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Preliminary results for practical ultrasonic testing of austenitic steel welds
Preliminary results for practical ultrasonic testing of austenitic steel welds G. Herberg, W. Moiler, O. Ganglbauer The welds of the austenitic reactor vessel of the liquid-metal fast-breeder reactor (Imfbr) SN R 300 were inspected by an ultrasonic method using transmitter-receiver angle-beam probes with longitudinal-wave ultrasound. The inspections showed that it is generally possible to find longitudinal defects, but not transverse defects. Furthermore, interpretation of the results for the type and size of defects is difficult, and phantom signals may lead to misinterpretation. Further developments and improvements are necessary.
Most components for the primary and secondary systems of the liquid-metal fast-breeder reactor, the SNR 300, are made in austenitic steels because of the high working temperature. The quality of the material used is similar to the AISI 304 ss. The safety of the plant seemed to call for inspection of the welds by two volumetric methods, x-ray and ultrasonics. This paper reports our experience with ultrasonic testing of the reactor vessel. The ultrasonic inspection of austenitic welds by angle-beam and shear-wave techniques has proved nearly impossible up to the present time. There are two reasons for this. First, the grain size is critical in ultrasonic inspection. Austenitic steel has a coarse dendritic structure and it is well known that the ultrasonic scattering in an austenitic structure increases with its coarseness. Secondly, the heat-affected zone is anisotropic; the kind, location, and direction of the crystals in the heat-affected zone influence the scattering and absorption of ultrasound. Reasonable results have therefore always proved unobtainable by conventional ultrasonics because of simultaneous ultrasonic attenuation and high scattering: that is, a low ratio of signal to noise. A ratio of at least 10 dB seems necessary for reasonable detection of defects.
Research The federal materials testing establishment, the Bundesanstalt for Matefialprofung (BAM), in Berlin has carried out applied research on the subject. It is reported that a transmitter-receiver system, in which the two probes generate longitudinal-wave ultrasonic beams crossing in a limited focal area solves the problem. It should be noted that the technique does not only apply to longitudinal waves. These BAM probes are improvements of the longitudinal-wave probes (transceiver) probes already developed to detect cracks under cladding; in both cases the two transducers Gotz Herberg and Walter Muller are at Interatom GmbH in Bensberg near Cologne, West Germany and Otto Ganglbauer is at VbestAlpine in Linz, Austria.
NDT INTERNATIONAL . OCTOBER 1976
Elevation
Plan
~
b Austenltlcweld
e
\
Fig. 1 BAM probes transmit and receive ultrasound into and from a small area of the weld
are inclined towards one another and cross in the focal area. This technique is an improvement over the pulse-echo technique because, by crossing the beams in the focal area, the scattering echoes from the structure derive only from the limited focal area. On the other hand this technique means that only a limited layer is inspected. In practice it means that the inspected layers are about 15 to 20 mm thick (Fig. !). For this reason BAM developed a range of transceivers, all at 2 MHz (Table !). Their large size of 40 × 50 mm requires that the objects to be tested have a plane surface. Experiments were carried out to find whether the depth of the focal point and the centre of each layer coincided as calculated. The transducers incline towards one another so that the transmitter beam accurately crosses the receiver beam. Using a delay block made of polymethyl metha-
239
40 to 60 mm. The ultrasonic results were compared with the earlier x-ray inspection.
Table 1. The BAM range of special transceivers
Designation
Angle [o]
Focal length [mm]
Field depth [mm]
1 TR-Long probe 2 ,, 3 ,, 4 ,,
70 70 60 50
18 65 75 82
0-15 15- 30 3 0 - 45 45-60
A
2-
/,,
/
~,./ / /
.,~"~'-,,,, ~ ' ~ . \ .
"x,.
/'iX"
4.
/
\.
