GB1587253A

GB1587253A - Article comprising brazed tubular members

Info

Publication number: GB1587253A
Application number: GB2326377A
Authority: GB
Original assignee: CERAMEL Ltd
Current assignee: CERAMEL Ltd
Priority date: 1978-05-11
Filing date: 1978-05-11
Publication date: 1981-04-01

Description

(54) ARTICLE COMPRISING BRAZED TUBULAR MEMBERS (71) We, CERAMEL LIMITED, a British Company of 32 Pope Road, Bromley, Kent, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an article in which a first tubular member is brazed to a second tubular member, one said member being a metal and the other member being either a metal or a ceramic material.

Metal parts are conventionally attached to other metal or ceramic material parts by brazing with a flux. For some uses the presence of a flux is undesirable. Thus, for example, in radioactive environments, the use of fluxes, such as borax, must be avoided. To overcome this problem it has been necessary, in the prior art, to form one of the members to be brazed of an "active metal" such as titanium. The titanium dissolves in the brazing material, conventionally a silver alloy having 7% copper, which during the brazing flows to fill up any spaces between the members being brazed and adheres to both members. The use of titanium for the construction of one of the members to be brazed is undesirable where a less costly and more easily workable material such as stainless steel would be preferred.

According to the invention we provide an article comprising a first tubular member made of steel connected with a second tubular member made of metal or ceramic material, the second member having one end entered into the first member, a solid device made of an active metal as hereinafter defined) surrounding the second member, and a metallic brazing material connecting said device to both members with said material forming a seal between said device and a peripheral surface of the second member and between said device and an annular surface of the first member which is transverse to the axis of the first member.

By "active metal" in the context of the present invention, is meant a metal which will dissolve in the brazing material to form a mixture which at the brazing temperature will flow and adhere to both the first and second members. While titanium is the preferred active metal, other metals may also behave in this way, for example, vanadium.

The invention also includes a method of making the article wherein the device is positioned around the second member one end of which is entered into the first member, and the metallic braze material is applied adjacent to the surfaces to be sealed without flux such that some of the metal of said device dissolves in the braze material.

The method according to the invention is particularly useful for brazing hollow cylindrical members which are positioned relative to each other in a telescopic manner thereby to seal the hollow interiors of the members from the outside. In such a case a ring of the active metal is provided which closely fits over the member having the lower diameter and may be accommodated in a countersink provided in the other member.

The method according to the invention is particularly useful for brazing metals such as stainless steel to ceramic materials such as alumina, in the manufacture of electrical components in which the ceramic is intended to act as an insulant.

An example of this is in the construction of an electrode assembly for a water level gauge, particularly for determining the level of water in steam boilers used, for example, in electricity generating stations.

Such electrodes are known. In use, a number of these electrodes are positioned in a chamber which is in communication with a steam boiler. The electrodes have their inner ends spaced at a predetermined distance from a wall of the chamber and serve to measure the level of water in the chamber by measuring the impedance between the electrode and the chamber. The known electrodes of this type are normally constructed of an electrode body formed of stainless steel which can be clamped to the outer wall of the gauge chamber, a titanium adaptor which passes through an aperture in the chamber wall, a ceramic insulant brazed to the titanium adaptor and a titanium tip brazed to the other end of the insulant. A titanium stud or stem passes along the length of the electrode body to the tip and is in electrical connection with the tip. The ceramic insulant positioned between the adaptor and the tip is necessary to insulate the tip from the electrode body.

In use the free end of the titanium tip is spaced at a predetermined distance (such as 1/4 inch) from a wall of the gauge chamber and by means of electronic circuitry known per se it is possible by measuring the impedance between the electrode tip and the chamber wall to determine whether the intervening material is water or steam. A number of these electrodes positioned one above the other in the gauge chamber can therefore be used to determine the level of water in the gauge and thus in the boiler itself. Again by means of circuitry known per se this information can be displayed in a suitable manner.

