Investigation on the limits of ultrasonic analysis methods ...

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Investigation on the limits of ultrasonic analysis methods of dissimilar welds Gerd AHLERS 1 , Bernd GRUHNE 2 , Hans-Jürgen BLEHER 2 , Thomas MÖHLENKAMP 3 , Jens GANSWIND 4 1 E.ON Kernkraft GmbH, Component Technology; Hannover, Germany Phone: +49 511 4304294, Fax: +49 511 4304377; e-mail: [email protected] 2 EnBW Kernkraft GmbH, Kernkraftwerk Neckarwestheim; Neckarwestheim, Germany; E-mail: [email protected], [email protected] 3 RWEpower, Kernkraftwerk Emsland,; Lingen, Germany; E-mail: [email protected] 4 VGB PowerTech e.V., Kernkraftwerke; Essen, Germany; E-mail: [email protected] Abstract NDE indications in primary coolant systems of light water reactors have to be further assessed corresponding to the valid German nuclear safety rules. One kind of this further assessment is to analyse the indication by ultrasonic analysis techniques e. g. SAFT-Analysis (Synthetic Aperture Focusing Technique), Analysis with Phased Array-Probes, Mode Conversion- Technique or possibly ToFD-Technique (Time of Flight Diffraction). The VGB-working group “Nondestructive Testing” initiated a study in order to evaluate the limits of the cited ultrasonic analyses methods with a focus on the application to dissimilar metal welds at components in the primary coolant system. For the evaluation four reference blocks had been selected, which are representative for PWR- and BWR-nuclear plants in operation. The reference blocks were used to test, optimize and assess different ultrasonic analysis methods according to their applicability for ISI. The study focused on indications at the inner wall (partially cladded surface) which were represented by machined longitudinal and transverse notches. The tests were carried out in two parts. The first part was intended to demonstrate the applicability of the searching technique and to find all relevant notches in their actual position relative to the weld. In the second part the suitability of the analysis techniques on these notches has been demonstrated. It was found out that ToFD-Technique could not be applied successfully on dissimilar metal welds. Therefore no further investigations were carried out with this technique. The presentation shows the results which were obtained during the investigations on the reference blocks. Good results from analysis techniques have been obtained for notches with a depth about 2 mm and deeper if these notches were located and scanned from the austenitic side. Below this limitation there was no difference between tip signal and base foot signal. Limitations for the other material areas were somewhat higher. Notches with a depth of 4 mm and above could also be analysed with good results. Keywords: ultrasonic analysis technique, SAFT-Analysis (SAFT), Phase Array technique (PA), Mode Conversion (MC), Time of Flight Diffraction (ToFD), dissimilar weld, search technique, analyse technique 11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic

Transcript of Investigation on the limits of ultrasonic analysis methods ...

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Investigation on the limits of ultrasonic analysis methods of

dissimilar welds Gerd AHLERS 1, Bernd GRUHNE 2, Hans-Jürgen BLEHER 2, Thomas MÖHLENKAMP 3,

Jens GANSWIND 4

1 E.ON Kernkraft GmbH, Component Technology; Hannover, Germany

Phone: +49 511 4304294, Fax: +49 511 4304377; e-mail: [email protected] 2 EnBW Kernkraft GmbH, Kernkraftwerk Neckarwestheim; Neckarwestheim, Germany;

E-mail: [email protected], [email protected] 3 RWEpower, Kernkraftwerk Emsland,; Lingen, Germany; E-mail: [email protected]