"-,,,',,
/'
Field depth E
8
-/
,4
/
~
I~ / 15
/
~ ~ 5 - 3 o m m / / ~_~.~__...-~
,
25
,
\
3,5 45 Woll thickness [mrn]
30 - 45 mm ~/"
,
55
,
65
N
"
7,5
The ultrasonic signals could be correlated only partially with the x-radiography. On the other hand, some defects detected by radiography could not be found by ultrasonics. Those areas of the welds which gave rise to ultrasonic signals but showed nothing on radiographs were ground to the depth calculated for the assumed defect and inspected by dye penetrant. Only one defect, a pore, which confirmed the ultrasound signals, was found. It is possible that small flaws in the material, such as fissures, were obscured by grinding, or that the heat caused by grinding dried the dye penetrant. It is also possible that a change of ultrasonic velocity from the base material to the weld caused a slight refraction which distorted indication of the depth of defect. As the result may have been influenced by these effects, it seems possible only exact sectioning can give an answer about the type and location of defects. The welds had to be ground before ultrasonic inspection and consequently undercuts in the weld area caused disturbances as the probes were partly tipped. Coupling was difficult in these cases, and caused spurious signals which could only be discounted as such after several tests at the same spot. A different phenomenon sometimes appeared to originate near the surface and was arbitrarily called fir tree (Fig. 5). Experimental work lead us to an explanation of the pheno-
Fig. 2 The amplitude of the response reaches a m a x i m u m in the focal area
crylate means that a definite focal area is achieved. Experiments showed that the focal areas were larger than expected, implying an overlapping (Fig. 2). This became apparent by comparing echo height with reflector distance made for each probe. This was accepted as the probable maximum which could be achieved whether at the centre of each layer or not, without further attenuation loss. ltence a more accurate estimation of the size of defects is possible. For the first ultrasonic tests Vtiest-Alpine produced a submerged welded plate of 50 mm wall thickness of quality similar to AISI 304 ss. Three holes of 2 mm diameter were drilled in the weld longitudinally at different depths. The holes were so placed that the velocity of sound through the weld was greater in one probe direction than from its opposite. Holes of 2 mm diameter were also drilled in the base material in the same direction and at the same depths. All holes were 35 mm deep. Ultrasonic investigation of the base material and weld showed the attenuation to be about 6 to 10 dB lower in the base than in the weld material. Shear-wave signals also appeared. However, these arrived behind the longitudinal-waves signal because of their lower velocity (Fig. 3).
Fig. 3 The oscilloscope shows t w o signals; the first t o arrive (left) is longitudinal-wave and the second shear-wave
The shear-wave signals were seen only as reflections from the holes and were disregarded. To avoid misinterpretation the oscilloscope screen was partly obscured to include only longitudinal-wave signals (Fig. 4). Results
For the ultrasonic investigation of the welds of the reactor vessel, test holes of 2 mm diameter were made in the welds of test pieces and tested ultrasonically. The lower signal was set at 60% of the screen height. All the welds were inspected in this way: submerged-arc and manual welds, AISI 304 ss,
240
Fig, 4 A paper mask covered the part of the screen showing shearwave response
NDT INTERNATIONAL. OCTOBER 1976
menon. The crystal "structure in the transitional area between the base material and the weld causes portions of an incident beam at a certain phase to be reflected with a phase shift of 2 or a multiple of 2. These reflected soundwaves may yield a combined signal of intensity equal to a real signal. Terraced structure could be the cause of such phase shift as these structures often occur in metallographic specimens of austenitic welds. This theory is supported by the fact that fir tree disappears when the angle or the direction of the incident beam is changed. Further work is necessary if this phenomenon is to be avoided. In addition, another test plate was fabricated with natural defects, including lack of fusion, slag lines, pores, and root defects. These defective areas were ultrasonically tested with the parameter fixed and all signals were photographed on the screen. It was found that the ratio of the height of echo between artificial and real defects does not def'me the type and size of the real defect. The height of the signal of unacceptable lack of fusion, for instance, is sometimes only slightly above the limit of registration. Consequently it is not yet possible to analyse defects by comparison of echo height for artificial and real defects.
