study on the ultrasound propagation in cast austenitic...

1 ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

E-Journal of Advanced Maintenance Vol.4 No.1 (2012) 5-25Japan Society of Maintenology

Study on the Ultrasound Propagation in Cast Austenitic Stainless Steel Kazunobu SAKAMOTO1,*, Takashi FURUKAWA2, Ichiro KOMURA2, Yoshinori KAMIYAMA2 and Tsuyoshi MIHARA3

1 Japan Nuclear Energy Safety Organization, 4-1-28, Toranomon, Minato-ku, Tokyo, 105-0001, Japan 2 Japan Power Engineering and Inspection Corporation, 14-1, Benten-cho, Tsurumi-ku, Yokohama, 230-0044, Japan 3University of Toyama, 3190 Gofuku, Toyama-shi, Toyama,930-8555, Japan. ABSTRACT

It is challenging to inspect cast austenitic stainless steel (CASS) piping weld by ultrasonic testing (UT) techniques due to its coarse and anisotropic grain structures resulted from manufacturing process.

To comprehend the unique ultrasound propagation into the CASS piping such as beam skewing, dispersion and unexpected attenuation which gives adverse effect on the UT, characteristics of microstructure and ultrasound propagation into the CASS piping was studied using CASS specimens simulating reactor coolant piping in Japanese Pressurized Water Reactor (PWR) plants. In this study, snapshots of the wave propagation in the CASS are also successfully obtained by visualization techniques and analyzed those results theoretically.

*Corresponding author, E-mail: [email protected]

KEYWORDS

Anisotropic, Cast austenitic stainless steel, CASS, Pressurized Water Reactor, PWR, Ultrasonic testing, UT, visualization

ARTICLE INFORMATION

Article history: Received 28, Nov 2011 Accepted 17, Apr 2012

1. Introduction

It is well known that volumetric inspection by ultrasonic testing (UT) for cast austenitic stainless steel (CASS), which is extensively used in the reactor coolant piping system of Japanese pressurized water reactors (PWR), is challenging due to beam skewing, dispersion and unexpected attenuation by its coarse and anisotropic crystal structures resulted from manufacturing process. The miss or false calling of the flaws and the deterioration of sizing performance are of great concerns in the UT for the material to maintain integrity of the components to the end of life, although no crack has been reported in Japan so far.

To cope with above difficulty, a variety of researches, such as UT technique and probe development, signal processing, study on the grain structures, have been carried out to cope with this problem [1-5]. With regard to the UT probes, large size probes which generate high energy ultrasound and using low frequency probe etc. have been proposed [6]. In Europe, round robin test was performed using CASS weld mockup under the Program for the Inspection of Steel Components (PISC) sponsored by European Communities [7,8]. In addition, a number of simulation models to predict the ultrasound propagation in anisotropic materials have been presented in recent years [9,10]. In these efforts to improve the inspection accuracy for CASS, understanding of the ultrasonic wave propagation characteristic is indispensable. But the precedent which clarified the elastic wave propagation characteristic in detail using equivalent material to actual plant does not yet exist.

Under such circumstances, observation of grain structure, measurement and visualization of the ultrasound propagation in the CASS are performed to understand the unique phenomena such as beam skewing, dispersion and unexpected attenuation precisely, using the CASS welded joint specimens and calibration blocks whose material, size, dimension and welding method are identical to Japanese PWR plants. In addition, this study is intended to provide inspection personnel with the fundamental information of ultrasound propagation in CASS for educational purpose.

K. Sakamoto, et. al Study on the Ultrasound Propagation in Cast Austenitic Stainless Steel

2

2. Materials

Although it is widely known that cast stainless steel used as the primary coolant piping in PWR plants has coarse columnar grain structures, previous studies identified that equiaxed and equiaxed-columnar mixed grain structures also existed in some cases [3].

In this study CASS specimens imitating the primary coolant piping in PWR with columnar grain structures and equiaxed grain structures are applied to understand the unique phenomena causing the difficulty of UT. Table 1 summarizes the specification of each specimen.

Fig. 1 shows the picture of the JTP-1, the CASS welded joint between Centrifugally CASS (CCASS) and Statically CASS (SCASS), whose material, size, dimension and welding method are identical to the most recent primary coolant piping in Japanese PWRs. JTP-1 was used for sound velocity, attenuation and noise measurement and visualization of ultrasound propagation, as well as electron backscatter Pattern (EBSP) analysis.

