FR2628997A1 - Prodn. of hermetic joint between ceramic insert and metal tube - using tie wire to maintain inward force on tubing applied by split rings of similar thermal expansion coefficient to that of insert - Google Patents

Abstract

A hermetic seal joint is produced between a thin metallic tube and a support ring of a ceramic component by producing a recess into which split rings of metal having a similar coefft. of expansion to that of component are located. Thin layers of support metal are located between the contacting surfaces of ring and splits and the assembly is heated to brazing temp. and as the tube is plastically deformed under the stress of rings a temporary wire support is tightened over the rings. USE/ADVANTAGE - Joining tube sections together with internal ceramic part such as a cathode support in an X-ray tube where high temps. prevail. The wire support prevents the tube from expanding as the rings undergo a certain amt. of creep at the brazing temp. (17pp Dwg.No.0/4)

Description

"Waterproof ceramic metal sealing process,
to withstand high temperatures "
The invention has tria: a sealed sealing process, withstanding high temperatures, between a thick metal tube and a metallized cylindrical annular seat of a ceramic part, where the ceramic part is assembled in position in the tube by lining the interface with an appropriate filler metal, a metal hoop ring with expansion coefficient matched to that of ceramic is inserted around the tube, at the height of the span, with the interposition of a thin sheet of filler metal, and the assembly is carried in a soldering chamber where it is heated so that the filler metal melts and wets the surfaces in contact.

 To obtain tight seals between ceramic insulators and metal parts, in particular to allow electrical ccannexations to pass through the walls of enclosures, either under high vacuum or under high pressures, with high mechanical resistance, it is common to carry out an assembly between an external tubular metallic part and a metallized annular bearing of a ceramic part of suitable diameter, and to braze the metallized bearing on the internal surface of the tubular metallic part, using a metal of appropriate intake.

 Since brazing involves heating at high temperature, and the assembly must undergo, either during assembly and completion of the equipment for which it is intended, or during the operation of this equipment, temperature variations noteworthy, the expansion coefficients of the ceramic and of the metal which constitutes the tube must be matched. Alloys, generally based on iron and nickel, have been developed, the coefficient of expansion of which is adjusted to that of ceramics. However, the adjustment of the expansion coefficients is only carried out within determined temperature ranges, where the intrinsic expansion coefficient of the metal is compensated for by structural modifications.

 Even small discrepancies between expansion coefficients jeopardize the sealing of the joint in different ways depending on the direction of the forces which occur on the weak element of the joint, in this case the ceramic filler metal interface. . This interface resists poorly to -ts in radial traction, and very well to efforts in radial compression.

 It is therefore necessary to choose the coefficient of expansion of the metal tube slightly higher than that of ceramic, so that, the molten filler metal being placed between ceramic and outer tube, the layer formed is put into compression from the solidification of the filler metal to room temperature, because the metal tube shrinks more than the ceramic during cooling. The joint is therefore hooped by the metal tube in elastic peripheral tension.

 However, one cannot go too far in the hooping, because one should not exceed the elastic limit of the metal tube under penalty of seeing the tube elongate plastically along its periphery, and the hooping effect seriously attenuate.

 However, at the brazing temperature, the metal tube is often annealed.

 Frequently hooping does not occur effectively until a temperature at which plastic deformation ceases, below the solidification temperature of the filler metal.

 However it is frequent that the assembly must be brought later to a high temperature; if this temperature is higher than the hooping start temperature mentioned above, the ceramic metal interface is put in tension, and the sealing of the joint is compromised.

 Thus, for example, if the metal ceramic seal is intended to constitute an insulating assembly for an electrode of a high vacuum tube, such as a cathode assembly of an X-ray tube, the metal tube sealed around the ground piece. te ceramic will be welded to other elements of the wall of the tube, and thereafter, when pumping the tube, the assembly is subjected to prolonged baking at a temperature which can exceed 500 ° C.

