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Ultrasonic sizing of defects in the austenitic cladding of reactor pressure vessels
Int. J. Pres. Ves. & Piping 55 (1993) 141-147
_ ~
Ultrasonic Sizing of Defects in the Anstenitic Cladding of Reactor Pressure Vessels
P. Kauppinen & P. S~irkiniemi Technical Research Centre of Finland (VTT), SF-02150 Espoo, Finland
ABSTRACT This paper reports the measurement procedure used for inspection of stainless steel cladding and sizing of flaws with inclined longitudinal transmitter-receiver probes. The special characteristics of these probes are discussed and a practical solution for calibration is presented. The results achieved in reactor pressure vessels inspections verify the applicability and accuracy of the technique described in this paper.
INTRODUCTION Conventional ultrasonic techniques are not able to reveal effectively the defects in or in the vicinity of stainless steel cladding in a ferritic pressure vessel steel. Inclined longitudinal transmitter-receiver probes (ILTR-probes) have been used for this purpose for almost 20 years. The application of these probes has normally been restricted to the detection of flaws only and the characterization of indications has been performed using different techniques? An experimental calibration and inspection procedure for the inspection of cladding and sizing of flaws has been developed and will be presented here. INSPECTION WITH ILTR-PROBES The major characteristics affecting the positioning and sizing of defects using ILTR-probes are the sound pressure distribution transverse to 141 Int. J. Pres. Ves. & Piping 0308-0161/93/$06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
142
P. Kauppinen, P. Siirkiniemi
/
-10
r~
-z0 i
1
-30
-4O 0 RELATIVE ANGLE OF DIVERGENCE
Fig. 1. Sound pressure distribution transverse to the beam. beam and the echo amplitude obtained at different distances along the centerline of the beam. The sound pressure distribution transverse to the beam is given in Fig. 1. A typical distance amplitude curve (DAC) along the centerline for an ILTR-probe is seen in Fig. 2. The shape of this curve is affected by many factors such as the inclination needed for the focussing, the attenuation and the shape of the reflector. Maximum amplitude is obtained at the focal distance, FD. The effect of the position of the probe on the echo amplitude obtained from side drilled holes is evaluated according to Fig. 3. In the first position, A, the side drilled hole lies exactly on the beam axis at a
4J ~_J
35
FOCAL DISTANCE
(FD)
w 251/
5
j
,
I
,
I
,
1
,
I
8 12 16 RELATIVE DISTANCE ALONG BEAM 4
CENTERLINE
Fig. 2. Sound pressure distribution along beam centerline.
Sizing of defects in RPV austenitic cladding
143
Z~VcL
/
!
L
Fig. 3. Dependence of the maximum amplitude on the location of the reflector (FD = focal distance).
distance larger than the focal distance, FD. If the probe is moved to a new position, B (dashed in Fig. 3), the side drilled hole will be outside the beam centerline axis but in this case at a position closer to the focal distance, FD. If now AV,r < AVe, a higher echo is gained from point B than from point A. 2 The m a x i m u m reflection on the C R T screen does not always mean that the reflector is on the beam axis. Consequently, the ILTR-probes may have a m a x i m u m amplitude axis which is different from the centerline axis. 3 This has also been mathematically proved by Rose and Singh. 4 To determine the m a x i m u m amplitude axis of the beam an experiment was made with a R T D - B A M 70°-L2-150f18 probe on ferritic material. 5 In this experiment echo amplitudes obtained from several side drilled holes at different depths were examined. The location of the probe was measured at the point where the maximum amplitude for each hole was gained. The result is shown in Fig. 4. The line drawn through the m a x i m u m amplitude positions does not coincide with the centerline axis but crosses it at about the focal distance. For practical inspections this observation has the following consequences: (i)
(ii)
The measurement of the probe index is associated with difficulties because the result depends on the radius (i.e. the probe to reflector distance) of the reference block (DIN 54120 and DIN 54122), as shown in Fig. 5. Because the probe index cannot be exactly determined the measured beam angle will also include errors. Even if the probe
P. Kauppinen, P. Siirkiniemi
144
SDH •2 mm
Reflector:
REFLECTION DISTANCE (mm) 0
"~E
0
_
10
20
~
I
~
1
4o
~glO 4o
~2~
Fig. 4,
(iii)
~
L
Beam centerline and the maximum amplitude axis.
index is correct, the measured beam angle depends on the distance to the reflecting surface. This has also been reported by de Raad and Dijkstra) The range to be inspected is decreased and the resolution is less than expected. This is illustrated in Fig. 6, in which the expected sound path range, SE, in the inspection is reduced to SR. Consequently, all reported defects seem to 'cluster' around the focal distance, FD.