Fig. 2 shows the picture of the CB-2 as a representative of CASS calibration block, CB-1 through 3 (SA351 Gr.CF8A), applied in this study. Sound portions of those calibration blocks were used for microscopic observation, sound attenuation and noise measurement.

Table 1. Selected specimens used for the study

Fig. 1. JTP-1 Specimen

(b) Macrograph of the axial-radial cross section

(a) Profile of JTP-1

Fig. 2. CB-2 Specimen

(b) Axial-radial cross section

(a) Configuration of Calibration block

CCASS

Weld

SCASS

Drill hole

Weld

See Fig. 2See Fig. 1Note

77.8mm73.0mm73.0mm77.8mmThickness

787.4mm (31inch)736.0mm (29inch)736.0mm (29inch)787.4mm (31inch)ID

JISG 5121 SCS14A

SA351 Gr. CF8MSA351 Gr. CF8MSA351 Gr. CF8MJISG 5121 SCS14AMaterial

EquiaxedEquiaxedColumnarColumnar-ColumnarGrain structure

Calibration blockCalibration blockCalibration blockPipe – Pipe(Piping joint mockup)

Type

CB-3CB-2CB-1JTP-1Specimen

See Fig. 2See Fig. 1Note

77.8mm73.0mm73.0mm77.8mmThickness

787.4mm (31inch)736.0mm (29inch)736.0mm (29inch)787.4mm (31inch)ID

JISG 5121 SCS14A

SA351 Gr. CF8MSA351 Gr. CF8MSA351 Gr. CF8MJISG 5121 SCS14AMaterial

EquiaxedEquiaxedColumnarColumnar-ColumnarGrain structure

Calibration blockCalibration blockCalibration blockPipe – Pipe(Piping joint mockup)

Type

CB-3CB-2CB-1JTP-1Specimen


3

3. Experiments and Discussion 3.1. Metallographic analysis 3.1.1. Microscopic observation of the microstructure

To understand the typical characteristics of CASS microstructures, microscopic views of JTP-1 which are the representative of the columnar grain structure and CB-2 which is the representative of the equiaxed grain structures are obtained as shown in the Fig. 3 and Fig. 4 respectively.

Both results show the ferrite phases, of which size is smaller enough than the wavelength of ultrasound in the material if 1-5 MHz longitudinal probes are applied, precipitate in the majority of austenite base. This implies that the reflection or refraction at the boundaries between ferrite phase and austenite phase can be neglected from the UT point of view.

3.1.2. EBSP analysis

In the course of manufacturing process of CASS, large size grains tend to grow to the specific direction, each of which is elastically anisotropic. Having those grain structures, the ultrasound propagation in the CASS piping used in the nuclear power plant exhibits unique phenomena. It is essential to comprehend the detailed grain structure information of CASS for understanding the characteristics of the ultrasound propagation in the material. For this purpose the texture and crystal orientation of each grain in the CASS specimens removed from JTP-1were measured using EBSP analysis.

The EBSP is the technique in the SEM to obtain the Kikuchi diffraction patterns from the surface of the highly polished test samples, of which fundamental is equal to TEM/Kikuchi diffraction patterns [11]. Using this technique, the crystal orientation maps of the CASS were acquired.

The result of EBSP analysis on axial-radial cross section of JTP-1 is presented in Fig. 5. The color map shows the orientation of each region as observed from the radial direction (normal to the page). The boundary of each color well corresponds to the microstructures observed by optical view. And the crystal orientation was equally distributed in the radial direction in both SCASS and CCASS. In this study, it is presumed that the grain structure of the target material is austenitic steel since

Fig. 3. Microscopic view of the JTP-1 cross section (Columnar grain structure)

Fig. 4. Microscopic view of the CB-2 cross section (Equiaxed grain structure)

100μm 100μm

CCASSSCASS

CCASS SCASS

Ferrite Phase

Ferrite Phase

Austenite Phase

AustenitePhase

100μm

Ferrite Phase

AustenitePhase


4

typical micrographic pictures of CASS piping used in the primary coolant piping of PWR plant shows the fine ferrite phases precipitation in the majority of austenite base (see Fig. 3 & Fig. 4).

