiled foil subassembly and secured by spot welds 64. A diametric bore 66 may be formed through the core 12 and the pin 23 inserted in the bore. A complete central core 12 is thus formed having the inner tube member 20 of the multi- piece tube section 18. Also, it will be noted that the ends of the tube member 20 extend axially beyond the ends of the core 12 because the width of the tail piece forming the tube 20 is greater than the width of the foil segments forming the central portion of the core 12 as explained above.

In Figs. 21 and 22, alternative embodiments of central

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core units are illustrated. In Fig. 21, the core 12a is shown with a pair of pins 23a, it being understood that three or more pins may be used depending on the design parameters of the catalytic converter. In Fig. 22, the tube 20b circumscribing the core unit 12b is a continuous closed tube separate from the coiled section of the unit 12b.

In accordance with the present invention, the annular leaf section and the central core are connected to each other by a tube section including two or more parts . In the embodiment illustrated in Fig. 1, for example, the two parts are represented by the inner tube 20 associated with the central core and the outer tube structure 22. It is contemplated however, that the multi-piece tube section interconnecting the annular leaf section and the central core may include a larger number of parts, such as separate tube sections in the outer tube structure. Variations in the outer tube structure are illustrated in Figs. 23-27 of the drawings .

In Fig. 23, the outer tube structure is designated by the reference character 22a and includes two axially spaced, closed cylindrical tube segments 22ax and 22ay. The tube segments 22ax and 22ay represent shorter lengths of the tube 22 in Fig. 23 and a combination of their length and axial spacing allows for the opposite ends of the segments to project axially from the leaves of the annular section 24.

It is noted further in connection with the embodiment of Fig. 23, that in a converter body having the generally radial

geometry shown in Fig. 1, opposite corners at the inner leaf ends represent critical points of connection to the central core 12. A result of the axially spaced, cylindrical tube segments 22ax and 22ay. is that those corners will be secured whereas a measure of torsional flexibility is provided between opposite corners at the inner leaf ends.

Figs. 24-27 represent further variations of the outer tube structure of the multipart tube section. Thus, in Fig.

24, the tube structure 22b is formed by a pair of cylindrical segments 22bm and 22bn. The arcuate cylindrical segments are spaced circumferentially from each other as shown. In Fig.

25, tubular structure is defined by pairs of circumferentially spaced tubular segments 22cx and 22cy axially spaced in the manner of the structure illustrated in Fig. 23. In Fig. 26, the tubular structure 22d is formed by three circumferentially spaced cylindrical segments 22dm, 22dn, and 22do. It is contemplated that this construction may be carried out by using four, five, or more of such circumferentially spaced arcuate cylindrical segments to provide the tube structure 22d. Finally, in Fig. 27, a variation of the embodiment of Fig. 23 is illustrated in which two closed tubular sections 22ex and 22ey are formed with radially projecting end flanges 22z.

Assembly of the outer tube structure with the annular leaf section 24 is depicted in Fig. 28 of the drawings. The outer tubular structure, in this instance, the tube 22 is first coated or otherwise provided on its outer periphery

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with a brazing compound 70 and then inserted into the opening 46 of the jacketed annular leaf section 24. A pair of electrodes 72 and 74, to which electrical energy may be applied, are inserted into the tube 22 in spaced relationship as shown in Fig. 28. In Fig. 28, the ends of the electrodes are bifurcated and designed to spring out against the inside of the tube. Alternatively, as shown in Fig. 29, the electrodes 72a and 74a are bifurcated and provided with conical end seats 73 biased axially by end forces, depicted by the arrows in Fig. 29, against a ceramic ball 75. In either case, the electrode ends are spaced by approximately 1/4 inch and advanced through the tube in increments of a length depending on the extent to which the inner ends of the leaves are to be joined to the outside of the tube. At each increment, the electrodes are pulsed with electrical current to heat about 3/8 inch of the tube.

The following example represents details of the brazing method. A 316 stainless steel tube having an outside diameter of .375" and a wall thickness of .035" is placed in the central opening of a converter body in contact with the inner ends of the foil leaves. The tube was covered with "Metglas 80 foil .001" in thickness in a spiral wrap providing an average foil thickness of about .002" layered on the tube. All surfaces were clean and degreased. A Taylor- Winfield resistive welder was set up with its transformer switch on #1 an programmed for 1 impulse of 60 cycle (1 second) duration at 46% current. A good braze of the inner

leaf ends to the outside of the tube was accomplished in an ambient air atmosphere without melting the tube or causing a flash to develop and without adversley affecting the coating on the leaves. After the outer tube structure 22 has been secured to the annular section 24, the central core 12 with the inner tube 20 attached thereto is inserted into the outer tube structure 22 as depicted in Fig. 30. The end margins of the inner and outer tubes are then secured such as by spot welding.

In Fig. 31, the inner tube 20 affixed to the central core 12 is provided with an end flange 20z whereas the annular section 24 is provided with the flanged and axially separated outer tube structure 22e described above with reference to Fig. 28. Central core 12 with the inner tube attached is inserted into the outer tube structure 22e as shown in phantom lines. Once inserted, a washer 22za is inserted over the projecting end of the inner tube 20 and secured in place such as by welding. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.