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Application of ultrasonic testing techniques on austenitic welds for fabrication and in-service inspection
Application of ultrasonic testing techniques on austenitic welds for fabrication and in-service inspection X. Edelmann This paper describes some practical considerations relating to the ultrasonic testing of weld joints in austenitic components, based on the author's experience with components manufactured for, and in service in, nuclear power stations. Problems caused by the effect of grain structure, spurious indications and attenuation of the ultrasound are discussed. The importance of acquiring adequate data from reference blocks is emphasized, and the uses and limitations of various types of angle probe are illustrated. Introduction The demand for safety and operational surveillance of nuclear power installations, in particular with regard to leaks in austenitic pipes which are mainly caused by intergranular stress corrosion cracking, is forcing engineers to find ways and means to test austenitic weld joints) In view of the advantages of ultrasonic testing, which is used with great success for weld joints from carbon steel, its application on austenitic welds seems to be obvious. Subsequent operational surveillance can be rendered even more effective by using experience of instrumental responses gathered during testing at the stage of fabrication. The author's company is a manufacturer of components for nuclear power plants. The laboratory for non-destructive testing gained its experience in ultrasonic testing of austenitic welds on components in fabrication which often undergo an ultrasonic examination. 2'3 Furthermore, in-service inspections have been performed on austenitic components of Swiss nuclear power plants, Boiling Water Reactors (BWR) as well as Pressurised Water Reactors (I'WR).4
Problems involved The difficulties in connection with ultrasonic testing of austenitic welds and parent metals are well-known. The problems are connected with the metallurgical structure. The elastic anisotropy of the different grains leads to scattering coupled with mode conversion. Disturbing echos are met as 'noise'. The sonic energy is attenuated, while scatter echos and grain boundary ('grass') patterns appear. It is important that the signal-to-noise ratio, which denotes the ratio between the evaluated ultrasonic indication of the actual signal and the noise due to scatter and interference echos, should be as high as possible. Differences in grain sizes and orientations influence testability. Weld zones are characterized by columnar grains which
behave quite differently compared to the grain structure in the parent metal. For ultrasonic testing a very clear distinction must be made between parent metal and weld metal, and between fabrication and in-service inspection.
Austenitic materials and weld joints When talking about the testability of austenitic structures, the term 'austenitic' covers a variety of materials. It includes stainless steels and nickel-chromium alloys, such as Inconel, Incoloy etc, as well as welds between dissimilar metals. The distinction between different materials and between parent metal and weld metal is very important; whereas parent metal may be rolled, drawn, forged or cast, a wide range of processes is used for welding: shielded metal .arc, submerged arc, TIG, EB, etc. The problems of testing differ according to material, production and process method. In the manufacturing process a component may undergo one or more heat treatments which can influence grain structure, especially the grain size. Fig. 1 shows a Nimonic forging with varying grain size and its effects. At two positions on the forging, an attempt was made to obtain four backwaU echos with a 4 MHz straightbeam probe. In one position the only indications which can be seen are those from the coarse-grained structure. In the author's experience, control of the grain configuration is at least as important as searching for flaws, eg laminations. Grain configuration can vary from component to component even within one workpiece. Great differences can be met. This may indicate an incorrect production process, eg incorrect forging or faulty heat treatment. The following example (Fig. 2) shows such an influence on grain configuration. A solution heat treatment was specified which, in this case, should not have influenced the grain size. Comparative ultrasonic measurements showed remarkable differences in sound transparency between the plate
0308-9126/81/030125-09 $02.00 © IPC BusinessPress 1981
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a
reduction of the testing frequency. Instead of investigating why these difficulties have occurred and what could be possible consequences for welding and for later use in service of the component, efforts may be concentrated, possibly unwisely, on sufficient sound transparency.
b
(2) I 20ram t
/
a
• i
Incorrect heat treatment and larger grain sizes can have a bad influence on the occurrence of intergranular stress corrosion cracking.