P R O C E D U R E F O R INSPECTION OF C L A D D I N G S ILTR-probes have been successfully used for the inspection of reactor pressure vessel cladding. The calibration was carried out so that the nominal probe index was used as a zero-point in the measurements. The position an and dn of the calibration holes are known (see Fig. 7) and Sn is read from the screen. During the inspection phase S. is read from the screen and corresponding an and d. values are obtained from
R 5 10 15 20 25 30 35
Fig. 5.
w
W-R
4 9 3 13 1.5 16.5 1.5 21.5 0.5 25.5 0 30 34.5 -0.5
The relationship between the measured index point and the radius R of reference block. The nominal index point is denoted by W.
Sizing of defects in RPV austenitic cladding
145
r
SE SR
-I
,
i
s
E
I I I I
E = Expected R =As measured
~D ~
= R
-
~ E
~
sR --=0.5 sE
Fig. 6.
Relation between expected sound path range (SE) and real range (SR).
the table in Fig. 7. Linear interpolation was used from intermediate values. In reactor pressure vessels (RPV) examinations, the defect size was measured by the technique described above for a large amount of defects and the inspection results were afterwards compared to the real dimensions obtained when the cracks were opened by grinding. The technique proved to be very reliable and accurate as can be seen in Fig. 8.
d
n
3.0
S
n
12.0
a
n
12.0
4.0 13.0 5.0 16.0 7.0!18.0
13.5 16.0 17.5
10.0 21.0
20.0
a t_
Fig. 7.
n
_1
Calibration procedure for defect location purposes.
146
P. Kauppinen, P. Siirkiniemi 0
40
LENGTH (mm) 80 120 160
200
240
0 A
E E
5
-r
Cladding c~ 10
Measured
by UT
Measured
by grinding
Fig. 8. The sizes of defects measured by ultrasonic technique and by penetrant testing
after grinding. CONCLUSIONS Based on the experiments described in this paper it is evident that the ILTR-probes cannot accurately be characterized using the calibration blocks D I N 54120 and D I N 54122. The m e a s u r e d index point and b e a m angle d e p e n d on the distance to the reflector. In RPV-examinations the defect size can, however, be reliably m e a s u r e d by the technique described in this paper.
REFERENCES 1. de Raad, J. & Sterke, A., Ultrasonic monitoring of sub-cladding cracks. Proc. Conf. Periodic Inspection of Pressurized Components. Institution of Mechanical Engineers, London, 1976. 2. S~irkiniemi, P. & Kauppinen, P., Some experiences in the use and characterization of inclined longitudinal transmitter-receiver probes. Eurotest expert meeting on Characterization of ultrasonic equipment, 9 December, 1980, Espoo, Finland. 3. Sasahara, T., Automatic sizing of intergranular stress corrosion cracking with IntraSpect/98. EPRI Topical Report NP-5409, 1987, Palo Alto, CA. 4. Rose, J. L. & Singh, G. P., An analysis of the dual-element angle beam transducer. Materials Evaluation 38(7) (1980) 38-43. 5. S~irkiniemi, P. & Kauppinen, P., A practical method for improving the
Sizing of defects in RPV austenitic cladding
147
accuracy of defect sizing with ILTR-probes (summary). Proc. 7th Int. Conf. on NDE in the Nuclear Industry, Grenoble, 29 Jan.-1 Feb. 1985, pp. 285-6. 6. de Raad, J. A. & Dijkstra, F. H., Einschallwinkel und Fehlerortung bei der Verwendung von SEL-Prtifk/3pfen. Sitzung des DGZfP-Fachausschusses: Ultraschallpriifverfahren. Deutsche Gesellshaft ftir Zersti3rungsfreie Prtifung, Berlin, 1980.