 Furthermore, special alloys for ceramic welding require very careful processing, both for the composition and for the treatments necessary to bring them to the physical structure which provides the desired coefficient of expansion. They are therefore expensive; furthermore the commercially available forms are. limited, and any order in an unusual form (such as a thick tube with determined dimensions) is accepted only from a high tonnage.

On the other hand, a classic iron / nickel alloy for ceramic metal seals, known as Dilver
P, is notably outside its isodilation range compared to ceramic already at 600 ° C.

 For a number of reasons, such as lower purchase cost, higher machinability, mechanical strength, ease of obtaining it in a suitable semi-finished form, resistance to oxidation and corrosion, it appeared desirable to replace a stainless steel type 18-8, for example, with so-called weldable alloys for ceramics.

 However, the major obstacle was the disparity in the expansion coefficients, 0.65 x 10 5 / K at 20 ° C for alumina ceramic, 1.7 x 10 5 / K for 18-8 stainless steel. With such values, for brazing with a filler metal melting towards 780-C (silver copper brazing), on a diameter of 30 mm the difference in diameter when brazing between alumina and stainless steel would reach approximately 0.25 mm. Filling such a large gap with filler metal would already be very difficult, and the disadvantages discussed above resulting from creep would be considerably considerable.

 It has previously been proposed to braze on a metallized cylindrical surface of a ceramic piece a tube of moderate thickness (less than a millimeter) of stainless steel type 18-8, by placing a ring of titanium hoop around the steel tube stainless to the right of the scope, with interpositicsn of a thin sheet of filler metal.

 When heating to bring to fusion, in an inert atmosphere is understood, the filler metal, the titanium ring, whose coefficient of expansion is 0.8 x 10 5 / K, prevents excessive expansion of the tube in stainless steel.

In practice, the stainless steel tube has its deformations imposed by the titanium hoop.

 Note that it is necessary that the thin sheet (25 to 50 micrometers thick) of filler metal line the entire interface between the stainless steel tube and the titanium hoop ring, because during heating for brazing, the tightening is such that the molten filler metal could not come to wet the surfaces in contact if it were not present in advance.

This gives watertight seals which are mechanically robust, and withstand already already elevated temperature rises. However, the performance appears to be insufficient in relation to the requests, for the following reasons
The wall thickness of the stainless steel tube must be less than that of the titanium hoop ring so that deformations during heating cycles are imposed by the hoop ring.

 This hoop ring is oversized, which can be a drawback in certain cases, for example if the assembly must be able to slide in a ring or a tube.

 It is necessary to keep a clearance between tube and hoop, to introduce the thin sheet of filler metal this clearance delays the tightening of the hoop ring during the rise in temperature, and therefore the effectiveness of hooping.

 The creep of titanium 3 the brazing temperature, under the expansion force of the stainless steel tube, is not negligible. This results in a lowering of the temperature at the start of hooping, already mentioned above, and which appears as the maximum temperature to which the seal can be exposed without risk later.

 The object of the invention is a method of sealing a metal tube tightly on a circular surface of a ceramic part, which does not have the drawbacks mentioned above.

 To this end, the invention discloses a sealed sealing process, withstanding high temperatures, between a thick metal tube and a metallized cylindrical annular bearing surface of a ceramic part, where the ceramic part is assembled in position in the tube. by lining the interface with an appropriate filler metal, cn is inserted around the tube, at the height of the span, a metal hoop ring whose coefficient of expansion agrees with that of ceramic with the interposition of a thin sheet of filler metal, and the assembly is carried in a soldering chamber where it is heated so that the filler metal melts and wets the surfaces in contact, characterized in that practical, in the outer surface of the metal tube, an annular groove in line with the location of the bearing surface, two split hoop rings are prepared with a height corresponding to that of the groove and suitable for being accommodated therein, the one inside and the other outside, last flush with the outer surface of the metal tube, there is, in addition to the sheet of filler metal between the throat bottom and the inner ring, a sheet of filler metal between the rings, arranged so that the slots are not coincidental, a temporary hoop formed of a few tight turns of a molybdenum wire is placed on the outer ring, and the temporary hoop is eliminated after the sealing has been carried out.