Testing o f austenitic welds during fabrication of a component Only grain indications
Variety of austenitic welds
Figures 3 and 4 show some macrographic sections through different types of welds as they can be found in BWRs and PWRs respectively. One can state that great differences
b
Four back echos
Probe: Krautkr~imer B4F Fig. 1
Effect of variation in grain size in a N i m o n i c forging
TIG mech, material : X 5 Cr Ni 18 9
Without heat treatment
With heat treatment I 20mm 1 Fig. 2 Influence of heat treatment on grain configuration in an austenitic weld j o i n t
which had been subject to heat treatment and that which had not. A study of the macrographic section showed the very different grain configurations, which led us to conclude that something had gone wrong with the heat treatment. In this connection two facts have to be pointed out: (1)
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Grain sizes in the parent metal from ASTM No. 2 (mean grain diameter 0.2 mm) to No. 1 (0.280 mm diameter), and 0 (0.4 mm diameter) and larger, together with a certain content of impurities such as phosphorus and sulphur can produce great problems during welding. Often, microfissures may occur to a large extent. These fissures are intergranular in the heat affected zone of the weld beginning perpendicularly at the fusion line. There are cases where, in the stage of inspection of the parent metal, testability can only be achieved by
20 mm
I Fig. 3
t
Typical austenitic welds in a Boiling Water Reactor
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The great number of probes makes it difficult to judge whether or not a certain probe is the best for a specific application. Therefore, reference blocks are absolutely necessary. Development of a testing technique
~ ~~i!® ~i!li~i ~
I Fig. 4
!
2
20ram
Typical austenitic welds in a Pressurised Water Reactor
A testing technique can be developed by using reference blocks. These reference blocks and reflectors are necessary both for longitudinal and transverse defects; the most suitable reflectors are side-drilled holes at different depths and, for defects close to or at the surface, notches. The best position for the side-drilled holes is at the transition line between the parent and weld material. Fig. 5 shows an example of a distance-amplitude-correction (DAC) curve from such holes on a weld joint between dissimilar metals. The indications from the various reflectors with the beam passing through the weld have been exposed successively on a photograph of the screen. The DAC curve from the opposite direction, ie only through the fine-grained base material, is quite different. A comparison of the two DAC curves gives the opportunity of assessing the absorption, scattering and refraction effects of the materials. In addition, a decision can be made as to whether the weld is to be examined with one procedure or in different depth zones with different probes. Spurious indications
occur both in parent as well as in weld metal. There are dissimilar metal welds, connecting different types of base metals. Carbon steels are buttered against the weld joint. It is obvious that each kind of weld joint has to be investigated separately.
When examining welds in the field, spurious indications are frequently met. These indications are caused mainly by the metallurgical structure conditions or by the weld seam geometry. When using longitudinal wave angle probes, the shear wave component can also be the cause.
Reference blocks
For the development of an examination method, one of the most important steps is the fabrication of an adequate number of reference blocks. These blocks must be of the same parent and welding materials as the object to be examined. Welding must also be carried out with the same welding techniques, the weld seam geometry must be the same, and welding parameters, welding position, weld temperature gradients etc must be kept practically identical. It is also of great importance that the reference blocks are heat treated in the same manner as the component.
Materials : X 20 Cr MoVI21
Inconel
10CrMo 9 I0
Advanced ultrasonic testing techniques
When choosing a suitable technique for a given examination problem, the use of conventional probes should always be considered first. If no success can be obtained, special techniques have to be applied. The possibilities can be divided into two groups: special probes and signal processing, each of which can bring improvements. The former may employ angle probes using longitudinal waves, short pulses, polarised shear waves, reduction of the testing area by transmitter/ receiver techniques, focussing probes etc, while the latter may involve signal analysis, signal averaging, adapted learning networks, frequency variation, etc. In Europe good results have been achieved with angle probes using longitudinal waves, often by means of the transmitter/receiver technique, s'6'7 A wide variety of longitudinal wave angle probes is available on the market. One difficulty in the practical application of these special probes is presented by the shear waves which accompany longitudinal waves at an oblique angle.
NDT I N T E R N A T I O N A L . JUNE 1981
I
20mm
I
"N---X-OAC through parent metal - 0 - - 0 DAC through weld metal Probe : RTD,TRL 4 5 ° 4 MHz f-30mm Fig. 5 Distance-amplitude-correction (DAC) curve f r o m side-drilled holes on the transition line between parent and weld metal
127
An example of a spurious indication is shown in Fig. 6. Here, during examination of a weld with 45 ° longitudinal waves, a part of the waves is deflected at the transition between the stainless steel parent metal and the lnconel weld and is incident perpendicular to the opposite surface. This effect can be confirmed by dabbing the spot in quetion on the opposite surface with contact medium, so that the indication on the screen is affected. As soon as the beam exit point of the probe is moved over the transition line, this indication disappears.