_ ~
Ultrasonic Sizing of Defects in the Anstenitic Cladding of Reactor Pressure Vessels
P. Kauppinen & P. S~irkiniemi Technical Research Centre of Finland (VTT), SF-02150 Espoo, Finland
ABSTRACT This paper reports the measurement procedure used for inspection of stainless steel cladding and sizing of flaws with inclined longitudinal transmitter-receiver probes. The special characteristics of these probes are discussed and a practical solution for calibration is presented. The results achieved in reactor pressure vessels inspections verify the applicability and accuracy of the technique described in this paper.
INTRODUCTION Conventional ultrasonic techniques are not able to reveal effectively the defects in or in the vicinity of stainless steel cladding in a ferritic pressure vessel steel. Inclined longitudinal transmitter-receiver probes (ILTR-probes) have been used for this purpose for almost 20 years. The application of these probes has normally been restricted to the detection of flaws only and the characterization of indications has been performed using different techniques? An experimental calibration and inspection procedure for the inspection of cladding and sizing of flaws has been developed and will be presented here. INSPECTION WITH ILTR-PROBES The major characteristics affecting the positioning and sizing of defects using ILTR-probes are the sound pressure distribution transverse to 141 Int. J. Pres. Ves. & Piping 0308-0161/93/$06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
142
P. Kauppinen, P. Siirkiniemi
/
-10
r~
-z0 i
1
-30
-4O 0 RELATIVE ANGLE OF DIVERGENCE
Fig. 1. Sound pressure distribution transverse to the beam. beam and the echo amplitude obtained at different distances along the centerline of the beam. The sound pressure distribution transverse to the beam is given in Fig. 1. A typical distance amplitude curve (DAC) along the centerline for an ILTR-probe is seen in Fig. 2. The shape of this curve is affected by many factors such as the inclination needed for the focussing, the attenuation and the shape of the reflector. Maximum amplitude is obtained at the focal distance, FD. The effect of the position of the probe on the echo amplitude obtained from side drilled holes is evaluated according to Fig. 3. In the first position, A, the side drilled hole lies exactly on the beam axis at a
4J ~_J
35
FOCAL DISTANCE
(FD)
w 251/
5
j
,
I
,
I
,
1
,
I
8 12 16 RELATIVE DISTANCE ALONG BEAM 4
CENTERLINE
Fig. 2. Sound pressure distribution along beam centerline.
Sizing of defects in RPV austenitic cladding
143
Z~VcL
/
!
L
Fig. 3. Dependence of the maximum amplitude on the location of the reflector (FD = focal distance).
distance larger than the focal distance, FD. If the probe is moved to a new position, B (dashed in Fig. 3), the side drilled hole will be outside the beam centerline axis but in this case at a position closer to the focal distance, FD. If now AV,r < AVe, a higher echo is gained from point B than from point A. 2 The m a x i m u m reflection on the C R T screen does not always mean that the reflector is on the beam axis. Consequently, the ILTR-probes may have a m a x i m u m amplitude axis which is different from the centerline axis. 3 This has also been mathematically proved by Rose and Singh. 4 To determine the m a x i m u m amplitude axis of the beam an experiment was made with a R T D - B A M 70°-L2-150f18 probe on ferritic material. 5 In this experiment echo amplitudes obtained from several side drilled holes at different depths were examined. The location of the probe was measured at the point where the maximum amplitude for each hole was gained. The result is shown in Fig. 4. The line drawn through the m a x i m u m amplitude positions does not coincide with the centerline axis but crosses it at about the focal distance. For practical inspections this observation has the following consequences: (i)
(ii)
The measurement of the probe index is associated with difficulties because the result depends on the radius (i.e. the probe to reflector distance) of the reference block (DIN 54120 and DIN 54122), as shown in Fig. 5. Because the probe index cannot be exactly determined the measured beam angle will also include errors. Even if the probe
P. Kauppinen, P. Siirkiniemi
144
SDH •2 mm
Reflector:
REFLECTION DISTANCE (mm) 0
"~E
0
_
10
20
~
I
~
1
4o
~glO 4o
~2~
Fig. 4,
(iii)
~
L
Beam centerline and the maximum amplitude axis.
index is correct, the measured beam angle depends on the distance to the reflecting surface. This has also been reported by de Raad and Dijkstra) The range to be inspected is decreased and the resolution is less than expected. This is illustrated in Fig. 6, in which the expected sound path range, SE, in the inspection is reduced to SR. Consequently, all reported defects seem to 'cluster' around the focal distance, FD.