 Thanks to these provisions, the metal tube can have, beyond the brazed zone, a thickness sufficient to be mechanically rigid and to accept machining alterations, while in the brazing zone, it will be thinned, by machining. throat enough for the expansion forces generated by the hoops to outweigh those generated by the metal tube.

 The hoops being housed entirely in the groove, the tube does not have any extra thickness in the sealing zone. In addition, the slots of the hoops allow the passage of these hoops around the tube to engage them in the groove; the hoops are however active due to their connection by brazing over the entire surface on which they rest, the slots also being out of radial coincidence to avoid a weak point.

 The hoop rings are not necessarily made of titanium, and can be of any metal whose expansion coefficient agrees with that of ceramic, and is, of course, compatible with the metal of the tube, since the temporary hoop acts in tightening at least up to a temperature where the hoop metal has a sufficiently high elastic limit.

 In addition, the temporary molybdenum hoop has the first effect of annihilating the radial clearances between the outer ring and the bottom of the groove, and between the inner and outer rings; then, the molybdenum having a coefficient of expansion lower than that of titanium, and even that of ceramic (4.7 x 10 1K), will produce an energetic tightening of the frets, increasing with the temperature, so that any creep of the frets is warned; in addition, molybdenum still has a very high elastic limit at the brazing temperature (the mely point of molybdenum is 2 610 C), so that the temporary hoop practically does not elongate plastically, and remains active until after the hoop rings have become fully effective.

 Preferably, 1. groove extends in depth over about two thirds of the thickness of the tube, the split hoop rings being of substantially equal thicknesses, and therefore equal to the remaining wall thickness of the metal tube under the throat. A total hoop thickness double that of the underlying tube wall is obtained.

 In a particular arrangement, the metal tube, before assembly, consists of two sections which are joined longitudinally in a cross section at an intermediate height of the groove.

 This arrangement can prove useful for ensuring the exact position of the ceramic piece in the tube, believed when the imposed shapes of the tube, on either side of the ceramic, make the machining of the tube in one piece. not desirable. Furthermore, the execution of the tube in two sections can facilitate the insertion of the split hoop rings. In any case, the sections are brazed together during the general operation of brazing the range area. In addition, each section is brazed independently to the ceramic. The sealing reliability of the seal is not affected.

Secondary characteristics and advantages of the invention will also emerge from the description which follows, by way of example, with reference to the accompanying drawings in which
Figure 1 shows in section a seal according to the invention, before performing brazing
Figure 2 shows, in exploded perspective, the different elements which constitute a seal according to the invention;
Figure 3 is a perspective view on a reduced scale of the seal in the same state as in Figure 1
4 shows a section on a large scale of a seal according to the inventic, n, completed.

 According to the execution mode> n shown in FIGS. 1, 2 and 3, to seal a conical insulator 1, made of alumina ceramic, such as an X-ray tube cathode holder, on a tube 2 made of 304 L stainless steel , of the family of 18-8 with a thickness close to 1.5 mm and a diameter of 30 mm substantially, a cylindrical bearing surface 10 of the insulator 1, which enters, is known in a known manner, with a molybdenum / manganese alloy in the inner bore of tube 2.

 Between the surface 10 and the internal face of the tube 2, a ribbon of a thin sheet (25 to 50 micrometers thick) of a filler metal, which is a conventional copper silver eutectic brazing alloy, has been inserted, at melting point 780'C.

 To make the description clearer, designate the structural elements by considering the tube placed vertically as shown in the figures, as it will appear during the brazing operation, although, in its destination, the orientation of the tube be free and arbitrary with respect to the vertical.

