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VISUALIZATION OF ULTRASONIC WAVE IN CAST AUSTENITIC STAINLESS STEEL

PIPING

Kazunobu Sakamoto, JNES, JAPAN; Tsuyoshi Mihara, University of Toyama;

Takashi Furukawa, Ichiro Komura, Yoshinori Kamiyama, JAPEIC, JAPAN

ABSTRACT

Japan Nuclear Energy Safety Organization (JNES) has been carrying out the research program entitled

“Nondestructive Inspection Technologies for the Cast Stainless Steel Piping” since 2009FY to comprehend

the unique ultrasonic wave propagation in Cast Austenitic Stainless Steel (CASS) and to confirm detection

and sizing capability for flaws in the material by currently available ultrasonic testing techniques. The

research is also intended to provide inspection staff with the fundamental information of ultrasonic wave

propagation in CASS, for educational purpose.

This report presents visualization results of ultrasonic wave propagation in CASS piping and its

comparison with simulation model developed in the research program to make sure the validity.

INTRODUCTION

It is well known that volumetric inspection by ultrasonic testing (UT) for cast austenitic stainless steel

(CASS), which is extensively used in primary loop 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 [1,2,3].

Numerical simulation models have been widely applied to studies of ultrasonic wave propagation

phenomena in anisotropic materials in recent years. In addition those simulation models are expected to be

used as tools for a setup of optimal inspection condition and verification of the inspection in the field as

well as improvement of inspection procedure, and the needs are growing increasingly in recent years [4,5].

To develop the accurate simulation model applicable to CASS material for those purposes, understanding

of the ultrasonic wave propagation characteristic is indispensable.

Under such circumstances, visualization of the ultrasonic wave propagation in the CASS are

performed to understand the unique phenomena such as beam skewing, dispersion and unexpected

attenuation precisely, using the CASS specimens whose material, size, dimension and welding method are

identical to Japanese PWR plants.

In addition a large scale finite element method (FEM) modeling technique for simulating the

ultrasonic wave propagation in CASS was developed to re-create not only the wave path but noise and

attenuation due to the coarse anisotropic crystal structure with accuracy to some extend. The simulation

results by developed model were compared with the actual wave propagation obtained through the

visualization technique.

SPECIMENS

Material applied in this study is presented in Figure 1. CASS welded joint mockup 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 was prepared.

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Figure 1 - Main coolant piping mockup

VISUALIZATION TEQUNIQUE OF ULTRASONIC WAVE PROPAGATION

Visualizing technique developed by T. Furukawa, et al. whose principle was illustrated in Figure 2 was

applied to observe the ultrasonic wave propagation in CASS [6]. 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, ultrasonic waves

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.

Figure 2 - Schematic image of the visualization

SIMULATION MODEL

FEM code

It is necessary to reproduce more precisely the ultrasonic wave propagation in CASS material which has

acoustical anisotropy. For this purpose, in the analysis using FEM, the nodes become the hundreds of

millions or more large-scale numbers. To satisfy these needs, the 3-dimensional FEM wave propagation

UT Pulse

Visualization Image

Probe (pitch)

Scanner (Y-Z axis)TP

DataSoftware

Probe (catch)

Scanner (Y-

Data

UT Equipment

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analysis cord "ComWave" (Itochu Techo-Solutions Corporation) was applied to calculate the ultrasonic

wave propagation.

With regard to the mesh size, it was considered as the voxel component of 0.1 mm x 0.1 mm x 0.1

mm so that it becomes small enough to the longitudinal wavelength in CASS and wedge with a frequency

of 1MHz. The number of analysis steps was set to 14ns in consideration of the duration of longitudinal

wave propagation in one component.

Figure 3 - Probe model (1.0M38 LA36)

An example of the probe model used in this analysis is shown in Figure 3. The emitting ultrasonic wave

was made giving the initial displacement to the FEM mesh corresponding to a transducer. Figure 4 shows

the simulation analysis of the acoustic field emitted in Pyrex glass from the 1.0M38 LA36 transducer.

Figure 4 - Simulation of generated elastic wave from the transducer (1.0M38 LA36)

Crystal model

In the simulation model of UT to the anisotropic material, the acoustic characteristics of the texture has

mainly been modeled from the bulk material point of view. For example, since the acoustic feature of the

texture shows transverse isotropy, the columnar grain of austenitic steel has been dealt with as a hexagonal

crystal model. Although this model is useful to evaluate the propagation route of the ultrasonic wave,

expression of the noise and attenuation is difficult.