For further understanding, the test samples were prepared from both statically and centrifugally casted portions as presented in Fig. 6, whose columnar grains oriented to the pipe outer surface.

Fig. 7 (a) shows the example of crystal mapping of SCASS from the outer surface point of view (radial direction). Fig. 7 (b) and (c) illustrate its pole figures and proportion of austenite to ferrite in the observation area, respectively. Austenite phase is the dominant in the observation surface as shown in Fig. 7 (c), same as the microscopic observation result (see Fig. 3). Based on these characteristics, crystal orientation of the austenite is discussed. As presented in the Fig. 7 (b), [001] direction is strongly aligned in the direction of columnar grain growth which is tilted about 20 degrees from the radial direction with random distribution to the transverse direction. The pole figures of [111] and [101] support this unique crystal orientation of the CASS.

Fig. 8 illustrates the analysis result of CCASS. Comparing to the result of SCASS, each grain at the observation surface is coarse as presented in Fig. 8 (a). Same as the result of SCASS, Fig. 8 (b) shows strong alignment of [001] in the direction of columnar grain growth with random distribution to the transverse direction, although the peak is not so strong since the number of sampled grain is smaller than the one of SCASS.

Fig. 5. EBSP analysis results on JTP-1

Fig. 6. Location of the sample taken from the JTP-1 for the EBSP analysis

SCASS CCASS

50m

m

20mm20mm

Weld

Observed area

(a) Macrograph of observation surface (b) Pattern quality map and the three inverse

pole figure map in the z direction (normal to the page)

SCASS CCASS

20mm 20mm

50

mm

Weld

SCASS CCASS CCASSSCASS

Observation surface

ND

TD

RD

SCASSSample

CCASSSample

111

001 101


5

Fig. 7. Crystal orientation map of SCASS and its pole figures by EBSP

(a) Crystal mapping

(b) Pole figures by intensity distribution Scale levels: number of count

(c) Proportion of austenite (Gamma) to ferrite (Alpha)


6

Fig. 8. Crystal orientation map of CCASS and its pole figures by EBSP

(a) Crystal mapping

(c) Proportion of austenite (Gamma) to ferrite (Alpha)

(b) Pole figures by intensity distribution Scale levels: number of count


7

3.2. Measurement of sound velocity As described previously, the columnar grain

oriented to the pipe outer surface corresponding to the temperature gradient of the solidification process direction. Sound velocity measurements were carried out to obtain the elastic constants, using JTP-1specimen. The block shaped specimens with the thickness of 20mm and 30mm in various directions were prepared from the JTP-1 bulk as presented in the Fig. 9.

The sound velocity was obtained by dividing the propagation time between the first and second bottom echoes emitted from the normal incidence probe by twice of the test piece thickness, referring to JIS Z 2353-1992 of Japanese Industrial Standards [12]. Hence, the sound velocity measured here was phase velocity, not energy flaw velocity. The specification of applied transducers is summarized in Table 2.

Fig.10 presents A-scan of the longitudinal sound velocity measurement in 45 degree direction of CCASS using PANAMETRICS V392 transducer (1.0MHz, 38mm diameter), as an example. The propagation time lag of first bottom echo and second bottom echo was computed by the cross correlation of both waveforms.

Fig. 11 and Fig. 12 show the measurement results, together with the calculated sound velocity curves described below.

Fig. 9 Removal of the specimens for sound velocity measurement

-0.1

0.0

0.1

Voltag

e [

V]

7060504030

time [s]x10-6

First bottom echo

Second bottom echo

Third bottom echo

Fig. 10 A-scan view of sound velocity measurement 45 deg. Direction of CCASS (R45, 20mm thickness)

Longitudinal wave Shear wave

0.5M38N 1M38N 2C20N 5C20N

1Z20SN 2Z20SN

*: Expressed in accordance with JIS Z2350

Table 2. Specification of transducer*


8

(a) Longitudinal (b) SV (c) SH

Fig. 11. Sound velocity relative to the long grain axis for grain in the axial-radial cross section

(a) Longitudinal (b) SV (c) SH Fig. 12. Sound velocity relative to the long grain axis for columnar

grain in the axial-circumferential cross section From the measurement result of the perpendicular to the grain direction, the material is able to be

regarded as a transversely isotropic from the bulk material point of view. With this precondition, the elastic sound velocities in the direction from the long grain axis (parallel to the columnar direction) can be described as a follows in general (see Fig. 13) [13];

2,000

3,000

4,000

5,000

6,000

0 30 60 90

Angle from long grain axis (deg.)