4 VGB PowerTech e.V., Kernkraftwerke; Essen, Germany; E-mail: [email protected]

Abstract

NDE indications in primary coolant systems of light water reactors have to be further assessed corresponding to the valid German nuclear safety rules. One kind of this further assessment is to analyse the indication by ultrasonic analysis techniques e. g. SAFT-Analysis (Synthetic Aperture Focusing Technique), Analysis with Phased Array-Probes, Mode Conversion-Technique or possibly ToFD-Technique (Time of Flight Diffraction). The VGB-working group “Nondestructive Testing” initiated a study in order to evaluate the limits of the cited ultrasonic analyses methods with a focus on the application to dissimilar metal welds at components in the primary coolant system. For the evaluation four reference blocks had been selected, which are representative for PWR- and BWR-nuclear plants in operation. The reference blocks were used to test, optimize and assess different ultrasonic analysis methods according to their applicability for ISI. The study focused on indications at the inner wall (partially cladded surface) which were represented by machined longitudinal and transverse notches. The tests were carried out in two parts. The first part was intended to demonstrate the applicability of the searching technique and to find all relevant notches in their actual position relative to the weld. In the second part the suitability of the analysis techniques on these notches has been demonstrated. It was found out that ToFD-Technique could not be applied successfully on dissimilar metal welds. Therefore no further investigations were carried out with this technique. The presentation shows the results which were obtained during the investigations on the reference blocks. Good results from analysis techniques have been obtained for notches with a depth about 2 mm and deeper if these notches were located and scanned from the austenitic side. Below this limitation there was no difference between tip signal and base foot signal. Limitations for the other material areas were somewhat higher. Notches with a depth of 4 mm and above could also be analysed with good results. Keywords: ultrasonic analysis technique, SAFT-Analysis (SAFT), Phase Array technique (PA), Mode Conversion (MC), Time of Flight Diffraction (ToFD), dissimilar weld, search technique, analyse technique

11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic

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Content

Abstract ...................................................................................................................................... 1

Content ....................................................................................................................................... 2

1. Introduction ........................................................................................................................... 3

2. Motivation ............................................................................................................................. 3

3. Object of the Investigation .................................................................................................... 3

4. Participants ............................................................................................................................ 4

5. Reference blocks ................................................................................................................... 5

6. Performance .......................................................................................................................... 6

7. Results ................................................................................................................................... 8

8. Conclusion ........................................................................................................................... 16

Abbreviations ........................................................................................................................... 18

References ................................................................................................................................ 18

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1. Introduction

Components of the primary system are tested periodically by NDT. Requirements like the extend of testing, the testing interval and the testing methods are described in the appendant German Nuclear Safety Standard (KTA-code) KTA 3201.4 “In-service Inspection and Operational Monitoring” [1]. The KTA-Codes are updated continuously, the last update of KTA 3201.4 started in 2005 and ended in 2010 with issue 11.2010. An English version is available.

2. Motivation

In the course of the revision of KTA 3201.4 requirements on Ultrasonic Testing (UT) were updated and extended. Thus, for all UT techniques, a confirmation of suitability referred to chapter 4.2.3.3.3 in [1] is required and the requirements of table 4-1 have to be fulfilled. Due to national and international events on dissimilar welds (e. g. from NPP “Biblis” 2001, “Wolf Creek” 2008) new requirements for UT-testing on dissimilar welds were added to this KTA-code. These requirements mentioned above are valid for ferritic welds just as well for the inspection of difficult to test materials like austenitic or dissimilar welds. Especially for the inspection of dissimilar welds the requirements of KTA 3201.4 could not be fulfilled just by one UT technique but it is useful to have a combination of several techniques in use. A reasonable combination will be given by longitudinal and transversal waves. One further requirements of KTA 3201.4 is to do an analysis in case of first detection or growth of an existing indication (chap. 8.2.1 (6) in [1]) to gain more detailed information about its type, location and size (primarily depth). This is a new requirement for the inspection of dissimilar welds and the former KTA-code. With KTA 3201.4 issue 11.2010, suitable analysis methods are to hold available for dissimilar welds as well.

3. Object of the Investigation

According to the new requirements of KTA 3201.4 the VGB-Working Group “Nondestructive Testing” (VGB-WG “NDT”) decided to start an investigation on ultrasonic analysis techniques in 2010. Object of the investigation was the confirmation of the capability of single ultrasonic analysis methods to characterize defined test reflectors focused on their depth inside of selected dissimilar welds reference blocks (VK) in two beam directions (longitudinal LF and circumferential QF) and also to demonstrate the limits of these selected analysis methods. For the investigation the Phased Array-Technique (PA), Synthetic-Aperture-Focus-Technique (SAFT), Wave Mode Conversion (MC) and if applicable Time of Flight Diffraction (ToFD) were selected as analysis methods. The VGB-WG “NDT” selected four dissimilar weld reference blocks (VK 1 to VK 4) with different weld composition and geometrical dimensions (see fig. 5-1). These represent a wide range of dissimilar welds used in nuclear power plants in Germany and Switzerland due to the Siemens/KWU-design. One side of base material is stainless steel (austenite), the other one is carbon steel (Ferritic) with an austenitic cladding on the inner surface with a thickness of about 5-6 mm. A detailed description of the reference blocks is given in chapter 5.