A preferred embodiment of the present invention relates to an improvement in the design of an electrode of this type, in which the adaptor and electrode body are made in one piece, preferably of stainless steel, one end of the ceramic insulant is inserted in a bore in the electrode body, a titanium ring is positioned about the ceramic insulant in the vicinity of the junction between the electrode body and the insulant and the titanium ring is brazed to the electrode body on the one hand and on the other hand to the ceramic insulant.

According to a further improvement of the electrode design, the centre stem has its lower end screwed into a tapped blind hole provided in the upper portion of the titanium tip.

The invention will now be described in more detail, purely by way of example, with reference to the accompanying drawing in which: Figure 1 shows a hollow stainless steel member brazed to a hollow ceramic member by a method according to the invention; Figure 2 shows an alternative method according to the invention for brazing a hollow metal member to a hollow ceramic member; Figure 3 shows an electrode according to the prior art; Figure 4 shows the electrode body of the electrode shown in Figure 3; Figure 5 shows the electrode adaptor of the electrode shown in Figure 3: Figure 6 shows the electrode tip of the electrode shown in Figure 3; Figure 7 shows the centre stud of the electrode in Figure 3; Figure 8 shows an electrode body according to the first feature of the invention together with an electrode insulant; and Figure 9 shows an electrode tip according to a second design improvement of the electrode assembly together with an electrode insulant and a lower part of the centre stud.

Referring to Figure 1 there is shown a tube 10 of stainless steel which is provided at one end with a recess 11 of greater diameter than the bore of the tube. Positioned in the recess 11 is one end of a ceramic tube 13, the end face 14 of which is supported against the bottom annular face or shoulder 16 of the recess 11 which is transverse to the axis of the tube.

On the end face 17 of the stainless steel tube 10, a second recess 19 is formed which is of larger diameter than the recess 11 and into which is positioned the aforesaid device in the form of a ring 20 of titanium. The ring 20 is fitted over the end of the ceramic tube 13 and has a square cross section approximately corresponding to the cross section of the countersink 20.

When these components have been assembled as shown, the ring 20 is brazed to the ceramic tube 13 and to the stainless steel tube 10. Some of the titanium of the ring 20 partly dissolves in the brazing material which then flows all over the ring and fills up the spaces 22 which inevitably exist between the ceramic tube 13, the titanium ring and the stainless steel tube 10, i.e. between the ring 20 and the peripheral surface of the tube 13 and between the ring 20 and the annular surface at the inner end of the recess 19.

A fluid tight seal is thereby produced serving to prevent the environment outside the components getting through to the interior 23.

Since only the ring 20 is brazed on to the ceramic tube 13, and is only in contact with it over a short axial length, the risk of surface cracking of the ceramic is reduced. These advantages both help to reduce the cost of the seal.

Referring to Figure 2, there is shown a tube 30 of stainless steel, which is provided at one end with a bore 31. Positioned loosely in the bore 31 is one end of a ceramic tube 33, the end face 34 of which is supported against the bottom face of shoulder 36 of the bore 11.

Against the end face 37 of the tube 30 there is positioned a ring 40 of titanium. The ring 40 is fitted over the end of the ceramic tube 33 and has a cross section which includes two planar faces 41, 42, which abut respectively against the stainless steel tube 30 and the ceramic tube 33, and an outer face 43 which may be curved or otherwise.

When the components have been assembled as shown, the ring 40 is brazed to the ceramic tube 33 and to the stainless steel tube 30. The titanium of the ring 40 partly dissolves in the brazing material which flows all over the ring and fills up the spaces 45 which inevitably exist between the ceramic tube and the stainless steel tube.

The seal provided by the method illustrated in Figure 2 has the same advantages as that described in connection with Figure 1.