Velo

city

(m/s)

VL(θ)

CCASS_30mm(t)_2MHz

CCASS_30mm(t)_1MHz

CCASS_20mm(t)_5MHz

CCASS_20mm(t)_2MHz

CCASS_20mm(t)_1MHz

CCASS_20mm(t)_0.5MHz

SCASS_30mm(t)_2MHz

SCASS_30mm(t)_1MHz

SCASS_20mm(t)_5MHz

SCASS_20mm(t)_2MHz

SCASS_20mm(t)_1MHz

SCASS_20mm(t)_0.5MHz2,000

3,000

4,000

5,000

6,000

0 30 60 90


Velo

city

(m/s)

VSV(θ)

CCASS_30mm(t)_2MHz

CCASS_30mm(t)_1MHz

CCASS_20mm(t)_2MHz

SCASS_30mm(t)_2MHz

SCASS_30mm(t)_1MHz

SCASS_20mm(t)_2MHz

2,000

3,000

4,000

5,000

6,000

0 30 60 90


Velo

city

(m/s)

VSH(θ)

CCASS_30mm(t)_2MHz

CCASS_30mm(t)_1MHz

CCASS_20mm(t)_2MHz

SCASS_30mm(t)_2MHz

SCASS_30mm(t)_1MHz

SCASS_20mm(t)_2MHz

2,000

3,000

4,000

5,000

6,000

0 30 60 90


Velo

city

(m/s)

VL(θ)

CCASS_20mm(t)_5MHz

CCASS_20mm(t)_2MHz

CCASS_20mm(t)_1MHz

CCASS_20mm(t)_0.5MHz

2,000

3,000

4,000

5,000

6,000

0 30 60 90


Velo

city

(m/s)

VSV(θ)

CCASS_30mm(t)_2MHz

CCASS_30mm(t)_1MHz

CCASS_20mm(t)_2MHz

2,000

3,000

4,000

5,000

6,000

0 30 60 90


Velo

city

(m/s)

VSH(θ)

CCASS_30mm(t)_2MHz

CCASS_30mm(t)_1MHz

CCASS_20mm(t)_2MHz


9

2

cossin)( 44

233

211 MCCC

VL

2

cossin)( 44

233

211 MCCC

VSV

244

266 cossin

)(CC

VSH

2224413

224433

24411 cossin)(4)cos)(sin)(( CCCCCCM

θ: angle from the long grain axis (z axis) ρ: 7.9 g/cm3

Using the sound velocity measurement results, following elastic constants were determined.

C11=265.8*109N/m2 C12=114.0*109N/m2 C13=128.5*109N/m2 C33=215.0*109N/m2 C44=117.1*109N/m2 C66=75.9*109N/m2

Here; C11 was obtained from equation (1) using VL(90°).

11

244114411

2

][)90(

CCCCCVL

C33 is obtained from equation (1) using VL(0°).

33

244334433

2

)]([)0(

CCCCCVL

C44 is obtained from equation (2) using VSV(0°).

44

244334433

2

)]([)0(

CCCCCVSV

C66 is obtained from equation (3) using VSH(90°).

66)90(

CVSH

C12 is obtained from equation (8) entering the C66 value obtained above.

661112 2 CCC

Once C11, C33, C12, and C44 are calculated, C13 can be obtained from equation (2) using the sound velocity other than VSV (0°) or VSV (90°). In this case, VSV (45°) was applied as follows,

X (1) Y (2)

Z (3)

θ

X (1) Y (2)

Z (3)

θ

Columnar Grain

Fig. 13 Axis of crystal system

z

(1)

(2)

(3)

(4)

(6)

(5)

(7)

(8)


10

2

)()2

(21

21

)45(

24413

23311443311 CC

CCCCC

VSV

For the sake of reference, the sound velocities in equiaxed grain structured CASS were also

obtained using CB-2. In this case, sound velocities of longitudinal wave in normal and 45 degrees direction from the OD surface were measured simply. As a result, it is presumed that CB-2 was isotropic since sound velocities of both directions were around 5,800m/s, which were almost identical to the normal stainless steel. 3.3. Measurement of the attenuation and noise

Not only the skew of ultrasound in anisotropic crystal structure but the high attenuation and noise are the key factor of the adverse effect of the UT for the CASS piping. Although a variety of past researches have revealed similar information, attenuation and noise were measured for fully understanding the characteristic of the ultrasound propagation in CASS specimen applied in this study specifically.