To enable practical use to the distinctive evaluation in defect detection or evaluation of the signal to

noise ratio in the simulation model, the cubic crystal model was applied. Following elastic coefficient of

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cubic crystal which is calculated from the sound velocity measurement results is used [7, 8].

C11:215.0, C12: 128.5, C44: 117.1

Grain boundary model

In the modeling of a grain boundary, the macrographic view of cross section of a system mock-up was

reproduced precisely. Figure 5 shows an example of the model created by this method, together with the

original macrostructure photograph. In this case, each grain is expressed in 4 gradation patterns.

Voronoi diagram decomposition algorithm was also applied for crystalline region division

technique to model a variety of crystal structures [5]. The example of the CASS piping weld-zone model,

which imitated and created the crystal structure shown in Figure 5(1), in this process is presented in Figure

6. Here, each grain is expressed in 4 gradation patterns.

The elastic coefficient and crystal directions were assigned to each crystal grain model created by

reproducing macrographic view and Voronoi division [9].

(1) Macrographic view of cross section

Angled probe model

(45degree nominal, 1MHz, φ38mm)

(2) Reproduced crystal structure model

Figure 5 - Modeling of grain structures by reproducing macrographic view

OD Surface

SCASS CCASS

Weld

ID Surface

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Figure 6 - CASS welding model by Voronoi decomposition

RESULTS

Figure 7 shows the snapshots of actual ultrasonic wave propagation and simulation result emitted from 45

degrees (nominal) probe into the SCASS. The solid and broken lines in the snapshots of Figure 7 (1) show

the theoretical energy flow direction and phase direction calculated by postulating the material as

transversally isotropic, respectively [10]. The solid line in the simulation result of Figure 7 (2), whose

grain structure was modeled by reproducing macrographic view, shows the theoretical energy flow, too. As

presented in Figure 7 (1), actual phase direction and energy flow conform to the theoretical one. The

simulation result presented in Figure 7 (2) shows the good agreement with the visualization result. And the

simulation result expressed scattered wave well.

Figure 8 illustrates the measurement and simulation results for the CCASS whose ultrasonic wave

was emitted from 45 degrees (nominal) probe. The solid and broken lines in the figures represent the

theoretical energy flow direction and phase direction. Figure 8 (1) shows the measurement result. And

Figure 8 (2) and (3) present the simulation result whose grain structures were modeled by reproducing

macrographic view and by Voronoi decomposition, respectively. Both simulation results show good

agreement with actual wave propagation presented in Figure 8 (1), with well reproduction of scattered

wave.

SUMMARY

The ultrasonic wave propagation in CASS was visualized by visualization techniques with relatively good

resolution. And the FEM simulation result showed good agreement with the actual ultrasonic wave

propagation.

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(1) Measurement result

(2) Simulation result

Figure 7 - Simulation and visualization results of ultrasonic wave propagation into SCASS

Shear wave

Longitudinal wave

OD surface

ID surface SCASS CCASS

41.6 deg.

Scattered wave

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Figure 8 - Simulation and visualization results of ultrasonic wave propagation into SCASS

REFERENCES

1) P. Lemaitre, T.D. Koble, “Report on the Evaluation of the Inspection Results of the Cast-to-Cast

PISC III Assemblies no. 41, 42 and Weld B of Assembly 43”, PISC III Report No. 34, European

Commission, 1995.

2) 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.

3) M. Anderson, et. al, “Assessment of Crack Detection in Heavy-Walled Cast Stainless Steel Piping

Welds Using Advanced Low-Frequency Ultrasonic Methods”, NUREG/CR-6933, prepared by

PNNL for the U.S. NRC, Washington, DC, 2007.

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Castings”, Proceedings of the 7th International Conference on NDE in Relation to Structural

Integrity for Nuclear and Pressurized Components, May 2009, pp329-335.

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Structures”, Proceedings of the 7th International Conference on NDE in Relation to Structural

Integrity for Nuclear and Pressurized Components, May 2009, pp336-344.

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Austenitic Stainless Steel Welds",Proceedings of 2nd International Conference on NDE in Relation

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7) B. A. AULD, “Acoustic fields and waves in solids, volume 1, second edition”, Krieger publishing

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E-Journal of Advanced Maintenance Vol. 4-1, May 2012.

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Piping”, Maintenolocy Vol.11 No.2, Japan Society of Maintenology, July 2012.

10) B. A. AULD, “Acoustic fields and waves in solids, volume 2, second edition”, Krieger publishing

company inc., 1990, pp1-61.