Phased Array Ultrasonic Inspection Technique for Cast

PHASED ARRAY ULTRASONIC INSPECTION TECHNIQUE FOR CAST AUSTENITIC STAINLESS STEEL PARTS OF NUCLEAR POWER PLANTS

Setsu Yamamoto Toshiba Corporation

Yokohama, Kanagawa, Japan

Jun Semboshi Toshiba Corporation


Azusa Sugawara Toshiba Corporation


Makoto Ochiai Toshiba Corporation


Kentaro Tsuchihashi Toshiba Corporation


Hiroyuki Adachi Toshiba Corporation


Koji Higuma Toshiba Corporation


ABSTRACT For safety operation of nuclear power plants, soundness

assurance of structures has been strongly required. In order to evaluate properties of inner defects at plant structures quantitatively, non-destructive inspection using ultrasonic testing (UT) has performed an important role for plant maintenances. At nuclear power plants, there are many structures made of cast austenitic stainless steel (e.g. casings, valve gages, pipes and so on). However, UT has not achieved enough accuracy measurement at cast stainless steels due to the noise from large grains. In order to overcome the problem, we have developed comprehensively analyzable phased array ultrasonic testing (PAUT) system.

We have been noticing that dependency of echo intensity from defect is different from grain noises when PAUT conditions (for example, ultrasonic incident angles and focal depths) were continuously changed. Analyzing the tendency of echoes from comprehensive PAUT conditions, defect echoes could be distinguished from the noises. Meanwhile, in order to minimize the inspection time on-site, we have developed the algorithms and the full matrix capture (FMC) data acquisition system. In this paper, the authors confirmed the detectability of the PAUT system applying cast austenitic stainless steel (316 stainless steel) specimens which have sand-blasted surface and 3 slits which made by electric discharge machining (EDM).

INTRODUCTION Recently, phased array ultrasonic testing (PAUT), which can vary its refraction angles and focal points by pulsing array probes separately, has been applied to various parts as a superior alternative technique for conventional single element probe testing. The authors have been applying a PAUT to in-service inspection of nuclear power plants [1-3]. There are many parts difficult to be inspected by UT owing to their complicated surface shapes and noises due to grain of cast stainless steels. Enhancement of the PAUT techniques, e.g. flexible array probe [4,5] and shape adaptive beamforming [6-9], enables inspection of more complex surface parts. On the other hand, inspection of a cast austenitic stainless steel has a problem due to its large grains. It is especially known high intensity noises due to the grain cause an oversight and defect sizing error. In this study, we have been noticing that PAUT conditions dependency of defect echo intensity is different from grain noise intensity. But, tendency analysis requires data under various UT conditions. As a result, the increase of the on-site inspection time is inevitable. In order to overcome the problem, a tendency analysis algorithm was combined with FMC [10] data acquisition.

Proceedings of the 2016 24th International Conference on Nuclear Engineering ICONE24

June 26-30, 2016, Charlotte, North Carolina

ICONE24-60256

1 Copyright © 2016 by ASME

PRINCIPLES Figure 1 shows a flow chart of this PAUT system. Flow

chart was divided into 2 main steps (on-site and off-site). Data acquisition step was conducted on-site. Data analysis step could be conducted off-site. -Data acquisition (On-site)

Ultrasonic wave is transmitted by each element that composes array probe. Reflected ultrasonic wave is received by all elements (from “element 1” through “element n”). Therefore n×n patterns for raw wave acquisition will be acquired. This method has been adapted to an under sodium viewer and 3D-SAFT inspection technique [11, 12].

Probe set and matching confirmationOn-site

Off-site

?N

Raw wave acquisition

Set0

Wave synthesizing

B-scope construction

Echo intensity extraction

False

True

dependency of echo intensity calculation

B-scope image merging ( )

Defect detection and sizing

es ~

Fig.1 Flow chart of this PAUT system

-Data analysis (Off-site) Figure 2 shows schematic diagram of off-site processing.

Fig.2 (a) shows B-Scope construction (from 0 to N ) and

(b) shows B-Scope image merging. Firstly, obtained raw waveform would be synthesized according to the delay times which depend on the PAUT conditions (linear scan of refraction angle ). Secondly, synthesized waveform was used to construct the linear scan B-Scope images. Thirdly, intensities of each echo from B-Scope image were extracted. The steps from wave synthesizing to echo intensity extraction, would be

repeated from 0 to N . Where 0 is an analyze start

angle, N is an analyze end angle and is an increment

pitch of angle. Analyzing the extracted intensity values of each B-Scope at refraction angle , dependency of echo intensities could be calculated.