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At first the participants of the investigation had to find the defined test reflectors (EDM-notches) with qualified ISI-Techniques (Search-Technique) on all material boundaries in both beam directions for longitudinal and circumferential inhomogeneities and to give a description of their location on all four reference blocks. KTA 3201.4 describes in its figure 4-2 the beam direction for longitudinal and in figure 4-3 for transverse examination of dissimilar welds (see figure 3-1). The results of measurements were documented in interim reports which were discussed between VGB members and by single participant of the study realtime to measurements. The documentation includes plots from characteristic notch-lines (as possible) and signal-noise-level.

Figure 3-1: Location of notches and beam directions for examination of dissimilar welds

(KTA 3201.4, fig. 4-2 c and fig. 4-3 [1])

a) Longitudinal defects

b) Transverse defects

The second step comprised a pre-investigation to select practical single ultrasonic techniques for selected analysis methods. The selection had to been justified (e. g. by references, existing qualification or simulation). With these analysis techniques, measurements on all four reference blocks were executed. These results as well were documented in interim reports and discussed between VGB members and the participating firms realtime to measurements. After all the over-all results were written in a final report.

4. Participants

The investigation was initiated by VGB members. Plant operators from EnBW, E.ON and RWE from Germany and Gösgen-Däniken AG from Switzerland attended the investigation during project time (executive board). For the investigation it was necessary to do measuements by mechanised ultrasonic equipment. The measurements were executed by six service suppliers from Germany and the Netherlands which are already in operation with NDE on nuclear power plants as illustrated in figure 4-1.

a) b)

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Figure 4-1: Participants of investigation

The investigation was planned for a period of two years starting in 2011. However, due to the order situation of service suppliers and unplanned events during shut down in some NPP, it was extended for another year.

5. Reference blocks

For dissimilar welds there exist many different weld compositions and geometrical dimensions. To cover a wide range of them, four reference blocks from the four executive plant operators were selected (VK 1 to VK 4). They differ concerning the buffering thickness and angle of fusion line between buffering and ferritic base material and also in wall thickness of the reference blocks themselves. Table 5-1 shows the base material and weld material of selected reference blocks. In Figure 5-1 an overview on the selected reference blocks used in the investigation is illustrated. Reference

block

Ferritic base

material

Austenitic base

material

Weld,

Buffering

Cladding

VK 1 15 NiCuMoNb5 (1.6368)

X6 CrNiNb18 10 (1.4553)

NiCr82 (2.4806)

X5 CrNiNb19 9 (1.4551)

VK 2 22 NiMoCr3 7 (1.6751)

X6 CrNiNb18 10 (1.4553)

NiCr82 (2.4806)

X5 CrNiNb19 9 (1.4551)

VK 3 20 MnMoNi5 5 (1.6310)

X6 CrNiNb18 10 (1.4550)

NiCr82 (2.4806)

X6 CrNiNb18 10 (1.4550)

VK 4 15Mo3 (1.5415)

X5 CrNiNb19 9 (1.4551)

NiCrFe3 (2.4807)

X5 CrNiNb19 9 (1.4551)

Table 5-1: Materials of reference blocks

Westing-

house

Intellige

NDT

Vattenfall NRG Cegelec ATUS

VGB WG „ZfP“

WG „UT-Analysetechniken“

VGB E.ON EnBW RWE

KK Gösgen-Däniken

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Figure 5-1: Overview of selected reference blocks

