GB2156982A

GB2156982A - Ultrasonic testing of metallurgical bond

Info

Publication number: GB2156982A
Application number: GB08412437A
Authority: GB
Inventors: Ian Cochran Campsie; Brian Edward Southern; Jack Smethurst
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1984-04-06
Filing date: 1984-05-16
Publication date: 1985-10-16
Also published as: GB8408900D0; GB8412437D0; GB2156982B

Abstract

The presence or absence of a metallurgical bond between a tube (21) and an internal sleeve (22) is tested by a probe (36) having a source (37) of ultrasound, a mirror (38) for directing the ultrasound laterally and for directing echo signals to transducer (37) which provides an output signal to a visual display (60a) or a chart recorder (60) driven by stepper motor (44B) in synchronism with rotary and axial drives (44a, 43) which give a helical scan. The bond may be brazing (26). The echo detector (58) may provide two variable time gates, one for an echo from the bond and one for a back wall echo from the interface of tube (21) to discriminate the echo signals. <IMAGE>

Description

SPECIFICATIONS Improvements in or relating to methods and apparatus for ultrasonic testing.

This invention relates to methods and apparatus for ultrasonic testing and in particular the testing of metallurgical bonds between a tube and an internal sleeve.

According to one aspect of the invention a method of testing for a metallurgical bond between a tube and an internal sleeve comprises directing ultrasound to the bond region from inside the sleeve, and using sensing means to sense souind reflected from the region.

The metallurgical bond may be brazing.

If there is a good (effective) metallurgical bond there is no reflected sound from the region.

The method may comprise discriminating the reflected sound from the region.

The discriminating may be by gating an output signal from the sensing means. There may be a first gate for a selected wave from the region and a second, later, gate for a back wall echo.

According to another aspect of the invention apparatus for testing for a metallurgical bond between a tube and an internal sleeve comprise a probe for insertion into the tube, the probe comprising a source of ultrasound, and a mirror for reflecting the ultrasound laterally of the sleeve axis.

The apparatus may comprise sensing means for sensing sound reflected from the bond.

The sensing means may produce an output signal, and means for discriminating the output signal to select sound reflected from the bond.

The discriminating means may comprise means for providing a first gate and a second, later, gate for the signal.

Means may be provided for adjusting the width of each gate and for adjusting the separation between the gates.

The apparatus may comprise means for giving a visual display of the signal.

Means may be provided for rotating the probe and moving the probe axially. Said means may comprise stepper motors. The invention may be performed in various ways and one specific embodiment with possible modifications will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is an axial section through part of a heat exchanger; Figure 2 is a schematic showing of test apparatus; Figures 3, 5c and 7c are part sections of tube/sleeve showing ultrasonic signals; Figures 4a and 4b illustrate signals and signal records; Figures 5a and 5c illustrate signals and signal records; Figures 6A to 6D show various braze conditions and ultrasonic signals; Figures 7and 7b illustrate signals and signal records; Figure 8 illustrates equipment; and Figure 9 is a block diagram of control equipment.

Figure 1 shows part of a tube-in-shell heat exchanger which has at least one tube plate 20 into which laterally spaced open-ended tubes 21 (only one shown) are welded at 23. In use, a first fluid is passed through the tubes 21 whilst a second fluid is passed through the shell to effect heat transfer between the fluids. If a leak develops in the tube at 23, it can be repaired by inserting a tubular sleeve 22 of approximately equal thickness to the tube 21 into the defective tube 21 and sealably bonding the end regions of the sleeve to the tube and to the tube plate to bridge the defective weld 23.

Such a heat exchanger may be used in the evaporator section of a nuclear fast reactor, for example with the first fluid being water emperature brazing, the brazing material being in annular grooves 25 in the sleeve between lands 24. The upper end 27 of the sleeve is explosively welded to the tube plate 20 by detonating a charge of explosive material disposed within the sleeve 22 to expand the sleeve radially into violent abutment with surface 21a of the bore in the tube plate thereby metallurgically bonding the sleeve to the tube plate.

Further details of these procedures are described in British Patent Specification 2032559.

It is desirable to test the effectiveness of the braze connection in the lower region 26 and the invention is concerned with this.