Defect

0

N

(a) B-Scope image construction (from 0 to N )

es

Merged area

(b) B-Scope image merging (from s to e )

Fig.2 Schematic diagram of off-site processing

Intensity of a noise would show random or continuous change which has no obvious peak. On the other hand, defect echo intensity would indicate obvious peak and change in a continuous pattern. According to dependency of echo intensities, defect echo and noises would be identified. Finally, several B-Scope images would be merged from

s to e .

Where, s is a merging start angle and

e is merging end

angle. Using merged B-Scope image, signal to noise ratio (SNR) and sizing result would be obtained.

Fig.2 (a)

Fig.2 (b)


EXPERIMENTAL SETUP

Figure 3 shows a schematic diagram of the experimental setup. The detector (T-PA256(3D) manufactured by Toshiba) has 256ch pulse generators and receivers. The detector and high-speed data transfer circuit can realize 256x256 parallel wave acquisition and high-speed data transfer from detector to PC (almost 500 MB/s). A 2-MHz, 128-elements, 0.85-mm pitch array probe was adopted. All signal processing and evaluations were conducted by the control PC with 4 GPUs.

Control signal

Waveformdata

PC

Detector(T-PA256(3D))

High-speed data transfer circuit

Array probe

GPU unit

Water

Specimen

Fig.3 Schematic diagram of experimental setup

Figure 4 shows a shape of a specimen. Top view and front view are depicted. The front view corresponds to B-Scope image. In this study, we fabricated 3 type specimens. All specimens were made of cast austenitic stainless steel (316 stainless steel) which has common length (250 mm) and width (120 mm). Only thickness t was changed in each specimen.

t

125

d

250

2020

2020

1010

120

a

b

c

5

[mm]

Top view

Front view

5

Fig.4 Shape of specimen

Specimen1 was t=35 mm, specimen2 was t=51 mm and specimen3 was t=70 mm. Photos of specimen3 are shown in Fig.5. Fig.5 (a) is an overall view of specimen3 and (b) is a tissue observation photo of specimen3. Observation area was surrounded by dotted lines in Fig.5 (a). Almost all grain sizes

(a) Overall view

10 mm

(b) Tissue observation

Fig.5 Photos of specimen3

were less than 10 mm as shown in Fig.5(b). Each specimen has 3 slits (named a,b, and c respectively) which are made by EDM. Detail of each slit property shown in table1. Each slit has a length of common to 20mm. Minimum slit depth is 1.5 mm at each specimen. This slit depth was sufficiently smaller than grain sizes. It would be used for detectability verification. The other slit depths were defined as t/6 and t/3 respectively. Slit a and c are placed on the center of specimen. To prevent the mistake at confirmation, only slit b was placed 5 mm away from specimen center. Measurement configuration, coordinates

Table 1 Property of specimens and slits

NameThickness

t [mm]Slit name

Slit depthsd [mm]

Specimen1

351-a 1.5 1-b 5.8 1-c 11.7

Specimen2

512-a 1.5 2-b 8.5 2-c 17.0

Specimen3

703-a 1.5 3-b 11.7 3-c 23.3

and definition of echoes are depicted in Fig.6. Fig.6 (a) is a measurement configuration and coordinates and (b) is a definition of echoes. Measurement was conducted under


immersion condition. Origin of x axis is defined as the center of probe and z axis is defined as specimen surface. Probe center was moved 55 mm away from specimen center and distance between probe and specimen surface was 45 mm. Bottom echo means reflection wave from specimen back surface, slit tip echo means refraction wave from tip of slit and slit corner echo means reflection wave from corner of slit root and back surface.

CL

45 Water

Specimen

55

x

z

０

Array probe

Slit a,c

Slit b

(a) Measurement configuration and coordinates

Slit tip echo

Slit corner echoBottom echo

(b) Definition of echoes

Fig.6 Measurement configuration, coordinates and definition of echoes

EXPERIMENTAL RESULTS -Detectability verifications

B-Scope images of slit 1-a are shown in Fig.7. Wave synthesizing refraction angle range was 200 , 60N .

Results of 53,50,47,44,41,38 are shown. Slit echo

intensity was increased as was increased (during 47~38 ). Thereafter, slit echo intensity was gradually

decreased. On the other hand, noises are observed continuously.

θ=41゜ θ=44゜

θ=47゜ θ=50゜

θ=38゜

θ=53゜

slit

noise

0

1 Signal intensity [A

rb. Unit]

x

z

Fig.7 B-Scope images of slit 1-a

Analyzing result of dependency of echo intensities is

shown in Fig.8. Red mark shows tendency of slit echo and yellow mark shows tendency of noises respectively. Peak value of slit echo was extracted as signal intensity. Noise intensity was calculated from averaged peak value of the areas which inside of dotted white lines in Fig.7 (5 mm ~ 10 mm depth. Largest grain noise should be appeared in this area). Slit echo intensity indicated obvious peak and change in a continuous pattern. On the other hand, noises shows continuous change but it had no obvious peak. According to these tendencies, slit echo and noise was clearly identified. But it was difficult to identify that the echo showed slit or not from B-Scope image owing to its low SNR.