Submerged-arc weld
Crack
Material X2 CrNIMoI8 12
Test results achieved with special probes The following two examples show the possibilities of the application of angle probes using longitudinal waves.
Magnification
Fig. 7 shows a section through an austenitic submerged arc weld in which is a 3 mm high longitudinal crack, together with the corresponding ultrasonic indication on the screen. The echo heights of reference holes, 2 mm in diameter, with the beam passing through the weld have been marked on the screen. For this type of defect, the most suitable probe is a small transmitter/receiver longitudinal wave 70 ° angle probe at 4 MHz. Such probes can also be used for the examination of cracks in or under cladding. Even surface cracks can be found in this way.
Crack
Probe Krautkramer WSY 70°TRL,4MHz
The next example is shown in Fig. 8 where a comparison is made among three different types of probes on a 45 mm thick automatically welded TIG Inconel weld. The investigation was made with longitudinal wave 45 ° probes from different manufacturers. Of course, each probe produces a different DAC curve from the 2 mm diameter side-drilled holes (7, 17, 27 and 37 mm from the surface on the weld/parent metal transition) with the beam passing through the weld. As noted previously, the photographs of the screen have been exposed successively, showing in each case all four indications. The probes were then set to find the 1 mm high lack of fusion defect shown, using the individual DAC settings. The resulting indication is shown for each probe. Due to the orientation of this defect, it is best found and gives the
Indications : X Reference holes 2ram
dia I
Flaw indication long.
Z Spurious indications trans,
I Fig. 7
2
Detection of crack in submerged-arc weld
strongest indication through the weld. Differences in the relationship between the DAC curve and the indication height for each probe are obviously due to the difference in probe characteristics. 20 mm
k
I
Apparent flaw
Stolnless steel
I B = Inconel butter weld IW = Inconel weld
As can be seen, some good results have been achieved by the application of special techniques at the stage of fabrication. But a general solution of the problem is still far away. Further experience must be gained in the very near future. Above all, a reasonable sensitivity setting must be determined for the various probes in relation to the flaws to be found. It is therefore important to make as many sections as possible through defects found and to follow closely and record any grinding-out of defects during repair. Information about the position, size and nature of defects thus investigate( can then be compared with the ultrasonic examination results. By a critical analysis of test results, we can make a step forward to a successful testing procedure. This requires, however, effective collaboration between manufacturer, customer and acceptance authorities.
In-service inspection Fig. 6 Spurious indication w i t h 4 5 ° longitudinal waves at the transit i o n between stainless steel parent metal and |nconel weld
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An in-service inspection is often performed under tess than
NDT I N T E R N A T I O N A L . JUNE 1981
Lack of fusion
Magnification IOx
DAC curves
Flaw indications
Sonatest,45°L, 2.25 MHz Fig. 8
Comparison of indications of defect in TIG Inconel weld from three different probes
ideal circumstances. Apart from the ambient conditions such as radiation and temperature, the components to be tested have not been designed and made to facilitate testing. For instance, the actual metallurgical structure may not be accurately known.
When applying longitudinal waves - which are able to cross the weld metal - special care has to be taken. For examination with the corner effect under 45 ° for flaws at the inner surface, some effects have to be taken into consideration. Fig. 10 shows a comparison of the amplitudes of reflections
Furthermore, geometric conditions at the inner and outer surface can bring problems. Often one has to decide during an inspection how to proceed because actual circumstances do not correspond exactly to those prescribed in the drawing. Particular difficulties are caused by the special type of expected flaws which can differ dramatically from those met during fabrication of a component.
Surface
l
The surface area can be covered by special transmitter/ receiver longitudinal (TRL) angle probes with an incidence angle of 70 ° or greater. With these probes it is often possible to detect cracks starting at the outer surface. It might also be possible to find cracks which had started at the inner surface, subsequently progressed but not yet penetrated to the outer surface. For the inner surface a 45 ° shear wave probe with a suitable frequency can be applied because the service-induced flaws occur mostly beside the root of the weld.
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Notch
(
70 ° dual search unit Ionqitudinal waves
An in-service inspection should - in the author's experience be concentrated on the areas near the inner and outer surfaces; Fig. 9 shows the technique applied with the help of a reference block. Suitable reference reflectors are notches at the inner and outer surface and side-drilled holes (SDH)in the fusion line between parent and weld metal.
