GB2212919A - Probe for an ultrasonic flaw detector welded to a support - Google Patents

Abstract

A probe 2 for ultrasonic flaw detectors is described that includes a transducer 5 sandwiched between electrodes 6a, 6b and either a protective plate 4 to protect the transducer or a metal body by which the transducer is directly attached to the object 1 to be tested, these two parts being bonded to each other with a heat-resisting brazing or soldering material, which also serves as one of the electrodes 6a of the transducer. Owing to its high bonding strength, the probe thus constituted is not only capable of serving both in the high temperature range and in the low temperature range, but also may be used in a mobile role or in a stationary mode in testing the object 1. Where a metal body is used for the mounting, this may be cylindrical or wedge-shaped. The transducer 5 may be a single lithium niobate crystal and the brazing material and electrodes may be a Al-Si-Mg alloy or a silver alloy. A coupling medium 3 may be of silicone oil, molten lead or liquid sodium and the object 1 may be a space station or a nuclear reactor pressure vessel. <IMAGE>

Description

"PROBE FOR AN ULTRASONIC FLAW DETECTOR" The present invention relates to a probe for an ultrasonic flaw detector.

Recently, there has been a general trend towards prolonging the service lifetime of various structures, and consequently the necessity of measuring the degree of ageing of such structures is increasing. Ultrasonic testing apparatus offers an effective means of assessing the remaining lifetime of a structure, for example, with regard to the degree of fatigue of the structure.

The conventional normal scan probe or straight beam testing probe for such an ultrasonic apparatus is illustrated in Figure 5 of the drawings, which is a schematic side view of such a probe in use.

As shown in the Figure, the probe b is slidably in contact with the object a to be tested, e.g. a structure, through a couplant, such as, for example, water or glycerol.

In this case, the probe b is constructed from a transducer f, which is made from an oscillator crystal, such as Pb(Zr,Ti)03, and which is sandwiched between a pair of electrodes e made by baking silver thereon at a low temperature of about 300 C, and which is bonded onto a ceramic protective plate d, which comes in contact with the object a to be tested through a couplant c, by means of an epoxy type resin adhesive g.

This kind of probe b, constructed as described above, has the disadvantage that, when the object a is a structure which may be heated to a high temperature, such as a nuclear reactor pressure vessel, the adhesive g is liable to separate on thermal expansion of the ceramic protective plate d, limiting the permissible service temperature range of the probe b to 150 rj 2000C, and thus restricting its application to those objects which will not become too hot.

When the object a is a structure which may become too cold, on the other hand, separation of adhesive g is liable to occur again on contraction of the ceramic protective plate d, limiting the permissible service temperature range in this direction too.

It is therefore an object of the present invention to provide a probe for an ultrasonic flaw detector in which this disadvantage is overcome and which has its permissible service temperature range extended both in the high temperature direction and in the low temperature direction.

A secondary object of this invention is to provide a probe for an ultrasonic flaw detector that is slidable on the object to be tested, and can thus be used in a mobile mode in testing the object.

Another secondary object of this invention is to provide a probe for an ultrasonic flaw detector that can be used in the stationary mode in testing the object.

According to the present invention, there is provided a probe for an ultrasonic flaw detector comprising a transducer sandwiched between a pair of electrodes and a member acoustically coupled or adapted to be acoustically coupled to an object to be tested, said transducer and said member being welded or brazed together (referred to hereinafter for simplicity merely as "brazed") to form a unified integral body by means of a heat-resisting brazing or soldering material disposed between the transducer and the member and serving as one of said electrodes.

In one embodiment which achieves the first and second objects as set out above, the member is a protective plate which is brazed to the transducer at one side and adapted to be slidably attached to the object to be tested at the other side through an acoustic coupling medium.

With such a construction in which a heat-resisting electrode is disposed between the protective plate and the transducer, and integrally brazed thereonto, the bonding strength is increased, the permissible service temperature range of the probe greatly increased in the directions of both high temperature and low temperature, and the reliability is improved.