Apparatus 30, Figure 2, is for ultrasonic testing of the effectiveness or integrity of the braze, ie testing for imperfect bonding of the braze material to the sleeve and to the tube.

The apparatus 30 comprises an elongate tube 31 for extending axially into the sleeve 22 and having mounted on it an upper steady 32 for engaging the upper surface 33 of the tube plate 20. A probe 34 is carried on the lower end of the tube 31 and the extent to which the probe is inserted into the tube 21 is determined by the position, which is axially adjustable, of a support 42 on the tube 31. An annular lower steady 35 is engageable in the sleeve and with the upper steady serves to limit any eccentric movement of the tube 31. The steadies may be made from PTFE or NYLON.The probe 34 comprises a hollow cylindrical shroud 36 closed at its ends and containing an axially mounted short pulse ultrasonic transducer 37 below which is a plane beam-focussing mirror 38 at an angle to the tube axis so as to reflect the sound beam sideways generally at right angles to the sleeve axis through a window 39 in the shroud 36 and towards the braze region 26. The resolution, ie the area of the inner surface of the sleeve engaged by the focussed ultrasonic beam 70, may, for example be 1 mum square. In operation the probe is inserted axially to the desired position opposite to the bottom of the braze region and is then rotated steadily in one sense with the transducer energised and signals reflected from the braze region are received and recorded for examination.

The tube 31 is connected at its upper end by a flexible coupling 40 to a shaft 41 which extends through the support 42 to a rotary drive unit stepper motor 44a for rotating the tube 31 through gearing 45a. An up and down drive unit stepper motor 43 is mounted on the support 42 and can rotate gearing ed by structure 48 adjustably held to the plate 20 by a magnetic clamping 49.

The stepper motors 43,44A are powered respectively by stepper motor drive units 50 through connections 51, 52 under the automatic control of unit 53 with manually operable control means shown schematically at 54. The gearing 45, 45A may be such that there are 200 steps for one 360" rotation. The gearing 45, 45A may be chosen to achieve an optimised helical scanning speed for the testing operation.

Variations in the relationship between the probe rotational speed and its pitch can be obtained by separately adjusting oscillators 50a in the drive units 50. An incremental scanning mode can be provided by sequencing the stepper motor drive units 50 so that the probe rotates one revolution for each increment of pitch or each pitch of axial movement.

The motors 43, 44A operate simultaneously so that a helical scan over the whole of the braze region 26 is effected; the speed of movement is a constant average speed.

The transducer 37 is energised by electrical connections 55 extending along the tube 31 and connected to slip rings 56 on shaft 41 which are connected by lines 57 to an ultrasonic test unit 58 which is electrically connected, at 59, to a facsimile recorder 60 and an oscilloscope visual display 60a.

The recorder 60 is driven by a stepper motor 44b through stepper motor drive unit 50 so that the movement of the recorder is synchronized with the rotary and axial testing movement of the transducer 37.

In use a water-tight plug 61 is inserted into the sleeve 22 and an ultrasonic coupling liquid 62, for example water containing a 1% by weight solution of sodium nitrite as a corrosion inhibiter, is poured into the sleeve above the plug or bung 61 for example to a level from 21 cm to 42 cm above the braze region 26. In a reactor the environment of the tube 21 is hot and this tends to cause bubbles to appear in the coupling liquid - the amount of bubbles, which may cause interference with the ultrasonic scanning, is reduced by using the bung 61 which limits the amount of coupling liquid used.

If desired a cylindrical brush can be carefully inserted into the sleeve to gently brush against the sleeve bore, down to just beyond the braze region, and then carefully withdrawn; this assists in removing unwanted bubbles.

The probe is now lowered into the sleeve 22, by lowering support 42 on stand 48, to a desired depth.

Control unit 53 is then operated to start the axial drive 43 and energize the transducer 37; on sensing the sleeve chamfer 122 a signal is developed to start simultaneously the rotary drive 44a and the recorder 60 which includes a device for producing a chart of the reflected signals from the braze region 26, the oscilloscope 60a for displaying the rf signals is connected to unit 58.

The test unit 58 will be referred to later and provides interface triggering to offset any eccentricity in the probe rotation, automatic gain control for signal levelling, and echo signal gating and monitoring.