 To ensure a correct and reproducible location of the insulator 1 in the tube 2, the latter is over-bored, at its lower part, to a depth of about 0.5 mm, to fit onto a substantially cylindrical caliber, which has a shoulder on which abuts the angle 21 of overbore, and an upper edge, above the shoulder, which forms rest for the lower edge of the insulator 1.

 A groove 20, with a cylindrical bottom, the depth of which is 1 mm, or two thirds of the thickness of the tube 2, and the height of which is here approximately 6 mm, has been provided in the outer surface of the tube 2, a little more than that of staff 10 (here 5 mm). The lower edge of the groove is situated a little above the angle 21 of overbore, so as not to cut the tube 2.

 We fill the bottom of the groove 20 with a thin sheet of filler metal, of the same kind as that which lines the worn interface 10-tube 2, laminated to a thickness of 25 to 50 jin, then we introduce a first hoop ring 3, having a longitudinal slot 30 in the groove 20. The inside diameter of the ring 3 is equal to the diameter of the tube 2 at the bottom of the groove 20 and its height corresponds to that of this groove. This hoop ring is made of TA6V titanium and its thickness is substantially half the depth of the groove 20, that is, here, 0.5 mm. The presence of the slot 30, joined to the elasticity of the titanium, makes it possible to thread the hoop ring 3 over the tube 2.

 Over the hoop ring 3, there is a second thin sheet of filler metal, then a second hoop ring 4, also provided with a slot 40, is engaged, over the inner hoop ring 3 and the sheet filler metal interlayer. The inside diameter of the outer hoop ring 4 corresponds to the outside diameter of the first 3, and its thickness is likewise substantially equal to half the depth of the groove (here therefore 0.5 mm). The height of the outer hoop ring 4 is also equal to that of the groove 20.

 It will be understood that the hoop rings can be made by rolling a titanium strip of appropriate length and width.

 It will be understood that the thicknesses of the fret rings 3 and 4, which are equal, are less than the half-groove depth of the thickness of the sheet of filler metal, so that the two rings 3 and 4 and the two sheets of apporen metal in place, the outer surface of the outer ring 4 exceeds that of the tube 2 only by the sum of the clearances at the various interfaces.

 Of course, the slots 30 and 40 are arranged in diametrical opposition approximately.

 Over the outer hoop ring 4, a temporary hoop is formed by rolling a molybdenum wire 5, 1 mm in diameter to form three or four tight turns. The tightening is completed by twisting together the two ends 50 of the molybdenum wire.

 It will be noted that the molybdenum wire used to constitute the temporary hoop must preferably have a diameter of between 1.5 mm and 0.7 mm. For diameters greater than 1.5 mm, the wire has excessive stiffness, does not easily marry the outer surface of the outer hoop ring 4, and does not lend itself effectively to twisting 50. Less than 0.7 mm, the wire may break under tension, and the tight maintenance of the twist 50 becomes random.

 In the soldering enclosure, under vacuum or in an argon atmosphere, the assembly is heated, at least in the section where the bearing 10 and the groove 20 are located, to a temperature of around 850 C. This operatic> n heating is quite conventional.

 The filler metal melts; on the one hand, at the interface 2-bearing tube 10, both the metallization of this bearing 10 and the inner surface of the tube 2 are wetted.

 On the other hand, in the groove 20, the molybdenum wire 5 has expanded less than the hoop rings 3 and 4, which, for their part, have expanded less than the tube 2. Like the tube 2, at the bottom throat 20, is relatively thin, and the elastic limit of stainless steel at the brazing temperature is relatively low, the tube 2 compensates by plastic deformation for the excess expansion which it undergoes. The deformations by creep of the hoop rings 3 and 4 are much more limited, so that the excess expansion of these rings relative to the temporary hoop 5 made of molybdenum results in an elastic tensioning of the temporary hoop. Of course, the peripheral length of the hoop rings 3 and 4 must be such that at no time do the lips of the slots 30 and 40 abut.