Also, since a material that has an acoustic impedance (namely, the product of the velocity of sound and the density) which is very much closer to that of the transducer, as compared with adhesives whose acoustic impedance differs greatly therefrom, is used for brazing, the ultrasonic properties, such as sensitivity of flaw detection, are also improved.

Moreover, since such a construction admits of free motion for the protective plate as attached or coupled to the object to be tested by a coupling medium, it is suited to mobile flaw detection.

On the other hand, the first and third objects referred to above are achieved by an embodiment in which a probe is constituted by sandwiching a transducer between a pair of electrodes, and these three elements being unified into an integral body on a metallic body which is attached directly to the object to be tested. Since the metallic body is directly attached to the object to be tested, thus fixing the probe thereto, this embodiment is suitable for stationary mode flaw detection.

The invention will now be further described with reference to Figures 1 to 4 of the drawings, in which: Figure 1 is a schematic side-sectional view of a first preferred embodiment of the invention; Figure 2 is a schematic side-sectional view of another preferred embodiment; Figure 3 is a partial schematic side-sectional view showing a modification of the embodiment of Figure 2; and Figure 4 is a schematic side-sectional view of yet another preferred embodiment.

Referring to Figure 1, an object to be tested for flaws is indicated at 1; it is a high temperature structure, such as nuclear reactor pressure vessel. To the object 1 to be tested, a probe 2 of an ultrasonic flaw detector 10 is applied so as to detect deterioration of or attack on the object 1, the manner of operation of the ultrasonic flaw detector 10 being such that it sends out ultrasonic waves from the probe 2 into the object 1 and converts the sound pressures of echos which vary depending on the size of flaws, into electrical signals, which are duly displayed in terms of voltage on a cathode ray tube.

In the preferred embodiment illustrated in Figure 1, the contact or coupling of the probe 2 to the object 1 is effected through a coupling medium 3, which may, for example, be silicone oil, or molten Pb, or can even be liquid Na, if the object 1 is a fast breeder reactor. In the case of silicone oil, it may be painted on the object's surface and in the case of liquid Na the reactor may be immersed in liquid Na.

The probe 2 comprises a transducer 5, a protective plate 4, which not only comes in contact with the object 1 through the coupling medium 3 but protects the transducer 5 from mechanical and thermal effects, and a pair of electrodes 6a and 6b, which are formed integrally with the transducer 5, and from which a pair of Pt wires 11 extend respectively to connect the probe 2 to the flaw detector 10. Here, the protective plate 4 may be dispensed with if the transducer 5 is sufficiently strong mechanically and thermally.

One important point here is that the electrode 6a of brazing material is provided between the transducer 5 and the protective plate 4 to bond these two by brazing them together and to function as a heat-resisting electrode layer A.

This is achieved in the present preferred embodiment by brazing a protective plate 4 which is made of a ceramic dispersed with TiC and/or TiN, i.e. a Ti type cermet, and a transducer 4, which is a single crystal of lithium niobate (LiNbO3) having an excellent heat resistivity (900N1,2000C), into a unified integral body, the layer of the brazing material serving as the electrode 6a, i.e. the heat resisting electrode layer A.

For this purpose, a brazing material of an Al-Si-Mg alloy (Si: 11.0 rv 13.0%, Fe: 0.8% or less, Cu: 0.25% or less, Mn: 0.10% or less, Mg: 1.0 ~ 2.0%) is used. This brazing material is a material that, by reducing the oxygen in the transducer 5 by means of its Al and Mg contents, has made brazing of ceramic to metal possible for the first time. The purpose of Si in this brazing material is to lower the melting point, and for this purpose may be replaced by Sn or other similar elements.

Moreover, the other electrode 6b is made from the same brazing material, and Pt wires 11, 11, are also attached to the electrodes 6a and 6b with the same material.