Fig. 5 The response f r o m a 2 M H z , 70 °, 20 mm probe showed an interfence pattern arbitrarily called fir tree Base materml testing
J , ~ II]1111111
IIIIIII1
Recent experience has shown that only round defects such as pores or holes, may be assessed by the present technique. There is a need for a catalogue of defects. Such a catalogue should ease analysis and assessment of signals, and thus identification of defects. For the investigation of transverse defects, holes of 4 mm were drilled in the weld vertically to the weld direction at depths of 10, 20, 30 and 40 mm. The holes extended into the base material to a depth of about 15 mm. Only the part of the holes in the base material was tested by ultrasonics; the signals were adjusted to fill the entire screen height. The holes in the welds were subsequently tested at the same parameters and the difference was measured; the decrease was 20 dB. The ratio o f signal to noise was only 2 dB, which means that this technique cannot detect transverse defects. This effect cannot be explained by the distinctive dendritic, structure alone, but is also due to the repeated transmission through the heat affected zone. This was demonstrated by the good through-transmission found in the transverse direction of the weld (that is the shortest distance through the weld) using a normal probe at 2 MHz. However, using the same probe and technique, but transmitting vertically through the weld, the sttenuation is so large that no echo was received (Figs 6, 7). It was also observed that the attenuation decreases considerably when the probe and the beam moves from the weld to the base material. The signal on the screen increases to full height. These experiments support the theoretical explanation given above and further
Weld testing Base
I i
500 mm Fig. 6 The researchers used a 2 M H z normal probe to test the weld and its surrounds
supports the view that transverse defects can only be detected if they extend into the base material.
Conclusion It must be noted that the overall attenuation in the austenitic weld differs from that in the base material by a factor of 10. The ratio o f signal to noise is about 6 to 10 dB for weld inspection where the probe is placed on the base material in contrast to a ratio of signal to noise of only 2 dB where the probe is placed on the weld as is necessary to inspect for transverse defects.
These and the other negative results mentioned do not rule out ultrasonic testing of austenitic welds but further development and improvement is necessary before ultrasonics can be considered suitable for this application. It must be stressed that this experience is relevant only to austenitic steel similar to AISI 304 ss. Literature
Kuhlow, B., Neumann, E. Ultrasonic' tcstin,g of austcnitic steel weld joints, IAEA-SM 195-24
Weld tesh ng BOO
Base-mater~al testing
IO0
Transverse
,oo
Transverse weld testing
r~
o~ _=¢:L E
o
a Fig. 7
Rear walt AmphfJer 40dB
A
•=- 50 E
0 /
ID
A
I St echo Amplifier 40 dB Decrease20 dB
o C
A
Weld Amplifier 48 dB
M
Rear wall
The response f r o m the three probe positions were quite distinctive
NDT INTERNATIONAL
. O C T O B E R 1976
241
Most components for the primary and secondary systems of the liquid-metal fast-breeder reactor, the SNR 300, are made in austenitic steels because of the high working temperature. The quality of the material used is similar to the AISI 304 ss. The safety of the plant seemed to call for inspection of the welds by two volumetric methods, x-ray and ultrasonics. This paper reports our experience with ultrasonic testing of the reactor vessel. The ultrasonic inspection of austenitic welds by angle-beam and shear-wave techniques has proved nearly impossible up to the present time. There are two reasons for this. First, the grain size is critical in ultrasonic inspection. Austenitic steel has a coarse dendritic structure and it is well known that the ultrasonic scattering in an austenitic structure increases with its coarseness. Secondly, the heat-affected zone is anisotropic; the kind, location, and direction of the crystals in the heat-affected zone influence the scattering and absorption of ultrasound. Reasonable results have therefore always proved unobtainable by conventional ultrasonics because of simultaneous ultrasonic attenuation and high scattering: that is, a low ratio of signal to noise. A ratio of at least 10 dB seems necessary for reasonable detection of defects.
Research The federal materials testing establishment, the Bundesanstalt for Matefialprofung (BAM), in Berlin has carried out applied research on the subject. It is reported that a transmitter-receiver system, in which the two probes generate longitudinal-wave ultrasonic beams crossing in a limited focal area solves the problem. It should be noted that the technique does not only apply to longitudinal waves. These BAM probes are improvements of the longitudinal-wave probes (transceiver) probes already developed to detect cracks under cladding; in both cases the two transducers Gotz Herberg and Walter Muller are at Interatom GmbH in Bensberg near Cologne, West Germany and Otto Ganglbauer is at VbestAlpine in Linz, Austria.