 The fusion of the filler metal in the groove, under high pressure due to the elastic tension of the temporary hoop, ensures intimate contact of the filler metal with the opposite faces of the throat bottom 20, of the hoop ring 3 and the fret fret 4, as well as in the slots 30 and 40.

 As the clamping effect of the temporary hoop 5 continues to be exerted from the brazing temperature to a temperature close enough to ambient for the titanium and stainless steel to have returned to their normal mechanical properties, it there is no hooping start temperature lower than the soldering temperature, so that the assembly will be able to withstand temperatures up to the softening of the filler metal without risk of loss of tightness. As, in the groove 20, the filler metal will have incorporated diffused titanium, the softening temperature of the filler metal will have increased.

 After brazing and cooling the assembly, it is removed from the enclosure, stripped of its temporary hoop, and cleaned on the surface to remove burrs of filler metal.

 It will be noted that, before carrying the assembly into the brazing enclosure, a rod of filler metal can be deposited between the tube 2 and the insulator 1, just above the span 10. This thus provides 'a reserve of filler metal, to feed a leave.

 I1 is possible, moreover, not to interpose a sheet of filler metal between the tube 2 and the bearing, and to feed the solder with only a rod of filler metal. However, this procedure has the disadvantage that the penetration by capillarity of the filler metal between the metallization of the surface 10 and the inner surface of the tube 2 can be locally prevented by hooping.

 A section along a plane passing through the axis of the tube of the finished seal is shown diagrammatically in FIG. 4. We see, from the inside to the outside, the ceramic insulator 1, its metallized surface 10, an interface 11 for soldering copper silver, the tube 2 with its groove 20, at the bottom of the groove a solder 31 uniting. the tube 2 at the inner hoop 3, and a solder 41 between the inner hoop 3 and the outer hoop 4, the outer surface of this outer hoop 4 coming flush with the outer surface of the tube 2.

 We can still see the solder 22 and 23 between the upper and lower sides of the groove 20 and the end edges of the hoops 3 and 4.

 Similarly, a section perpendicular to the axis of the tube would reveal solder in the slots 30 and 40.

 It will be noted that the brazing process is part of the sealing structure, as it can be revealed by metallographic sections. We can recognize the effect of the temporary hoop 5 in the thinness of the soldering of the slots 30 and 40, because, if the split fret rings had not been kept tight until the soldering consolidation, the uncontrolled expansion of the tube 2 would have resulted in a significant widening of the slots.

 So far, it has been considered that the tube 2 is longitudinally continuous in the brazing zone.

However, it will be understood that the tube could consist of two sections which would be joined in a straight section 6 passing inside the groove 20. Each section would be brazed directly on a corresponding area of the span 10, in a sealed manner. During the brazing operation, filler metal penetrating into the joining interface would bond the sections to each other at the end, while the hoop rings would consolidate the connection at the end.

 The separation into two sections of the tube 2 could make it possible to block the bearing rim 10 between shoulders formed in the two sections, in order to obtain a reciprocal positioning of the tube 2 and of the insulator 1 even more precise and robust. In addition C> n could provide flares of the tube 2 on either side of the brazing zone, without hindering the establishment of the split hoop rings 3 and 4, if these flares appeared desirable.

 Similarly, if the tube 2 did not have to exceed the insulator 1, the groove 20 would be reduced to a thinning of the tube 2 at the end, the lower flank of the groove 2 being eliminated.

 It will also be understood that, in FIGS. 1 to 4, the tube 2 and the ceramic insulator 1 have only been shown in their parts relating to the brazing zone, and that the corresponding parts, ndant to the ray tube X have peculiarities which would have been unnecessarily described.