Inside of these blocks are many EDM-notches on the outer and the inner surface, but for this investigation just the inner surface was on interest. Existing notches were used and no additional notches were inserted in. As a consequence to this, characteristic notch-lines are not possible for each material boundary due to number of notches. To fulfil the requirements of KTA 3201.4, there must be at least three to four notches at each material boundary (reference notch, one lower and one deeper than reference notch and in some case a sufficiently deep notch may be used for simulating a through-wall crack). The aim of the investigation was to check the limits of existing analysis techniques and not to do an extensive qualification of the testing techniques. This is the reason why it was not necessary to insert additional notches in according with the requirements of the KTA-code. As demanded in KTA 3201.4 table 4-2 the depth of reference notch depends on the wall thickness of the examined component. For wall thicknesses above 20 mm and less or equal to 40 mm the depth of the reference notch is 2 mm. For wall thickness above 40 mm the depth of reference notch is 3 mm. For the VGB investigation existing notches are used and no more notches are machined additionally.

6. Performance

The ultrasonic test device had to met some general requirements (related to DIN 25435-1 [2]): - UT-tester to send and receive ultrasonic lambs - Usage of standard-probes and/or PA-probes - Pulse-echo system (IE)

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- Direct coupling with water or ultrasound coupling paste - Usage of mechanical operating systems - Operating system with data recording and analysis software All measuring data (search technique and analyse technique) had to be illustrated in BCD-images.

Figure 6-1: Material boundaries: base material (austenitc = AU, ferritic = FE), weld

material (weld = SN, buffering = PU, cladding = PL)

At first all EDM-notches inside the inner wall of reference blocks had to be found and localised in comparison to their actual position (on base material FE or AU, on interfaces between base material and weld material SN, buffering PU, cladding PL), see figure 6-1. Therefore qualified In-Service Inspection (ISI)-techniques were used - and optimized if needed. ISI-techniques contain standard-probes like angle beam probes (e. g. MWK45-2, 45ET2 or 65SEL2) or PA-probes (ET1, SEL2) with 16 or 32 elements. The results of measurements were documented in interim reports and data were illustrated in colored BCD-images (compare to code VGB-R515 [3]). The documentation includes plots from characteristic notch-line (as possible) and signal-noise-level. After having found all notches in their actual positions, the second step could be started. This means, that the found notches had to be analyse. Therefore three analysis methods, which all used crack tip diffraction, were selected for the investigation program: a) Phased Array (PA) b) Synthetic Aperture Focus Technique (SAFT) c) Mode Conversion-techniques (MC). A fourth one, Time of Flight Diffraction (ToFD), was planned, but a pre-study of this analysis method showed no good results on dissimilar welds. The main reason is the sound scattering in combination with sound deflection due to material boundaries. Therefore no more investigations were made to this analysis method. The analysis results were also documented in interim reports and data were illustrated in the same way as for search technique. The depths of notches were measured by the different analysis techniques and compared to a list with known reference depth of notches.

Austenitic Ferritic

Austenitic

PL/FE

PU SN AU

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Figure 6-2 shows an ultrasonic test equipment used from one participant during the investigation. Some assistance was necessary to mount the robot system

Figure 6-2: Example of an ultrasonic test equipment used during investigation;

a) Mounted mechanical UT-equipment on a reference block (on the right side); the

blue colored pipe is just an assistance

b) Ultrasonic test device; operating PC and software in the front

7. Results

In general, all ISI search techniques and selected analysis methods showed, that those reflectors on the inner surface of dissimilar welds could be found. Signal-to-Noise level was high enough for all notches in all material boundaries to make an evaluation. For the search technique no good evaluation of the echo level by just one single technique was possible due to the background echo of the weld materials. Therefore, combinations of several techniques were used. The longitudinal wave mode has a good sound property for coarse-grain materials likes austenite or dissimilar welds. But it shows a bad characteristic notch-line. Thus it is important to use the transversal wave mode as an additional technique. These two UT techniques together in combination and also in combination to characteristic pattern displayed of Mode Conversion or crack tip signal enabling a sure and repeatable evaluation of the results. For the analysis methods it is not possible to make an evaluation on the depth expansion for all EDM-notches especially the notches in the interface SN to PU. For these notches an analysis is just possible if the notches are deeper than 4 mm. For smaller one the sound scattering and sound deflection on the material boundaries have a greater influence so it is not possible to differ. On austenitic base material side an analysis on depth expansion was possible for notches deeper than 2 mm. There is also no differ between tip signal and base signal (main echo) below this limit. 7.1 Search technique