JIS Z 2353-1992 of Japanese Industrial Standard stipulates the measurement method of attenuation coefficient. However the scope of this standard is a case with a frequency of larger than 2MHz, and it is difficult to measure in accordance with this method due to high attenuation of CASS. In addition, it was assumed that the attenuation changes with position. Hence the measurement method suited to CASS material was applied referring the basic philosophy of JIS standard. The attenuation value was obtained by dividing the ratio of the first bottom echo (B1) to second bottom echo (B2) emitted from the normal incidence probe by twice of the test block thickness. Since the comparison among the test blocks was the purpose of this study, loss of energy by reflection and diffusion in the bottom and the surface was not considered. For each specimen, 201 point data at 1mm pitch were obtained, and then the maximum and mean values were compared.

With regard to the noise measurement, maximum and Root Mean Square (RMS) values of amplitude in the range of 3/4T±10mm of beam path distance were obtained. Then the noise level was expressed by percentage of Distance Amplitude Correction (DAC%) as the ratio of echo amplitude from the side drilled hole at 3/4T to obtained values. For each specimen, 201 point data at 1mm pitch were obtained, and then the maximum and RMS values were compared among each specimen.

PANAMETRICS V392 transducer (1.0MHz, 38mm diameter) and micro-Tomoscan were applied for the data acquisition.

Fig. 14 presents A-scan and B-scan view when the measurement was carried out for CCASS of JTP-1 as an example. The propagation time lag of B1 and B2 was computed by the cross correlation of both waveforms. The frequency spectrums of B1 and B2 obtained by Fast Fourier Transform were presented in Fig. 15. The center frequencies of both echoes were about 1MHz. Since it was thought that the frequency dependency of attenuation was small from the fact that center frequency was almost consistent, the attenuation coefficient was calculated from the decrease in amplitude between B1 and B2.

The measurement results of each test block are summarized in Fig. 16. It is apparent that the attenuation of CASS is larger than the one of normal stainless steel (e.g. SUS304), especially in the columnar grain structures. In addition, the data dispersion of columnar grained CASS is larger than the one of equiaxed CASS, means the attenuation varies with place. This is one of the obstacles of the UT for columnar grained CASS. However it should be noted that even the equiaxed CASS, large attenuation can be seen.

As for the noise level, Fig. 17 demonstrates that UT for the CASS, especially columnar grained, is more difficult than for normal stainless steel same as field experiences.

(9)


11

Fig. 14. A-scan and B-scan view of echoes emitted from the normal incidence probe into JTP-1 (CCASS)

(a) First bottom echo (B1) (b) Second bottom echo (B2)

Fig. 15. Frequency spectrums of first and second bottom echoes

*: reference

First bottom echo (B1)

Second bottom echo (B2)

B2

B1

Range of noise measurement

0

0.05

0.1

0.15

0.2

JTP-1

(CC

ASS)

CB

-1

CB

-2

CB

-3

SU

S304

(Bas

e m

eta

l)*

Average

Maximun

Att

enuat

ion

(dB

/m

m)

Columnar

Equiaxed

Columnar

JTP-1(CCASS)

CB-1 CB-2 CB-3SUS304

(Base metal)*

Maximun 0.148 0.168 0.103 0.085 0.04

Average 0.117 0.118 0.076 0.073 0.04

StandardDeviation

0.013 0.022 0.009 0.004 -

Average+2σ 0.142 0.162 0.094 0.082 -

Center frequency Center frequency

(MHz) (MHz)

(%

)

(%

)

Fig. 16. Attenuation in the CASS


12

3.4. Visualization of ultrasound propagation

Ultrasonic wave propagation into the CASS piping was visualized using JTP-1 specimens. 3.4.1. Measurement technique

In this study, visualizing technique developed by T. Furukawa, et al. was applied. Fig. 18 illustrates the principle of the technique [14].