 However, it is specified that the pumping of the finished tube involves baking at 700 ° C., and that the study pieces withstood temperature cycles with peaks at 750 ° C., without deterioration of the sealing of the ceramic metal seal.

 It will be appreciated that the method of the invention makes it possible to execute leaktight seals on ceramic insulators of very large diameter, by eliminating the risks of radial tensioning of the ceramic tube interface; the radial thickness of the interface is substantially the same regardless of the diameter of the seal, while the effects of differential expansion are proportional to this diameter. Incidentally, the execution of the hoop rings by rolling a band does not limit the peripheral length of these rings, which would not be the case if the hoop rings were pulled in the mass.

 It goes without saying that the invention is not limited to seals of alumina / stainless steel 18-8, but encompasses all of the ceramics which can be welded to metals, and of the alloys used in the execution of leaktight equipment; ceramics and alloys which can be chosen because of their particular properties, thermal Magnetic raw.

 Furthermore, molybdenum is the typical metal for executing the temporary hoop, because of its low coefficient of expansion, its high elastic limit at the brazing temperature, and its ability to allow itself to be shaped properly without any particular precautions.

 However, the invention cannot be strictly limited to the application of molybdenum to the constitution of the provisional hoop, if another metal or alloy having equivalent coefficient of expansion, elastic limit and forming capacity properties, taking into account the properties particular components of the proposed seal. For example, some molybdenum-tungsten alloys could be used.

 In general, the invention is not limited to the examples described, but embraces all the variant embodiments thereof, within the scope of the claims.

Claims (9)

 1. Process. waterproof seal, withstanding high temperatures, -between a thick metal tube (2) and an annular cylindrical bearing surface (10) metallized with a ceramic part (1), where the ceramic part (1) is assembled in position in the tube (2) by lining the interface with a suitable filler metal, around the tube (2), at the height of the seat (10), a hoop ring (3, 4) made of metal is inserted coefficient of expansion granted to that of ceramic with the interposition of a thin sheet of filler metal, and the assembly (1, 2) is carried in a brazing enclosure where it is heated so that the metal d '' contribution enters in fusion and wets the surfaces in contact, characterized in that one practices, in the external surface of the metal tube (2), an annular groove (20) in line with the location of the bearing surface (10) , two split hoop rings (3, 4) are prepared, split in height corresponding to that of the groove (20) and able to be accommodated in superposition, one inside and the other outside, the latter (4) flush with the outside surface of the metal tube (2), in addition to the sheet of filler metal between the bottom of the groove (20) and the inner ring (3), there is a sheet of filler metal between the rings (3 and 4), arranged so that the slots (30, 40) are not in coincidence, a temporary hoop (5) is formed on the outer ring (4) tight turns of a molybdenum wire, and the temporary hoop (5) is eliminated after the sealing has been carried out.  2. Method according to claim 1, characterized in that the ceramic is an alumina.  3. Method according to claim 2, characterized in that the metallization of the surface (10) of the part (1) in alumina ceramic is carried out in molybdenum / manganese.  4. Method according to any one of claims 1 to 3, characterized in that the metal tube (2) is of stainless steel type 18-8.  5. Method according to any one of claims 1 to 4, characterized in that the filler metal is a silver / copper brazing alloy.  6. Method according to any one of claims 1 to 5, characterized in that said groove (20) extends in depth over approximately two thirds of the thickness of the tube (2), the split rings (3, 4) of hoop being of substantially equal thicknesses.  7. Method according to any one of claims 1 to 6, characterized in that the metal tube (2), before assembly, consists of two sections which are joined longitudinally along a cross section (6) at an intermediate height of the groove (20).  8. Method according to any one of claims 1 to 7, characterized in that the molybdenum wire of the temporary hoop (5) has a diameter between 0.7 and 1.5 mm.  9. Method according to any one of claims 1 to 8, characterized in that the temporary hoop (5) is held tight by a twist (50) executed with the two ends of the hoop wire (5) together.