As described in chap. 3 the defined test reflectors (EDM-notches) had to be found with qualified ISI-Techniques on all material boundaries in both beam directions (LF and QF) at

a) b)

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first. For all notches a description of their actual location on all four reference blocks had to be given. This stopover was necessary to do the next step – to analyse (to characterize the depth) the founded reflectors. All participants reached this intermediate target and gave a good description of the EDM-notches located on the inner surface of the reference blocks. The noise-to-signal level was high enough for all search techniques to make an evaluation of the data. In some cases the display of the weld root and also the background noise from the buffer/weld interface was the characteristic signal on which the location of the reflectors could be fixed. 7.1.1 Beam direction from austenitic base material

For this beam direction the test section involves the AU and SN as well as the interface to the buffering (PU) as shown in figure 3-1 a) along with figure 6-1. The relevant notches were found at their actual positions. In some cases there were also seen notches on the ferritic base material side.

Figure 7-1.1: Example for Search Technique 45SEL2 +Y at VK3

Depending on the wave mode and the beam angle a better sound transmission for the weld material will be possible. For a 45° longitudinal wave probe search technique with beam direction from austenitic base side is shown in figure 7-1.1, that all notches in the boundary between SN and PU are visible but not on the ferritic side. In addition to this for a 60° transversal wave probe search technique with same probe frequency and beam direction just the notches on the austenitic base side are visible, see figure 7-1.2. Together with these two techniques the requirements of KTA 3201.4 are performable.

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Figure 7-1.2: Example for Search Technique 60T2 +Y at VK3 7.1.2 Beam direction from ferritic base material

For this beam direction the test section involves the PL/FE and PU as well as the interface to the weld (SN) as shown in figure 3-1 a) along with figure 6-1. The relevant notches were found at their actual positions. In some cases there were also seen notches on the austenitic base material side. With this search technique, a 54° transversal wave probe with 0.5 MHz probe frequency, in figure 7-1.3 is shown, that all relevant notches and also the deeper one on the austenitic base side are visible. In this case this technique can be used as a confirmation of the results for real reflectors in the opposite beam direction.

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Figure 7-1.3: Example for Search Technique 54T0.5 –Y at VK3

7.1.3 Conclusion

The requirements of KTA 3201.4 table 4-1 are valid for ferritic welds just as well for the inspection of difficult to test materials, like austenitic or dissimilar welds. For dissimilar welds it is not possible to fulfill these requirements, especially for table 4-1 case 1, with just one UT-technique. To fulfill the requirements it is useful to have a combination of several techniques, e.g. a combination of longitudinal and transversal waves or with different beam angles. Figure 7-1.4 shows the position of the EDM-notches of VK 2 (test section was the inner surface). Some examples of characteristic notch lines of two 2 MHz-transversal probes with an angle beam of 45° and 60°, recorded at VK 2, are illustrated in figure 7-1.5.

Figure 7-1.4: Notch sketch of VK 2

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Figure 7-1.5: Example of characteristic notch line of VK 2 inner surface (notches A to

M), blue line = beam direction from austenitic base material

a) Transceiver probe MWK45-N2

b) Transceiver probe MWK60-N2

7.2 Analyse Technique

The deflection of ultrasound from a crack tip backward to the probe is known as crack tip diffraction. These crack tip signals only have a weak echo level compared to all other reflection signals from defective area and from angle mirror because of their non-directional character due to the cylindrical wave. With adequate images crack tip signals from reflectors inside the dissimilar weld could be visible. Because there is a temporal delay between the main echo and the crack tip echo, a statement about the reflector depth could be made. Figure 7-2.1 illustrates this issue.