INTERNATIONAL
and near surface
Side drilled holes
Principles of in-service inspection
NDT
Krautkr~Jmer VRY, 45°TRL~2MHz
RTD,45*TRL,f = 30,4MHz
1981
Root oleo incl.
volume
Side drilled holes
45 ° search unit shear : through parent metal long,: through weld metal
Fig. 9
Illustration of in-service inspection technique, using a
reference block
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Grain size
Saturation o,,, o--
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Network of cracks grain disintegration
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Long waves
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Depth of notch ( r a m ) Fig. 10 C o m p a r i s o n of amplitudes of reflections from reference notches of different depths in a test block, for shear and longitudinal waves
from notches of different depths in a test block 50 mm thick. A 2.25 MHz crystal, size 0.5 x 0.75 in (13 x 19 mm), with two wedges producing shear waves and longitudinal waves at an incidence angle of 45 ° in steel is used. The results using the shear wave probe show an increase in amplitude with increasing notch depth. The longitudinal (compressional) angle probe produces indications which do not even allow dimensioning, by means of amplitudes of notches, let alone cracks. A very important fact for application of shear wave probes is the region of saturation which begins in this case at a notch depth of about 3 mm. These results are very important for the sensitivity setting for an in-service inspection. The behaviour of notches in the sound field of a probe has to be taken into strict consideration. If this is not the case, large errors in the test result can be the consequence. The problem of saturation is highlighted if we consider a notch depth of 10% of sample thickness, according to ASME Code Section XI. In our case this is a notch depth of 5 mm, which is in the saturation region as far as the applied probe is concerned. This means that the 5 mm deep notch gives the same ultrasonic indication as an infinitely deep notch - equivalent to a through-wall flaw! I ntergranular
stress corrosion cracking (IGSCC)
Ultrasonic testing for IGSCCidentification still creates great problems. The effects considered in all of the previous paragraphs are unfortunately valid also for components with IGSCC. Fig. 11 shows the most important factors which limit testability and have great influence on the test result. One meets different grain sizes again from component to component and even within one component, with corresponding influence on attenuation. The crack may have unfavourable shape and orientation, which makes it a bad reflector. In the limit, the occurrence of a crack network
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Shape
of crack
Fig. 11 Factors i n f l u e n c i n g t h e results of tests f o r i n t e r g r a n u l a r stress c o r r o s i o n c r a c k i n g
may, together with grain disintegration, prevent any reflection because the grain structure is no longer capable of transferring ultrasonic energy. For correct ultrasonic testing one should have a reference block as similar as possible to the object to be tested. One cannot use an arbitrary piece of austenitic steel with reference reflectors. Furthermore, a comparison of ultrasound attenuation from the reference block to different locations on the object to be tested has to be performed. Such a comparison can be made with two identical shear wave probes set in V-path (Fig. 12), both on the reference block and on the test object. If the different sound transparency is not corrected, the examination result can be worthless. Another point which has to be taken into account is the often less than ideal geometry. The objects to be tested seldom have machined inner and outer surfaces. Thickness can change, counterbores, excessive penetration at the root of the weld and the weld bead surface limit the possibilities of ultrasonic testing. In many cases these factors are identified at the beginning of an actual examination only. Each weld must be investigated as an individual problem, which demands a lot of the testing personnel. In the author's company a special training programme has been established. Another fact has to be kept in mind: even today it is not possible to give a reliable value for crack depth by ultrasonic means because a crack can be a very bad reflector due to its form, which is not known in advance!