Experiments have shown that a heat-resisting electrode layer A brazed in an Ar atmosphere at a temperature of 5800C, and held for 30 minutes thereat, and under a pressure of 0.02 kgf/mm2 (19 x 10 Pa), is capable of raising the permissible service 0 temperature of the probe 2 to as high as 55Or##'600 C. Also in a durability test, which has been continuously conducted for about one year in a furnace held at 3350C, no anomalies, such as separation, have as yet been detected.

In the instance described above, the example was taken from high temperature structures such as a nuclear reactor pressure vessel, as the test object 1 but, owing to its great bonding strength, the present probe 2 may be applied to low temperature structures, such as liquid nitrogen (LNG) tanks; and even more, there is an advantage in applying it to LNG tanks of using liquid nitrogen itself as the coupling medium 3.

This has been verified by an experiment conducted in liquid nitrogen (- 1960C), in which no separation had taken place.

Also it is quite feasible, owing to the high bonding strength mentioned above, to apply the probe 2 of the invention to space structures, such as a space station, whose temperatures vary inordinately between that when subjected to direct irradiation from the sun and when it comes into shadow.

In the application of the invention as described above, silver-alloy brazing was also tried. In the construction with a single crystal or lithium or niobate (NiNbO3) as the transducer 5, and with a cermet plate as the protective plate 4, the bonding surfaces of the transducer 5 and the plate 4 were provided with a thin film of Cu or Ni, formed thereat by ion-plating, and then were subjected to a heat treatment at 8000C for 1 hour so as to strengthen the bonding of the thin films thus formed to the transducer 5 and to the protective plate 4. Subsequently, brazing was conducted in an Ar atmosphere at 6800C for 15 minutes at 2 4 0.02 kgf/mm2 (19 x 104 Pa) with a silver-alloy (Ag: 45%, Cu: 16%, Cd: 24%, Zn: substantially the remainder) as the brazing material.

This method also produced a probe 2 which was quite satisfactory both at high temperatures and at low temperatures.

Thus, by forming a heat-resisting electrode layer A of, for example, an Al-based alloy, an Ag-based alloy or a Pb-based alloy, as the electrode 6a between the transducer 5 and the protective plate 4, with 6a, 5 and 4 being brazed together, the upper and the lower limits of permissible service temperature of the probe 2 can be greatly raised and lowered, respectively, widening the range of the application of the probe 2.

Figures 2 and 3 show other preferred embodiments of this invention.

In the preferred embodiment illustrated in Figure 2, there is provided a metal cylinder 7 through which the ultrasonic waves can propagate. A heat-resisting electrode layer A is formed between the metal cylinder 7 and the transducer 5 by brazing with the electrode 6a the two elements 5 and 7 into a unified integral body. Another electrode 6b is brazed onto the other side of the transducer 5.

Here, the conditions of brazing together with metal part 7 and the transducer 5 are much the same as in the previous examples, except that, when silver-alloy brazing is used, the ion-plating on the metal part 7 may be dispensed with.

As for the metal part 7, a metallic material that is amenable to bonding to the test object 1 and possesses an acoustic impedance that is approximately the same as that of the test object 1 is selected. For example, when the test object 1 is a nuclear reactor pressure vessel, which is in general made from steels, mild steel may be used for the metal part 7. For another example, when the object 1 is a space station, which is constructed from various metallic materials, a metal that is suited to the particular part should be selected therefor.

The metal part 7 is welded to the test object 1 by a weld 8, for which arc welding is preferred, although other welding methods, such as stud welding, are also feasible.

Moreover, inasmuch as the metal part 7 is used for the sole reason that direct welding of the transducer 5, which is itself a ceramic, onto the test object 1 is difficult, what is required here is that a metal part 7 be present for the sake of convenience of welding. The shape of the metal part 7 is therefore not limited to that of a cylinder. Also, although a long piece is preferably used for the metal part 7 because of its function of relieving thermal stresses by radiation, this is not an essential requirement.