Figure 7-2.1: Crack tip diffraction in general

7.2.1 Crack tip diffraction with Phased-Array probes

Due to the focusing of the beam bundle of the PA-Technique a high resolution is available as well as a high probability of detection. The focus point is located in the test section on the inner surface. The application of PA-technique differs between the participants. Some use the PA probes with swivel angle, others use it just with fixed angle (comparable to single angle transceiver probe), but all had focused to the test section on the inner surface. In most cases swivel angel were operated for the search technique and to evaluate and analyse the data a fixed angle or a limited swivel angles were used.

a) b)

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Figure 7-2.2: Example for PA-Technique with a 2 MHz SEL-probe and 32x1 elements;

on the right side notch no. 4 (nominal depth 10 mm) is illustrated from the crack tip

diffraction

7.2.2 Crack tip diffraction with SAFT

To analyse with the SAFT analysis method it is necessary to use a defocused sound beam with a wide range sound beam and to acquire data as HF data. Normally the measurements were done with 45°-standard probe with 2 MHz testing frequency (e.g. MWK45-2) as some participating firms had applied. The recorded data were analysed with the SAFT algorithm which gave a reconstruction of the structure of the test section. An example of a SAFT reconstruction is illustrated in figure 7-2.3. In some cases the participating firms applied PA-probes for the data measurement. Therefore it was necessary to defocus the sound beam. This is contraire to the normal using of PA-Probes where the sound beam is focused on the test section to achieve a high resolution (see also chap. 7.2.1). It is not reasonable to analyse data collected with a focused PA-technique, because the SAFT algorithm had nothing to focus on. The results from the SAFT analysis were comparable with these from the PA-Technique compared with crack tip diffraction. The measured depths were just as well and figure 7-2.4 illustrates an example of the comparison.

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Figure 7-2.3: Example of SAFT on notch no. 16 (nominal depth 7 mm) on VK 3 with

PA-Probe (45EL-32x1 2MHz); beam direction from ferritic side; measured depth is

7.8 mm

Figure 7-2.4: Comparison between original data measured with defocused PA-probe

and SAFT analysis of notch no. 4 on VK 3; beam direction from austenitic side;

measured deepness is comparable (approx. 10 mm).

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7.2.3 Crack tip diffraction with Mode Conversion-technique

To generate a mode conversion it is necessary to apply a longitudinal transceiver probe which also sends out a sheer transversal wave of about 33° beam angle. Normally a 70° or 65° SEL-probe is in operation. In addition to the main echo typical spurious echo and corresponding effects are displayed. The spurious echo could be differ in spurious echo 1 (NE 1) and spurious echo 2 (NE 2), see fig. 7-2.5. Spurious echo 1 resulted from a reflection of the longitudinal wave, which was converted from the transversal wave on the inner surface, on the defective area of a deep reflector. To receive this NE 1 a defective area reflector with a depth greater than 1 mm is necessary. Below this limitation there will be no NE 1 to receive. For the base material side a clear characteristic notch line was visible for reflectors deeper than 1 mm. But for the weld material zone and due to the geometry of the reference blocks no suitable characteristic notch lines were received. The spurious echo 2 resulted from the creep wave which was converted from the sheer transversal wave on the inner surface. This NE 2 is in interaction with reflectors which were in contact with the inner surface and which were displayed with high amplitude.

Figure 7-2.5: Mode Conversion: main echo HE (blue), spurious echos NE 1 (black) and

NE 2 (red)

The Mode Conversion technique was just applicable from the austenitic base material side because of the interface of the cladding on the ferritic base material side no mode conversion was feasible. Two general statements could be given:

- If spurious echo 1 is displayed than there is a reflector with a depth above a critical deepness and with a specific defective area. Otherwise there is no deep reflector.

- If spurious echo 2 is displayed than there is a reflector with contact to the inner surface. Otherwise there will be a ligament between reflector and inner surface.

With these two statements it is possible to make an estimate about the depth of a reflector. For this investigation it was possible to differ between reflectors with a defective area which traced back to reflector depths above 3-4 mm and reflectors blow this limitation due to the existing or missing of the spurious echo 1 and 2.