NDT I N T E R N A T I O N A L . JUNE 1981
Gain setting
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Fig. 1 2 Arrangement of shear wave probes to establish sensitivity correction for different ultrasound attenuation in reference block and test piece
Thick wall cast components
A special problem is the ultrasonic testing of cast components. In Westinghouse type PWRs the main cooling pipes are made of austenitic steel in a thickness range of 60-85 mm. At the Beznau power plants I and II an in-service inspection of austenitic welds of the primary coolant system was performed in 1979. Cast elbows welded together with different types of austenitic pipe materials were encountered. A report about this problem has been published. 4 Here, the most important results will be mentioned only. Fig. 13 shows the attenuation for different frequencies in a 40 mm thick piece of austenitic casting - which is typical for elbows. It is obvious that 1 MHz is the most suitable frequency. But great differences in attenuation may occur in the various components. Fig. 14 shows the reference block with reference reflectors, side-drilled holes and notches, for cast components welded together. Examination of the surface and near surface region is done with a special 70 ° TRL-probe. A 2 mm deep reference notch in the cast material at the surface and the 5 mm diameter SDH at a depth of 15 mm produce indications with a good signal to noise ratio for sound waves incident through the weld metal (Fig. 15). This probe has been developed for the detection of subcladding cracks. 8 The volume and the root area are examined by a special transmitter/receiver probe with longitudinal waves of 45 ° at a frequency of 1 MHz. 9 The indications of the 5 mm dia. SDHs at 15, 30 and 45 mm depth for sound waves incident through weld and parent metal can be seen in Fig. 16. In this case the sound transparency through the cast parent metal is worse than through the weld metal. This demonstrates that there exist cases where the problems in the parent metal are more pronounced than in the weld metal. The applied probe was the best available for the cast material in the above mentioned elbows. But even this probe is not sufficient for a thickness of 60 mm as can be seen in Fig. 17. The upper screen trace shows the noise level and the lower trace shows an indication which could be associated with the 4 mm deep notch. The notches of 1,2 and 3 mm, and in
NDT INTERNATIONAL
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Austenitic costing ASTM A551 CF8M thickness 40 mm
~ ~
......
20 mm I
I
Fig. 13 Ultrasound attenuation for different frequencies in an austenitic casting
actual testing also the 4 mm notch, would not be found. With shear waves there is no possiblity of detecting either the side drilled holes or the notches through the cast parent
Ce~ting G-X5CrNIMo 1810 A- ~kSIGr CF8M
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Reference reflectors
Notches Fig. 14 Reference block for austenitic weld of cast components in main cooling system of Pressurised Water Reactor
131
Through parent metal
Through weld metal
B2
Indication of the 2mm deep surface notch through cast parent metal B3
Vin~otte probe V 4 5 ° L SE 85F IMHz Fig. 16 Indications from testing of volume and root area of reference block (shown in Fig. 14) with 45 ° TRL probe
Notch (4ram deep) at inside surface Indication of the 5ram dia SDH at 15mm depth through weld metal Probe: RTD BAM, 70°SEL, subcladding crack probe Fig. 15 Indications f r o m testing of near surface area of reference block (shown in Fig. 14) with 70 ° TRL probe
metal. These facts together with those"demonstrated in Fig. 10 (notches as reference reflectors) forced us to concentrate the imservice inspection on those components which have a better sound transparency. Such components with a much better sound transparency than that of the reference blocks of Figs. 13 and 14 were found during in-service inspection and were carefully investigated.
Noise indications
Summary
There are a lot of cases where ultrasonic testing of austenititic components is possible, but a general solution is far away. Development has to be performed on a case-by-case basis. It is very important that each factor influencing the test result is taken into consideration. One has to differentiate between parent metal and weld metal. Manufacturing process, welding technique and heat treatment are very important influences on testability. Control of the grain configuration by means of measurements of the sound transparency may indicate an incorrect manufacturing process. Differentiation between fabrication and in-service inspection is most important. For both, adequate reference blocks are absolutely necessary. On these blocks the testing
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Notch indication Vin~otte probe V45°L SE 85F IMHz Fig. 17 Indications from inner surface of reference block (shown in Fig. 14) using 45 ° TRL probe
NDT INTERNATIONAL. JUNE 1981
problem can be analysed and a suitable testing technique and sensitivity laid down. In older plants the manufacturing of adequate reference blocks is a difficult task because similar materials and welding techniques to those used in the plant have to be repeated for the reference blocks. This procedure is necessary in order to produce the same ultrasonic response characteristics both in the plant and in the reference blocks. Unforeseen difficulties on the components to be tested can complicate the examination. Often, a definite technique can be set down only in course of the actual examination especially for in-service inspection - because of geometrical and metallurgical problems. These facts demand a lot of the testing personnel. Special training, to acquaint them with the problems encountered in the ultrasonic testing of austenitic welds, is therefore necessary.
References 1 2 3 4
sium on New Methods of Non-destructive Testing of Materials and their Application Especially in Nuclear Engineering, Saarbracken 17-19 September 1979 p 177
5
6
Conclusions Experience has to be gathered with the new testing techniques on actual flaws. Exact documentation and critical analysis of findings will allow a judgement of possibilities and limitations and will also help in taking corrective action in the development of testing techniques. Further research on the testing of strongly attenuating cast structures and on techniques of distinguishing false indications from real flaws is urgent. These developments must be tested in practice. Testability must be considered in the stage of fabrication of a component, in relation to geometry, accessibility, metallurgy and surface condition. Most important is a careful pre-service inspection of new components.