Figure 3 shows a good bond condition. In this case the ultrasonic beam 70 from the transducer produces two reflected beams, beam 71 from the inner face 22a of sleeve 22 and beam 72 from the outer face 21b of tube 21; there is no reflected beam such as 73 from the interface 22b between sleeve 22 and tube 21. Beams 72(a), 72(b), 72(c) are re-reflections between faces 22a and 21b.

Similar indications are shown in Figures 5A to 5C for a non-bond condition, ie signal 73 present. Signals 78 are multiple re-reflections from the non-bond interface 22b ie part of the initial reflected beam from face 22b is reflected back from surface 22a and then again reflected from face 22b, and so on. The corresponding chart record is at 79.

The unit 58 is provided with a gating device so that it only responds to the reflected signal for a predetermined gating period, which is adjustable, starting at a predetermined time (which is also adjustable) after initiation of sensor beam 70. The gating period is set so that a reflected signal is only received if beam 73 is present ie the gating period starts after signal 71 reaches the transducer 37 and ends before signal 72 reaches the transducer 37. This is shown in Figure 4 which illustrates the display at oscilloscope 60a and where the total reflected beam is shown at 74 and the gate period at 76. Signals 71 and 72 are shown. The corresponding record on the recorder chart 60 is shown at 77.

The unit 58 gain and gating controls are set or calibrated using a signal 70 of known amplitude using a braze specimen with a small non-bond, or a drilled hole in the sleeve.

With the above single gate, a disadvantage is that if there is a loss of signal (ie irregularity in surface 22a so that none of signal 70 enters the sleeve 22) or if there is diffusion of the signal at the edges of grooves 25 Figure 1, this produces an indication on the chart which indicates a good bond. To deal with this only the chart areas corresponding to the braze areas between lands 24 but not grooves 25 are considered, but it is preferable to provide a gating system such that all areas which the chart indicates are bonded are in fact bonded. Such an arrangement is illustrated in Figures 6A to 6D. A second gating module is added to the unit 58 to permit a selection of signals from bond and non-bond regions Figs. 7A to 7C.

It will be observed that because the sleeve 22 and the tube 21 are of similar thicknesses the re-reflected signal 78 has a harmonic 78A (Fig. 5) which appears as a response in the same position as signal 72 (Fig.

4). Figs. 6A to 6D illustrate how this phenomen is overcome.

Thus in Figure 6A a non-bond region 80 means there is a signal in both gates, giving a logic of 1,1.

In Figure 6B with an irregularity at the interface there is a signal only in gate 1 (logic 1,0). In Figure 6C (a good bond) there is no signal in gate 1 but a signal in gate 2 (logic 0,1). In Figure 6D (signal loss) there is no signal in either gate (logic 0,0).

The signals from the two gates are fed to a logic summing unit operating as follows: Signal condition Gate 1 Gate 2 Output Chart indication Non-bond-Fig 6A 1 1 0 Black Non-bond-Fig 6B 1 0 0 Black Bond-Fig 6C 0 1 1 White Loss or diffusion of signal-Fig 6D 0 0 0 Black Thus white on the chart means a good bond.

The recorder chart is such that as the probe rotates step-by-step, the chart, which is moved in step, indicates white for a good bond opposite the mirror and black for a non - or unacceptable bond. A microprocessor 87 could be arranged to read the finished chart and indicate whether, say, 80% of the area was white or give a numerical readout of the proportion of chart area which is white, to give an indication of whether the braze as a whole meets requirements. The microprocessor could be used to control the movement of the stepper motors and store the ultrasonic information concerning the braze onto magnetic tape.

To reduce temperature problems, calibration is effected using couplant liquid heated by an immersion heater so as to be at the tube test temperature, and a spare probe is kept pre-heated by immersing in this liquid.

The gate periods are set wide enough to accommodate thinning of the tube/sleeve due to roiling out or eccentricity in the brazed joint with reference to the centre axis of the tube. Eccentricity in probe rotation is accommodated by interface gating to the water echo.

The periods of gates 1 and 2, the period between them and the times at whichh wheels in channels 84.

The tank of heated liquid couplant 62 is at 85 and calibration equipment and spare probe are mounted at 86 in the tank 85.