The ultrasonic pulse is emitted by transmitting probe which is set on the edge of specimen. Then the cross section of the specimen is scanned using receiving probe to catch the elastic displacement on the surface. And synchronous display of the received ultrasonic signals enableｓ visualization of the wave propagation. In this study, ultrasounds generated by PANAMETRICS V392 transducer (1.0MHz, 38mm diameter) are visualized whose nominal angles of incidence are 0 and 36 degrees respectively, using acrylic resin wedge.

Fig. 18. schematic image of the visualization of ultrasound propagation

UT Pulse

Visualization Image

Probe (pitch)

Probe (catch)

Scanner (Y-Z axis)

TP

DataSoftware

UT Equipment

UT Pulse

Visualization Image

Probe (pitch)

Probe (catch)

Scanner (Y-Z axis)

TP

DataSoftware

UT Equipment

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

40.0%

45.0%

50.0%

JTP-1

(CC

ASS)

CB

-1

CB

-2

CB

-3

SU

S304

(Bas

e m

eta

l)*

RMS

Peak value

Nois

e leve

l (D

AC

%)

Columnar Equiaxed

*: reference

JTP-1(CCASS)

CB-1 CB-2 CB-3SUS304

(Base metal)*

Peak value 22.3% 21.0% 17.6% 6.2% 1.4%

RMS 7.6% 6.7% 5.5% 2.1% 1.0%

Average of Peak 16.2% 13.3% 10.9% 4.5% 1.2%

Average of RMS 6.4% 4.7% 3.6% 1.6% 0.9%

Fig. 17. Noise level in the CASS


13

3.4.2. Result and discussion

Slowness surface, which is the plot that represents the inverse of the sound velocity (phase velocity), is the useful tool to understand the elastic wave propagation in an anisotropic material [15]. Based on the measurement result of the sound velocity and obtained elastic constants, the slowness surface of this transversely isotropic austenite material is described as Fig. 19. Since [001] direction has good correlation with columnar direction as observed by EBSP, simplified austenitic columnar grain structure of all which grew in the same [001] direction is postulated in the cross section to obtain the slowness surface.

In addition, wave skew which is the difference between the phase direction (normal to wave-surface) and energy flow direction is plotted in the Fig. 20 as a function of phase direction relative to the long grain axis [16]. Theoretical phase direction, which conforms to Snell’s law, and energy flaw of the elastic wave propagated into the anisotropic material were calculated using these figures.

Fig. 19. Slowness surface of JTP-1 with the (100) plane

Fig. 20. Wave skew v.s. phase direction relative to the long axis for JTP-1

1/VL1/VSH

1/VSV

1/VL1/VL1/VSH1/VSH

1/VSV1/VSV

-50.0

-40.0

-30.0

-20.0

-10.0

0.0

10.0

20.0

30.0

40.0

50.0

0.0 15.0 30.0 45.0 60.0 75.0 90.0

Phase direction relative to the long grain axis (deg.)

Skew angle (deg.)

Longitudinal

SV

SH

(s/km)


14

Fig. 21 shows the snapshots of ultrasound propagation emitted by normal incidence probe in the CCASS synchronizing the grain structures of the cross section, together with the theoretical wave propagation analysis result using slowness surface and the diagram of wave skew angle v.s. phase direction relative to the long grain axis. The red line in the snapshots of Fig. 21 (a) shows the calculated energy flow direction by Snell’s low. The wave in the CCASS propagated relatively straight as predicted theoretically. The longitudinal wave front spread widely and complicated dispersion was observed behind the wave front. Although it is qualitative consideration, attenuation in this case is higher than one of normal stainless steel due to those phenomena.

Fig. 22 shows the snapshots of ultrasound propagation emitted by normal incidence probe in the SCASS and its wave propagation analysis result. In this case, wave skew angle was calculated to be 16.9 degrees since about 15 degrees tilt of columnar grain growth orientation existed in the target material as shown in Fig. 22 (a). The red line and blue lines in the snapshots of Fig. 22 (a) show the calculated energy flow direction and theoretical phase direction, respectively. Actual beam propagation corresponded well with the theoretical energy flow. High attenuation of the left side of the wave from the theoretical energy flow center was also observed. On the other hand, attenuation of right side of energy flaw center was relatively low.