NDT I N T E R N A T I O N A L . JUNE 1981
Lapides, M.E. 'Improved ultrasonic inspection method for stainless steel piping' EPRI NP-1153 Special Report (August 1979) Edelmann, X. and Hornung, R., 'Erfahrungen im Prtffen von austenitischen Schweissverbindungen', Material und Technik 5 No 1 (1977) p 19 Edelmann, X., 'The application of ultrasonic testing of austenitic weld joints', Materials Evaluation 37 No 10 (1979) p 47 Edelmann, X., 'Wiederkehrende Prfffung mit UltraschaU an austenitischen Schweissverbindungen im Hauptkt/hlmittelsystem von Druckwasserreaktoren', Conf Proc International Sympo-
7 8
WUstenberg,H., Just, T., MOhrle, W., and Kutzner, J., 'Zur Bedeutung fokussierender PrtffkOpfefor die Ultraschallprafung yon Schweissn~hten mit austenitischem Gefttge', Materialprtlfung 19 No 7 (1977) p 246 Just, T., ROmer, M., Neumann, E., Matthies, K., and Kuhlow, B., 'Leistungsfa'higkeit verschiedener Ultraschallprtlftechniken an austenitischen Probeschweissungen mit nattlrlJchen Tesffehlern',Materialprttfung 19 No 12 (1977) p 488 Herberg, G., Laufer, W. et al, 'Statusbericht zur Ultraschallprdfung an austenitischen SchweissnaTtten',Materialprttfung 20 No 3 (1978) p 120 WUstenberg,H. and Schulz, E., 'Versuche zur Feststellung plattierungsnaher Reflexionstellen an Reaktorteilen mit Ultraschall' (DGZfP-Vortragstagung, Saarbrtlcken, 1972)
9
Caussin, P. and Cermak, J., 'Performances of the ultrasonic examination at austenitic steel components' Conference on Periodic Inspection o f Pressurized Components, London 8-10 May 1979 (I Mech E, 1979) C 49/79 p 207
Author Mr Edelmann is with Sulzer Brothers Limited, Dept NDT - 1513, CH-8401, Winterthur, Switzerland.
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Problems involved The difficulties in connection with ultrasonic testing of austenitic welds and parent metals are well-known. The problems are connected with the metallurgical structure. The elastic anisotropy of the different grains leads to scattering coupled with mode conversion. Disturbing echos are met as 'noise'. The sonic energy is attenuated, while scatter echos and grain boundary ('grass') patterns appear. It is important that the signal-to-noise ratio, which denotes the ratio between the evaluated ultrasonic indication of the actual signal and the noise due to scatter and interference echos, should be as high as possible. Differences in grain sizes and orientations influence testability. Weld zones are characterized by columnar grains which
behave quite differently compared to the grain structure in the parent metal. For ultrasonic testing a very clear distinction must be made between parent metal and weld metal, and between fabrication and in-service inspection.
Austenitic materials and weld joints When talking about the testability of austenitic structures, the term 'austenitic' covers a variety of materials. It includes stainless steels and nickel-chromium alloys, such as Inconel, Incoloy etc, as well as welds between dissimilar metals. The distinction between different materials and between parent metal and weld metal is very important; whereas parent metal may be rolled, drawn, forged or cast, a wide range of processes is used for welding: shielded metal .arc, submerged arc, TIG, EB, etc. The problems of testing differ according to material, production and process method. In the manufacturing process a component may undergo one or more heat treatments which can influence grain structure, especially the grain size. Fig. 1 shows a Nimonic forging with varying grain size and its effects. At two positions on the forging, an attempt was made to obtain four backwaU echos with a 4 MHz straightbeam probe. In one position the only indications which can be seen are those from the coarse-grained structure. In the author's experience, control of the grain configuration is at least as important as searching for flaws, eg laminations. Grain configuration can vary from component to component even within one workpiece. Great differences can be met. This may indicate an incorrect production process, eg incorrect forging or faulty heat treatment. The following example (Fig. 2) shows such an influence on grain configuration. A solution heat treatment was specified which, in this case, should not have influenced the grain size. Comparative ultrasonic measurements showed remarkable differences in sound transparency between the plate
0308-9126/81/030125-09 $02.00 © IPC BusinessPress 1981
NDT INTERNATIONAL. JUNE 1981
125
a
reduction of the testing frequency. Instead of investigating why these difficulties have occurred and what could be possible consequences for welding and for later use in service of the component, efforts may be concentrated, possibly unwisely, on sufficient sound transparency.
b
(2) I 20ram t
/
a
• i
Incorrect heat treatment and larger grain sizes can have a bad influence on the occurrence of intergranular stress corrosion cracking.