Figure 9 is a biock diagram of the controls. An ultrasonic transmitter/pulse genrator 250 triggers a send/receive unit 251 to emit, say, 5000 pulses per second. The echoes are received by the unit 251, pass to a first amplifier 252, having conventional automatic gain control from unit 253, then to a second amplifier 254 which outputs to a gate delay generator 255 and unit 253. The generator 250 and generator 255 output to the unit 253. The amplifier 254 and generator 255 output to gated detectors 256, 257 which output to a double gating detector 258 which outputs to event controller 53 which outputs to a two pen printer or quantizer 259 and thence to recorder 60. The recorder moves in synchronism with the stepper motors.

The gate delay generator 255 when pulsed by unit 250 is readied so that when it receives as echo signal from amplifier 254 it activates detectors 256, 257 to open and close gates at pre-set or calibrated times. Signals in the gates are detected by the double gating detector 258 to produce the appropriate record on recorder 60.

Claims

1. A method of testing for a metallurgical bond between a tube and an internal sleeve comprising directing ultrasound to the bond region from inside the sleeve, and using sening means to sense sound reflected from the region.

2. A method as claimed in Claim 1, comprising discriminating the reflected sound from the region.

3. A method as claimed in Claim 2, in which the sound is discriminated by gating an output signal from the sensing means.

4. A method as claimed in Claim 3, including providing a first gate for a selected wave from the region, and a second, later, gate for a back wall echo.

5. Apparatus for testing the integrity of a metallurgical bond between a tube and an internal sleeves substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

5. Apparatus for testing for a metallurgical bond between a tube and an internal sleeve comprises a probe for insertion into the tube, the probe comprising a source of ultrasound, and a mirror for reflecting the ultrasound laterally of the sleeve axis.

6. Apparatus as claimed in Claim 5, including means for sensing sound reflected from the bond.

7. Apparatus as claimed in Claim 6, in which the sensing means produces an output signal, and means for discriminating the output signal to select sound reflected from the bond.

8. Apparatus as claimed in Claim 7, in which the discriminating means comprises means for providing a first gate and a second, later, gate.

9. Apparatus as claimed in Claim 8, comprising means for adjusting the width of each gate and for adjusting the separation between the gates.

10. Apparatus as claimed in any of Claims 5 to 9, comprising means for giving a visual display of the signal.

11. Apparatus as claimed in any of Claims 5 to 10, including means for rotating the probe and for moving the probe axially.

12. A method of testing for a metallurgical bond between a tube and an internal sleeve substantially as hereinbefore described.

13. Apparatus for testing for a metallurgical hond between a tube and an internal sleeve substantially as hereinbefore described with reference to and as shown in Figs 2 to 7 and 9 of the accompanying drawings.

Amendments to the claims have been filed, and have the following effect: (b) New or textually amended claims have been filed as follows: CLAIMS

1. A method of testing the integrity of a metallurgical bond between a tube and an internal sleeve comprising directing ultrasound from inside the sleeve towards the bond region, gating the reflected ultrasound to isolate echos emanting from the bond region and the outer periphery of the tube respectively, scanning the ultrasound circumferentially and axially with respect to the sleeve and tube, and producing a display of the scanned area such that each position in the display of the scanned area is identified according to whether or not, for the corresponding position in the scanned area, an echo is isolated from the outer tube periphery without an accompanying echo from the bond region.

2. A method as claimed in Claim 1 including discriminating, at each such position in the scanned area, between the following conditions: (a) the presence of echos from both said bond region and outer periphery; (b) the presence of an echo from said bond region with any accompanying echo from said outer periphery; (c) the presence of an echo from said outer periphery without any accompanying echo from said bond region; and (d) the absence of any echo from both said bond and said outer periphery.

3. Apparatus for testing the integrity of a metallurgical bond between a tube and an internal sleeve comprising an ultrasonic probe assembly for insertion into the sleeve to direct ultrasound towards the bond region, means for rotating the probe and moving it longitudinally so as to scan the ultrasound circumferentially and axially with respect to the sleeve and tube, the probe being arranged to receive the reflected ultrasound, first and second gating means for effecting time discrimination of the reflected ultrasound received by the probe so as to isolate respective echos emanating from the bond region and the outer periphery of the tube, and display means foranying echo being captured by the first gating means.

4. A method of testing the integrity of a metallurgical bond between a tube and an internal sleeve, substantially as hereinbefore described with reference to the accompanying drawings.