The prior art electrode shown in Figures 3 to 7 comprises an electrode body 110 in the form of a stainless steel cylindrical tube provided at its lower end with a flange 111. In use this flange is clamped to a wall of a water level gauge chamber shown by dotted lines 113, with a gasket 114 positioned therebetween.

Into the lower end of the electrode body 110 there is inserted a tubular titanium adaptor 116 which extends, in use, into the interior of the gauge chamber. The adaptor includes an annular shoulder 117 (Figure 5) the upper surface of which is brazed to the lower surface of the flange 111- by means of a brazing material which 'conventionally is silver containing 7% copper.

Into the upper end of the electrode body 110 a tubular insulant 120 is inserted. A similar tubular ceramic insulant 122 is inserted in the lower end of the titanium adaptor 116.

These insulants serve to locate a titanium centre stud 123 in spaced relationship from the electrode body 110 and insulated therefrom.

The insulants are formed of high purity (e.g.

99.5%) alumina and are resistant to attack by the high pressure steam environments to which they are, in use, subjected. The electrode in sulant 122 is brazed to the titanium adaptor 116 by means of a silver brazing material to form a fluid-tight seal.

Towards the lower end of the titanium adaptor 116 a groove 119 is provided so that, in use, any steam condensing on the adaptor is encouraged to drip off-the adaptor and is hence kept away from the seal between the adaptor and the ceramic electrode insulant 122.

To the lower end of the electrode insulant 122 is brazed the upper end of a tubular titan ium electrode tip 125 which, in a similar man ner to the titanium adaptor 116, is provided with a drip groove 126 (see Figure 6). The lower end of the tip 125 is provided with a central hole 128 which is drilled and tapped and into which is screwed the lower end of the centre stud 123 to make an electrical connection therewith. To prevent the access of steam or water the stud is brazed at 129 to the titanium tip 125.

Referring to Figure 8 there is shown a stainless steel electrode body 131 in accord ance with the invention which includes an outer flanged portion 132 which corresponds to the electrode body 110 of Figure 1, and an integral inner portion 134 which substan tially corresponds in shape to the titanium adaptor 116 of Figure 1. Thus, the lower por tion 134 is formed with a drip groove 135 and is provided at its lower end with a bore 137 in to which the ceramic insulant 122 is fitted.

The lower end of the portion 134 is provided with a countersink or annular groove 138 into which a titanium ring 140 is fitted and brazed to both the electrode body 131 and the ceramic insulant 122 by means of silver brazing material. The ring 140 may be brazed by a static or a rotating brazing step. The titanium ring 140 is a tight fit on the ceramic insulant 122.

By providing the electrode body and adaptor in one piece the seal between these com ponents is avoided which not only improves the reliability of the electrode by removing the risk of a metal/metal braze leak but also reduces the cost. Further, by using stainless steel in place of titanium for the adaptor and by carrying out a single machining step the cost is still further reduced.

The fitting of the ceramic insulant in the bore 137 of the electrode body need not be tight, as any tolerance will be taken up by the titanium ring 140 and the brazing material.

An accurate shaping of the adaptor to the ceramic insulant as was necessary in the prior art is now therefore avoided. Only the ring 140 is a tight fit on the ceramic and the short axial length of this tight fit reduced the risk of surface cracking in the ceramic.

It is not essential to provide the countersing 138 to accommodate the titanium ring 140.

Instead, the titanium ring may simply abut the lower end face of the electrode body 131.

Since the brazing step is, in any case, carried out with the electrode upside down (relative to the views shown in the Figures) an adequate seal between the titanium ring and the electrode body on the one hand and the ceramic material on the other is assured. The titanium ring 140 may be provided with any cross sectional shape desired.

Referring to Figure 9, there is shown the tip 141 according to the design improvement of the electrode assembly. The tip 141 is formed of solid titanium and is generally cylindrical with an unbroken lower end face 142, a drip groove 143 provided adjacent its upper end and an aperture 144 in its upper end face into which is inserted the lower end of the electrode insulant 122. The floor of the aper ture 144 is provided with a tapped blind hole 146 into which is screwed the tapped lower end 147 of the stainless steel stud 123.