NDT INTERNATIONAL . OCTOBER 1976
Elevation
Plan
~
b Austenltlcweld
e
\
Fig. 1 BAM probes transmit and receive ultrasound into and from a small area of the weld
are inclined towards one another and cross in the focal area. This technique is an improvement over the pulse-echo technique because, by crossing the beams in the focal area, the scattering echoes from the structure derive only from the limited focal area. On the other hand this technique means that only a limited layer is inspected. In practice it means that the inspected layers are about 15 to 20 mm thick (Fig. !). For this reason BAM developed a range of transceivers, all at 2 MHz (Table !). Their large size of 40 × 50 mm requires that the objects to be tested have a plane surface. Experiments were carried out to find whether the depth of the focal point and the centre of each layer coincided as calculated. The transducers incline towards one another so that the transmitter beam accurately crosses the receiver beam. Using a delay block made of polymethyl metha-
239
40 to 60 mm. The ultrasonic results were compared with the earlier x-ray inspection.
Table 1. The BAM range of special transceivers
Designation
Angle [o]
Focal length [mm]
Field depth [mm]
1 TR-Long probe 2 ,, 3 ,, 4 ,,
70 70 60 50
18 65 75 82
0-15 15- 30 3 0 - 45 45-60
A
2-
/,,
/
~,./ / /
.,~"~'-,,,, ~ ' ~ . \ .
"x,.
/'iX"
4.
/
\.
"-,,,',,
/'
Field depth E
8
-/
,4
/
~
I~ / 15
/
~ ~ 5 - 3 o m m / / ~_~.~__...-~
,
25
,
\
3,5 45 Woll thickness [mrn]
30 - 45 mm ~/"
,
55
,
65
N
"
7,5
The ultrasonic signals could be correlated only partially with the x-radiography. On the other hand, some defects detected by radiography could not be found by ultrasonics. Those areas of the welds which gave rise to ultrasonic signals but showed nothing on radiographs were ground to the depth calculated for the assumed defect and inspected by dye penetrant. Only one defect, a pore, which confirmed the ultrasound signals, was found. It is possible that small flaws in the material, such as fissures, were obscured by grinding, or that the heat caused by grinding dried the dye penetrant. It is also possible that a change of ultrasonic velocity from the base material to the weld caused a slight refraction which distorted indication of the depth of defect. As the result may have been influenced by these effects, it seems possible only exact sectioning can give an answer about the type and location of defects. The welds had to be ground before ultrasonic inspection and consequently undercuts in the weld area caused disturbances as the probes were partly tipped. Coupling was difficult in these cases, and caused spurious signals which could only be discounted as such after several tests at the same spot. A different phenomenon sometimes appeared to originate near the surface and was arbitrarily called fir tree (Fig. 5). Experimental work lead us to an explanation of the pheno-
Fig. 2 The amplitude of the response reaches a m a x i m u m in the focal area
crylate means that a definite focal area is achieved. Experiments showed that the focal areas were larger than expected, implying an overlapping (Fig. 2). This became apparent by comparing echo height with reflector distance made for each probe. This was accepted as the probable maximum which could be achieved whether at the centre of each layer or not, without further attenuation loss. ltence a more accurate estimation of the size of defects is possible. For the first ultrasonic tests Vtiest-Alpine produced a submerged welded plate of 50 mm wall thickness of quality similar to AISI 304 ss. Three holes of 2 mm diameter were drilled in the weld longitudinally at different depths. The holes were so placed that the velocity of sound through the weld was greater in one probe direction than from its opposite. Holes of 2 mm diameter were also drilled in the base material in the same direction and at the same depths. All holes were 35 mm deep. Ultrasonic investigation of the base material and weld showed the attenuation to be about 6 to 10 dB lower in the base than in the weld material. Shear-wave signals also appeared. However, these arrived behind the longitudinal-waves signal because of their lower velocity (Fig. 3).