P R O C E D U R E F O R INSPECTION OF C L A D D I N G S ILTR-probes have been successfully used for the inspection of reactor pressure vessel cladding. The calibration was carried out so that the nominal probe index was used as a zero-point in the measurements. The position an and dn of the calibration holes are known (see Fig. 7) and Sn is read from the screen. During the inspection phase S. is read from the screen and corresponding an and d. values are obtained from
R 5 10 15 20 25 30 35
Fig. 5.
w
W-R
4 9 3 13 1.5 16.5 1.5 21.5 0.5 25.5 0 30 34.5 -0.5
The relationship between the measured index point and the radius R of reference block. The nominal index point is denoted by W.
Sizing of defects in RPV austenitic cladding
145
r
SE SR
-I
,
i
s
E
I I I I
E = Expected R =As measured
~D ~
= R
-
~ E
~
sR --=0.5 sE
Fig. 6.
Relation between expected sound path range (SE) and real range (SR).
the table in Fig. 7. Linear interpolation was used from intermediate values. In reactor pressure vessels (RPV) examinations, the defect size was measured by the technique described above for a large amount of defects and the inspection results were afterwards compared to the real dimensions obtained when the cracks were opened by grinding. The technique proved to be very reliable and accurate as can be seen in Fig. 8.
d
n
3.0
S
n
12.0
a
n
12.0
4.0 13.0 5.0 16.0 7.0!18.0
13.5 16.0 17.5
10.0 21.0
20.0
a t_
Fig. 7.
n
_1
Calibration procedure for defect location purposes.
146
P. Kauppinen, P. Siirkiniemi 0
40
LENGTH (mm) 80 120 160
200
240
0 A
E E
5
-r
Cladding c~ 10
Measured
by UT
Measured
by grinding
Fig. 8. The sizes of defects measured by ultrasonic technique and by penetrant testing
after grinding. CONCLUSIONS Based on the experiments described in this paper it is evident that the ILTR-probes cannot accurately be characterized using the calibration blocks D I N 54120 and D I N 54122. The m e a s u r e d index point and b e a m angle d e p e n d on the distance to the reflector. In RPV-examinations the defect size can, however, be reliably m e a s u r e d by the technique described in this paper.
REFERENCES 1. de Raad, J. & Sterke, A., Ultrasonic monitoring of sub-cladding cracks. Proc. Conf. Periodic Inspection of Pressurized Components. Institution of Mechanical Engineers, London, 1976. 2. S~irkiniemi, P. & Kauppinen, P., Some experiences in the use and characterization of inclined longitudinal transmitter-receiver probes. Eurotest expert meeting on Characterization of ultrasonic equipment, 9 December, 1980, Espoo, Finland. 3. Sasahara, T., Automatic sizing of intergranular stress corrosion cracking with IntraSpect/98. EPRI Topical Report NP-5409, 1987, Palo Alto, CA. 4. Rose, J. L. & Singh, G. P., An analysis of the dual-element angle beam transducer. Materials Evaluation 38(7) (1980) 38-43. 5. S~irkiniemi, P. & Kauppinen, P., A practical method for improving the
Sizing of defects in RPV austenitic cladding
147
accuracy of defect sizing with ILTR-probes (summary). Proc. 7th Int. Conf. on NDE in the Nuclear Industry, Grenoble, 29 Jan.-1 Feb. 1985, pp. 285-6. 6. de Raad, J. A. & Dijkstra, F. H., Einschallwinkel und Fehlerortung bei der Verwendung von SEL-Prtifk/3pfen. Sitzung des DGZfP-Fachausschusses: Ultraschallpriifverfahren. Deutsche Gesellshaft ftir Zersti3rungsfreie Prtifung, Berlin, 1980.