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Figure 7-2.6: Example for Mode Conversion on VK 1;

1: spurious echos NE1 (trans – creep wave)

2: spurious echos NE2 (trans – long - long wave)

3: main echo (long wave from 60°)

4: crack tip echo

7.2.4 Time of Flight Diffraction Technique

A described already in chapter 6 for using the ToFD technique as an analysis method, it is essential to know about the basics of this technique. The application of ToFD is associated with a simple geometry with coplanar surfaces like plates or tubes and with low-alloy steel. The biggest disadvantage for this technique is the sound diffraction due to the sound scattering [4]. It is also necessary to use a high test frequency (normally 5 MHz and above) to get a divergent sound beam and as well a high resolution. Just this high test frequency is not useful for testing coarse-grain materials likes austenite or dissimilar welds. To test coarse-grain materials, a low test frequency below 2 MHz is to apply. The reasonable low test frequency is contrary to the aspired high resolution. A further reason why ToFD cannot be applied reasonably to apply on dissimilar welds is the sound scattering in combination with sound diffraction due to the interfaces between the different materials (FE to PU, PU to SN and SN to AU) and the different sound velocities. There is no directly sound path from the transmitter to the receiver which is necessary for a meaningful analysis. Pre-studies of this analysis method showed no good results for dissimilar welds especially to the cone-shaped geometry of the reference blocks respectively components to test. Therefore no more investigations were made with this analysis method.

8. Conclusion

The choice of analysis methods for depth measurements had to be adapted on the specific testing event. There is no approach which could cover or be a solution for all uncertain events. As shown in this investigation there are many methods available to make a depth analysis on foundings, which are more or less comparable due to the measurements results. In general it is to consider, that there had to be used more than one ultrasonic technique (especially for the search technique).

1 2

3

4

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The depth resolution power on dissimilar welds depends on the beam direction, the beam angle, the test frequency and the wave mode. To get a high depth resolution it will be helpful to use a low test frequency not lower than 0.5 MHz and not above 2 MHz. Due to the sound scattering and the sound diffraction on the weld interfaces it was not possible to get a better depth resolution than 4 mm for the test section between SN and PU and also on the cladded ferritic base material side. On the austenitic base material side it was possible to analyse reflector depth above 2 mm. For all investigated analysis methods it will be supportive to make a pattern matching between the reflector display from reference block and the display from the component. As well it will be supportive to use the Mode Conversion as discussed in chapter 7.2.3. In principle the longitudinal main wave from the Mode Conversion technique is provided for the detection of inhomogeneities (search technique). To assign of reflector depth the Mode Conversion with the spurious echo NE 1 and NE 2 is applied as an analysis method. The influence of various materials (base material and weld material), sound scattering, sound diffraction and component geometry must be respected due to the utilisation of Mode Conversion technique. In application of the Mode Conversion technique it is possible to make an estimate about the depth of a reflector. The way to make an analysis is often preset by the practiced search technique. If to use standard probes for the search technique it is obvious to use SAFT as analysis technique. Otherwise if to use phased-array probes as a search technique it is better to make the analysis also with these phased-array probes compared with crack tip diffraction. Last but not least there is the pattern matching which could be used together with all these invested analysis methods particularly with regard to the testing of dissimilar welds and his complexity of material interfaces.

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Abbreviations

NDE Non-destructive examination NDT Non-destructive testing VGB VGB PowerTech e.V. Europäischer Fachverband für Strom und Wärmeerzeugung WG Working group KTA Kerntechnischer Ausschuss PA Phased Array SAFT Synthetic Aperture Focusing Technique MC Mode Conversion ToFD Time of Flight Diffraction ISI In-Service Inspection PWR Pressurized Water Reactor BWR Boiling Water Reactor LF Längsfehler QF Querfehler VK Vergleichskörper

References

[1] KTA 3201.4, german issue 2010-11 Components of the Reactor Coolant Pressure Boundary of Light Water Reactors, Part 4: In-service Inspections and Operational Monitoring [2] DIN 25435-1:2005-09 Wiederkehrende Prüfungen der Komponenten des Primärkreises von

Leichtwasserreaktoren, Teil 1: Mechanisierte Ultraschallprüfung [3] VGB-R 515, Richtlinie Auswertung von Messergebnissen bei mechanisierten Ultraschallprüfungen, VGB

PowerTech GmbH, Erste Ausgabe 2001 [4] ISO 10863 Non-destructive testing of welds, Ultrasonic testing, Use of time-of-flight diffraction

technique (TOFD), 2011-09