Fig. 3 The oscilloscope shows t w o signals; the first t o arrive (left) is longitudinal-wave and the second shear-wave
The shear-wave signals were seen only as reflections from the holes and were disregarded. To avoid misinterpretation the oscilloscope screen was partly obscured to include only longitudinal-wave signals (Fig. 4). Results
For the ultrasonic investigation of the welds of the reactor vessel, test holes of 2 mm diameter were made in the welds of test pieces and tested ultrasonically. The lower signal was set at 60% of the screen height. All the welds were inspected in this way: submerged-arc and manual welds, AISI 304 ss,
240
Fig, 4 A paper mask covered the part of the screen showing shearwave response
NDT INTERNATIONAL. OCTOBER 1976
menon. The crystal "structure in the transitional area between the base material and the weld causes portions of an incident beam at a certain phase to be reflected with a phase shift of 2 or a multiple of 2. These reflected soundwaves may yield a combined signal of intensity equal to a real signal. Terraced structure could be the cause of such phase shift as these structures often occur in metallographic specimens of austenitic welds. This theory is supported by the fact that fir tree disappears when the angle or the direction of the incident beam is changed. Further work is necessary if this phenomenon is to be avoided. In addition, another test plate was fabricated with natural defects, including lack of fusion, slag lines, pores, and root defects. These defective areas were ultrasonically tested with the parameter fixed and all signals were photographed on the screen. It was found that the ratio of the height of echo between artificial and real defects does not def'me the type and size of the real defect. The height of the signal of unacceptable lack of fusion, for instance, is sometimes only slightly above the limit of registration. Consequently it is not yet possible to analyse defects by comparison of echo height for artificial and real defects.
Fig. 5 The response f r o m a 2 M H z , 70 °, 20 mm probe showed an interfence pattern arbitrarily called fir tree Base materml testing
J , ~ II]1111111
IIIIIII1
Recent experience has shown that only round defects such as pores or holes, may be assessed by the present technique. There is a need for a catalogue of defects. Such a catalogue should ease analysis and assessment of signals, and thus identification of defects. For the investigation of transverse defects, holes of 4 mm were drilled in the weld vertically to the weld direction at depths of 10, 20, 30 and 40 mm. The holes extended into the base material to a depth of about 15 mm. Only the part of the holes in the base material was tested by ultrasonics; the signals were adjusted to fill the entire screen height. The holes in the welds were subsequently tested at the same parameters and the difference was measured; the decrease was 20 dB. The ratio o f signal to noise was only 2 dB, which means that this technique cannot detect transverse defects. This effect cannot be explained by the distinctive dendritic, structure alone, but is also due to the repeated transmission through the heat affected zone. This was demonstrated by the good through-transmission found in the transverse direction of the weld (that is the shortest distance through the weld) using a normal probe at 2 MHz. However, using the same probe and technique, but transmitting vertically through the weld, the sttenuation is so large that no echo was received (Figs 6, 7). It was also observed that the attenuation decreases considerably when the probe and the beam moves from the weld to the base material. The signal on the screen increases to full height. These experiments support the theoretical explanation given above and further
Weld testing Base
I i
500 mm Fig. 6 The researchers used a 2 M H z normal probe to test the weld and its surrounds
supports the view that transverse defects can only be detected if they extend into the base material.
Conclusion It must be noted that the overall attenuation in the austenitic weld differs from that in the base material by a factor of 10. The ratio o f signal to noise is about 6 to 10 dB for weld inspection where the probe is placed on the base material in contrast to a ratio of signal to noise of only 2 dB where the probe is placed on the weld as is necessary to inspect for transverse defects.
These and the other negative results mentioned do not rule out ultrasonic testing of austenitic welds but further development and improvement is necessary before ultrasonics can be considered suitable for this application. It must be stressed that this experience is relevant only to austenitic steel similar to AISI 304 ss. Literature
Kuhlow, B., Neumann, E. Ultrasonic' tcstin,g of austcnitic steel weld joints, IAEA-SM 195-24
Weld tesh ng BOO
Base-mater~al testing
IO0
Transverse
,oo
Transverse weld testing
r~
o~ _=¢:L E
o
a Fig. 7
Rear walt AmphfJer 40dB
A
•=- 50 E
0 /
ID
A
I St echo Amplifier 40 dB Decrease20 dB
o C
A
Weld Amplifier 48 dB
M
Rear wall
The response f r o m the three probe positions were quite distinctive
NDT INTERNATIONAL
. O C T O B E R 1976
241