The 36 degree (nominal) probes are commonly used in ISI program for the primary coolant piping in Japanese PWRs based on the result of the research effort and field experiences by utilities and venders. Hence, the results of the wave propagation in the CCASS from the 36 degree (nominal) probes are presented in Fig. 23 as examples of the research. In the case of this study, the wave skew which is calculated from the difference of theoretical energy flow direction and phase direction in CCASS was +7.7 degrees. Good agreement between the calculated energy flow and actual longitudinal wave was observed in Fig. 23 (a).

The results of the wave propagation in the SCASS from the 36 degree (nominal) probes are presented in Fig. 24 and Fig. 25. In good agreement with the theoretical calculation, a large deviation of the propagation angle to 36 degrees and inconsistency between wave surface and energy flow are observed in SCASS when the wave propagated from right to left as shown in Fig. 24 (a). On the other hand, little wave skew which has good agreement with theoretical calculation (-1.0 degrees) was observed when the wave propagated from left to right as shown in the snapshots of Fig. 25.

These successfully obtained snapshots with good resolution clearly express that ultrasound is strongly affected by the acoustically anisotropic columnar grain structure. And theoretical beam skew predicted by the slowness surface postulating that the material was transversely isotropic represented the actual wave propagation well, as described in the all cases of this study.


15

3.5μs 7.0μs

Fig. 21. Ultrasound propagation in the CCASS (Normal incidence)

CCASSCCASS

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50

1/VL(θ)

1/VL Wedge

Incident angle

Normal

Phase direction

CCASS：Normal incidence

Phase directionin the wedge

Phase direction inthe CCASS

Slowness surface of longitudinal wave in CCASS

CCASSCCASS

-20.0

-10.0

0.0

10.0

20.0

0 15 30 45 60 75 90Phase direction relative to the long grain axis (deg.)

Ske

w a

ngl

e (

deg.

)

longitudinal wave0->0

Phase direction: 0° Difference of phase direction & long grain axis: 0° Wave Skew: 0° Energy flow direction (red line): 0°

(a) Snapshots of the ultrasonic wave propagation Red line: Theoretical Energy Flow and phase direction (0°)

(c) Skew angle calculation of energy flow

(b) Slowness surface (s/km)


16

Fig. 22. Ultrasound propagation in the SCASS (Normal incidence)

SCASSSCASS

-20.0

-10.0

0.0

10.0

20.0


Ske

w a

ngl

e (

deg.

)

longitudinal wave(0) 15->16.87

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50

1/VL(θ)

1/VL Wedge

Incident angle

Normal

Phase direction


Phase direction inthe CCASS

SCASS：Normal incidence

Energy flow direction(16.9 deg.)

SCASSSCASS


Phase direction: 0° Difference of phase direction & long grain axis: 15° Wave Skew: 16.9° Energy flow direction: 16.9° (Right)

(a) Snapshots of the ultrasonic wave propagation Red line: Theoretical Energy Flow and phase direction (16.9°) Blue line: Theoretical phase direction (0°)


3.5μs 10.0μs


17

Fig. 23. Ultrasound propagation in the CCASS (LA36)

CCASSCCASS

-20.0

-10.0

0.0

10.0

20.0


Ske

w a

ngl

e (

deg.

)

longitudinal wave37.5->7.71

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50

1/VL(θ)

1/VL Wedge

Incident angle (in the wedge)

LA36 (-37.5)

Phase direction

CCASS：Angledincidence fromRight to Left (LA36)


Phase directionin the CCASS

CCASSCCASS

Phase direction: 37.5° Difference of phase direction & long grain axis: 37.5° Wave Skew: +7.7° Energy flow direction: 45.2°



(a) Snapshots of the ultrasonic wave propagation Red line: Theoretical Energy Flow and phase direction (45.2°) Blue line: Theoretical phase direction (37.5°)

4.5μs 9.0μs


18

Fig. 24. Ultrasound propagation in the SCASS (LA36, Right to Left)

SCASSSCASS

-20.0

-10.0

0.0

10.0

20.0


Ske

w a

ngl

e (

deg.

)

longitudinal wave(34.4) 19.4->16.13

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50

1/VL(θ)

1/VL Wedge


LA36 (-34.4)

Phase direction

SCASS：Angled incidence from Right to Left (LA36)





Phase direction: 34.4° Difference of phase direction & long grain axis: 19.4° Wave Skew: +16.1° Energy flow direction: 50.5°

SCASSSCASS


3.5μs 7.0μs


19

Fig. 25. Ultrasound propagation in the SCASS (LA36, Left to Right)

-20.0

-10.0

0.0

10.0

20.0


Ske

w a

ngl

e (

deg.