● Slit echo● Noises

20 25 30 35 40 45 50 55 60

Refraction angle [°]

0.2

0.3

0.4

0.5

0.6

0.7

Ech

o in

tens

ity [

Arb

. U

nit]

Fig.8 Analyzing result of dependency of echo intensities

In order to improve SNR of B-Scope image, image

merging was conducted. Merging range was 42s , 47e .

The center of this range was defined as which has slit echo peak. Merged B-Scope images are shown in Fig.9. Fig.9 (a) shows normal PAUT B-Scope image ( 45 ) and (b) shows merged B-Scope image. SNR of normal PAUT was 1.2, merged one was 4.4. Where, SNR means slit echo intensity divided by noise intensity. Table 2 shows SNR of each specimen and


adapted refraction angles. All results of normal PAUT SNR were less than 2. Conducting image merging, SNR was improved up to 2. From these results, detectability of our PAUT system was verified.

Slit Slit

(a) (b)

0

1 Signal intensity[A

rb. Unit]

x

z

(a) Normal PAUT (b) Merged image

Fig.9 B-Scope images of slit 1-a

-Sizing capability verifications B-Scope images of slit 1-b are shown in Fig.10. Refraction

angle range was 200 , 60N .

Result of 53,50,47,44,41,38 are shown. Same

as slit 1-a echoes, slit tip and corner echo was increased as was increased. Slit corner echo indicate highest intensity at 44 , highest intensity of tip echo was 47 . After that, the intensities of slit coroner and tip echoes were gradually decreased.

Table 2 SNR of each specimen

Slit name

Normal PAUT Merged image

[゜] SNR [゜] SNR

1-a 45 1.2 42~47 4.4

2-a 35 1.4 32~37 6.5

3-a 23 1.2 20~26 2.5

es ~

θ=41゜ θ=44゜

θ=47゜ θ=50゜

θ=38゜

θ=53゜

corner

noise

0

1 Signal intensity [A

rb. Unit]

tip

x

z

Fig.10 B-Scope images of slit 1-b

Analyzing results of dependency of echo intensities are shown in Fig.11. Red mark shows tendency of slit corner echo, Blue mark shows slit tip echo and yellow mark shows tendency of noises respectively. Calculation method of each intensity was similar as Fig.8. Slit corner echo indicated clear peak at

38 (bottom echo was superimposed on slit corner echo during 30~20 ). And slit tip echo indicated almost same tendency. As a small difference, slit tip had an obvious peak around 45 . On the other hand, noises show continuous change without obvious peak.

● Slit corner● Slit tip● Noises

20 25 30 35 40 45 50 55 60

Refraction angle [°]

0.2

0

0.4

0.6

0.8

Ech

o in

tens

ity [

Arb

. U

nit]

Fig.11 Analyzing result of dependency of echo intensities

Merged B-Scope images are shown in Fig.12. Fig.12 (a)

shows normal PAUT B-Scope image ( 39 ), (b) shows merged B-Scope image ( 37s , 42e ). It is difficult to

identify the slit tip echo from normal PAUT B-Scope image owing to its low SNR. On the other hand, slit tip echo was clearly observed in merged image. Using merged image, slit depth sizing was conducted. As a sizing method, highest position of slit corner and tip echoes were adapted as slit corner and tip position. Slit depth was calculated by depth difference between corner and tip. Sizing results are shown in Table 3. Each result indicated that measurement error was less than 2 mm. From this result, this PAUT system achieved high precise sizing in cast austenitic stainless steel.

Slit

(a) (b)

0

1 Signal intensity[A

rb. Unit]

Slitx

z

(a) Normal PAUT (b) Merged image

Fig.12 B-Scope images of slit 1-b


Table 3 Sizing result of each slit

Slit name Nominalslit depth [mm]

Measuredslit depth [mm]

Measurementerror [mm]

1-b 5.8 6.3 +0.5

1-c 11.7 11.9 +0.2

2-b 8.5 8.1 -0.4

2-c 17.0 15.8 -1.2

3-b 11.7 12.1 +0.4

3-c 23.3 22.7 -0.6

CONCLUSION In order to improve the detectability and sizing capability

of UT for cast austenitic stainless steel, we have developed new PAUT system. Analyzing the dependency of echo intensities, this system can identify the defect echo and noises. Merging the B-Scope images, SNR of defect echoes were significantly improved. Using this system, slits in cast austenitic stainless steel were clearly observed and slit sizing error was less than 2 mm. In the future, capability of this system would be verified using actual defects like a fatigue crack or stress corrosion cracking.

.

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Phased Array Ultrasonic Inspection Technique for Cast