The lower end of the ceramic insulant 122 is brazed to the titanium tip 141, the brazing material being allowed to run between the tip and the insulant into the aperture 144 to take up any tolerance therebetween and to stick the stud 123 securely into the tip 141.

A comparison of Figure 9 with Figures 3 and 6 shows that the improvement enables the tip to be brazed to the electrode insulant and the stud in a single step whereas two brazing steps were previously necessary. Also in the prior art the brazing of the tip to the stud was critical to prevent the ingress of water and/or steam. The seal is no longer critical since the seal between the electrode insulant and the tip will be sufficient.

A further advantage of this aspect of the invention is that the blind hole 146 is easier to form and tap than the hole provided in the conventional tip shown in Figures 3 and 6.

The second aspect of the invention enables the tip 141 to be alternatively formed of stainless steel if desired, but it is considered to be preferable to maintain the use of titan ium since the titanium to steel seal between the tip and the stud is easier to produce than a corresponding steel to steel seal. In such a case it would be necessary to form the stud of titanium.

The brazing steps referred to throughout this specification are generally carried out at about 1000 C (the melting point of titanium by comparison is about 1750 C). It is to be remembered that the brazing steps described are for sealing and are not intended to give the electrode assembly mechanical strength.

WHAT WE CLAIM IS: 1. An article comprising a first tubular member made of steel connected with a second tubular member made of metal or ceramic material, the second member having one end entered into the first member, a solid device made of an active metal (as hereinbefore defined) surrounding the second member, and a metallic brazing material connecting said device to both members with said material forming a seal between said device and a peripheral surface of the second member and between said device and an annular surface of the first member which is transverse to the axis of the first member.

2. An article as claimed in Claim 1 wherein the first member is the electrode body of a water level gauge, the second member is ceramic and an electrode tip is connected to the other end of the second member.

3. An article as claimed in Claim 2 wherein the electrode body is formed integrally with a flange to seal with a water container and with a projecting steel adaptor adapted to extend into the gauge chamber and constituting said first member to which said second member is secured.

4. An article as claimed in Claim l, 2 or 3 wherein the device is engaged in a recess in the first member of larger diameter than the bore of the first member.

5. An article as claimed in any of the preceding claims, wherein the device is a solid ring having an inner annular surface which seats around the second member and an end planar surface which seats against a similar surface on the first member.

6. An article as claimed in any of the preceding claims wherein the device is made of titanium.

7. An article as claimed in Claim 2 or 3 having a titanium electrode tip secured to the other end of the second (ceramic) member, and a titanium centre stud has its lower end screwed into a tapped blind hole provided in the upper portion of the titanium tip.

8. A method of making an article as claimed in any of the preceding claims wherein the device is positioned around the second member one end of which is entered into the first member, and the metallic braze material is applied adjacent to the surfaces to be sealed without flux such that some of the metal of said device dissolves in the braze material.

9. An electrode assembly substantially as hereinbefore described with reference to and as illustrated in Figures 8 and 9 of the accompanying drawings.

10. An article as claimed in Claim 1, substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. this specification are generally carried out at about 1000 C (the melting point of titanium by comparison is about 1750 C). It is to be remembered that the brazing steps described are for sealing and are not intended to give the electrode assembly mechanical strength. WHAT WE CLAIM IS:

1. An article comprising a first tubular member made of steel connected with a second tubular member made of metal or ceramic material, the second member having one end entered into the first member, a solid device made of an active metal (as hereinbefore defined) surrounding the second member, and a metallic brazing material connecting said device to both members with said material forming a seal between said device and a peripheral surface of the second member and between said device and an annular surface of the first member which is transverse to the axis of the first member.

10. An article as claimed in Claim 1, substantially as hereinbefore described.