)

longitudinal wave(38.2) 53.2->-1.04

SCASSSCASS

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50

1/VL(θ)

1/VL Wedge


LA36 (38.2)

Phase direction

SCASS：Angled incidence from Left to Right (LA36)



Phase direction: 38.2° Difference of phase direction & long grain axis: 52.2°Wave Skew: -1.0° Energy flow direction (red line): 37.2°



SCASSSCASS


4.5μs 9.0μs


20

4. Conclusion

To comprehend the unique ultrasound propagation into the CASS piping such as beam skewing, dispersion and unexpected attenuation which gives adverse effects on the UT, characteristics of microstructure and ultrasound propagation into the CASS piping was studied using CASS specimens simulating reactor coolant piping in Japanese Pressurized Water Reactor (PWR) plants.

In this report, crystal orientation maps of columnar grained CASS by EBSP and fundamental characteristics of ultrasound in CASS such as sound velocity, attenuation and noise are studied.

In addition, the snapshots of the wave propagation in CASS are also successfully obtained by visualization techniques with high resolution. The unique ultrasound beam propagation such as skew due to the coarse grained and acoustically anisotropic crystal structures was observed through the technique. The tendency of skew of energy flow in CASS agreed with the theoretical one obtained from slowness surface when postulating the material as transversally isotropic.

The information obtained in this study would help inspection personnel understand the characteristics of ultrasound propagation in CASS. A precise simulation model applicable to the CASS piping will be developed as future work using the knowledge obtained in this study.

References

[1] Y. Kurozumi, H. Ishida, “Detection Sensitivity and Sizing Ability of Defects in Cast Stainless Steel with

Newly Developed Automatic Ultrasonic Inspection System”, Journal of the Institute of Nuclear Safety System, 2004, No 7, pp182-197.

[2] M.T. Anderson, S.E. Cumblidge, S.R. Doctor, “Low Frequency Phased Array Techniques for Crack Detection in Cast Austenitic Piping Welds: A Feasibility Study”, Material Evaluations/January 2007, pp55-61.

[3] M. Anderson, S.Crawford, S. Cumblidge, K. Denslow, A. Diaz, S. Doctor, “Assessment of Crack Detection in Heavy-Walled Cast Stainless Steel Piping Welds Using Advanced Low-Frequency Ultrasonic Methods” U.S. Nuclear Regulatory Commission, NUREG/CR-6933, March 2007.

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[8] P. Lemaitre, T.D. Koble, “Report on the Evaluation of the Inspection Results of the Wrought-to-Cast PISC III Assemblies 51 and Weld A of Assembly 43”, PISC III Report No. 35, European Commission, 1995.

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[10] F. Jenson, L. Ganjehi, et. al, “Modeling of Ultrasonic Propagation in Cast Stainless Steels with Coarse Grained Structures”, Proceedings of the 7th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, May 2009, pp336-344.

[11] Yutaka Sato, Hiroyuki Kokawa, “Automated EBSP Analysis and Crystal Orientation Mapping”, Journal of the Japan Welding Society, 68-8 (1999), pp16-20.

[12] Japanese Industrial Standards Committee, “Method for assessing the overall performance characteristics of ultrasonic pulse echo testing instrument”, JIS Z 2353-1992.

[13] B. A. AULD, “Acoustic fields and waves in solids, volume 1, second edition”, Krieger publishing company inc., 1990, pp191-264.

[14] T. Furukawa, H. Yoneyama, Y. Horii, N. Uesugi, “Measurement of Ultrasonic Wave Propagation in


21

Austenitic Stainless Steel Welds”, Proceedings of 2nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, JRC, New Orleans, May 24-26, 2000, B195-B201.

[15] B. A. AULD, “Acoustic fields and waves in solids, volume 2, second edition”, Krieger publishing company inc., 1990, pp1-61.

[16] J.A. Ogilvy, “A Model for Elastic Wave Propagation in Anisotoropic Media with Applications to Ultrasonic Inspection through Austenitic Steel”, British Journal of NDT, January 1985, pp13-21.

study on the ultrasound propagation in cast austenitic...

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