In another application, the transducer 5 is brazed to the metal attachment 7 beforehand, and the cylinder 7 together with the transducer 5 is welded to the test object 1 at the construction site so as to enable the object to be monitored. For example, the probe 2 of this invention may well be used for monitoring the changes in the bearing wall thickness of an LNG pump as immersed in an LNG tank so as to detect anomalous rotation of the pump. As another example, the probe 2 of the invention may equally be used for monitoring the changes in the wall thickness in a high temperature structure at portions that are subjected to severe wear.

Figure 3 shows a further modification of the embodiment of Figure 2, in which a cermet plate 12 is disposed between the transducer 5 and the metal part 7, which is made of mild steel, and the bondings between the cermet plate 12 and the transducer 5, and between the cermet plate 12 and the metal cylinder 7 are made by brazing at A and B of, for example, the aforementioned Al-base heat-resisting brazing material, respectively. Owing to this construction, in which the appreciable difference in the coefficient of thermal expansion existing between the mild steel constituting the metal part 7 and the lithium niobate transducer 5 is effectively alleviated by causing a cermet, which is a substance that is intermediate between metals and ceramics, to intervene between the two parts so that bonding therebetween is ensured.

Although the normal scan probe has been referred to in the foregoing examples, an angle probe can be used instead. For example, in a case similar to that illustrated in Figure 2, the metal material may be attached obliquely to the object 1 so that the ultrasonic waves are introduced obliquely into the object 1 during monitoring of the magnitude of flaws, such as cracks. Figure 4 illustrates another example. In this case, a wedge 9 is provided between the probe 2 and the object 1 so as obliquely to attach the probe 2 to the object 1.

Finally, even though brazing was the sole method of bonding referred to in the aforegoing examples, soldering may be used equally well, if and when the intended service temperature admits of the use of such material.

Claims (11)

1. A probe for an ultrasonic flaw detector comprising a transducer sandwiched between a pair of electrodes and a member acoustically coupled or adapted to be acoustically coupled to an object to be tested, said transducer and said member being welded or brazed together to form a unified integral body by means of a heat-resisting brazing or soldering material disposed between the transducer and the member and serving as one of said electrodes.

2. A probe as claimed in Claim 1, wherein said member is a protective plate brazed or welded to said transducer at one side and adapted to be slidably attached to the object to be tested through an acoustic coupling medium at the other side.

3. A probe as claimed in Claim 1, wherein said member is a metallic body directly attached to the object to be tested.

4. A probe as claimed in any one of Claims 1 to 3, wherein said heat-resisting brazing or soldering material is an Al-based, Ag-based, or Pb-based alloy.

5. A probe as claimed in Claim 4 as dependent on Claim 2, wherein said transducer is made from lithium niobate, said heat-resisting brazing or soldering material is an aluminium alloy containing magnesium, and said protective plate is made of cermet.

6. A probe as claimed in Claim 4 as dependent on Claim 3, wherein said transducer is made from lithium niobate and said heat-resisting brazing material is made from an aluminium alloy containing magnesium.

7. A probe as claimed in Claim 3 or Claim 6, wherein said metallic body is attached to the object to be tested by welding and is made of a metal having an acoustic impedance that is close to or the same as that of the material constituting said object to be tested.

8. A probe as claimed in Claim 7, wherein said metallic body is made from steel and wherein a plate made from cermet is disposed between said steel and said transducer, said cermet being brazed or soldered to said steel, on the one hand, and to said transducer, on the other hand, each with said heat-resisting brazing or soldering material to form a unified integral body.

9. A probe as claimed in Claim 1, wherein said member is constituted by the heat-resisting brazing or soldering material constituting said one electrode.

10. A probe for an ultrasonic flaw detector substantially as hereinbefore described with reference to any of Figures 1 to 4 of the drawings.

11. An ultrasonic flaw detector including a probe as claimed in any one of Claims 1 to 9.