Testing o f austenitic welds during fabrication of a component Only grain indications
Variety of austenitic welds
Figures 3 and 4 show some macrographic sections through different types of welds as they can be found in BWRs and PWRs respectively. One can state that great differences
b
Four back echos
Probe: Krautkr~imer B4F Fig. 1
Effect of variation in grain size in a N i m o n i c forging
TIG mech, material : X 5 Cr Ni 18 9
Without heat treatment
With heat treatment I 20mm 1 Fig. 2 Influence of heat treatment on grain configuration in an austenitic weld j o i n t
which had been subject to heat treatment and that which had not. A study of the macrographic section showed the very different grain configurations, which led us to conclude that something had gone wrong with the heat treatment. In this connection two facts have to be pointed out: (1)
126
Grain sizes in the parent metal from ASTM No. 2 (mean grain diameter 0.2 mm) to No. 1 (0.280 mm diameter), and 0 (0.4 mm diameter) and larger, together with a certain content of impurities such as phosphorus and sulphur can produce great problems during welding. Often, microfissures may occur to a large extent. These fissures are intergranular in the heat affected zone of the weld beginning perpendicularly at the fusion line. There are cases where, in the stage of inspection of the parent metal, testability can only be achieved by
20 mm
I Fig. 3
t
Typical austenitic welds in a Boiling Water Reactor
NDT INTERNATIONAL
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The great number of probes makes it difficult to judge whether or not a certain probe is the best for a specific application. Therefore, reference blocks are absolutely necessary. Development of a testing technique
~ ~~i!® ~i!li~i ~
I Fig. 4
!
2
20ram
Typical austenitic welds in a Pressurised Water Reactor
A testing technique can be developed by using reference blocks. These reference blocks and reflectors are necessary both for longitudinal and transverse defects; the most suitable reflectors are side-drilled holes at different depths and, for defects close to or at the surface, notches. The best position for the side-drilled holes is at the transition line between the parent and weld material. Fig. 5 shows an example of a distance-amplitude-correction (DAC) curve from such holes on a weld joint between dissimilar metals. The indications from the various reflectors with the beam passing through the weld have been exposed successively on a photograph of the screen. The DAC curve from the opposite direction, ie only through the fine-grained base material, is quite different. A comparison of the two DAC curves gives the opportunity of assessing the absorption, scattering and refraction effects of the materials. In addition, a decision can be made as to whether the weld is to be examined with one procedure or in different depth zones with different probes. Spurious indications
occur both in parent as well as in weld metal. There are dissimilar metal welds, connecting different types of base metals. Carbon steels are buttered against the weld joint. It is obvious that each kind of weld joint has to be investigated separately.
When examining welds in the field, spurious indications are frequently met. These indications are caused mainly by the metallurgical structure conditions or by the weld seam geometry. When using longitudinal wave angle probes, the shear wave component can also be the cause.
Reference blocks
For the development of an examination method, one of the most important steps is the fabrication of an adequate number of reference blocks. These blocks must be of the same parent and welding materials as the object to be examined. Welding must also be carried out with the same welding techniques, the weld seam geometry must be the same, and welding parameters, welding position, weld temperature gradients etc must be kept practically identical. It is also of great importance that the reference blocks are heat treated in the same manner as the component.
Materials : X 20 Cr MoVI21
Inconel
10CrMo 9 I0
Advanced ultrasonic testing techniques
When choosing a suitable technique for a given examination problem, the use of conventional probes should always be considered first. If no success can be obtained, special techniques have to be applied. The possibilities can be divided into two groups: special probes and signal processing, each of which can bring improvements. The former may employ angle probes using longitudinal waves, short pulses, polarised shear waves, reduction of the testing area by transmitter/ receiver techniques, focussing probes etc, while the latter may involve signal analysis, signal averaging, adapted learning networks, frequency variation, etc. In Europe good results have been achieved with angle probes using longitudinal waves, often by means of the transmitter/receiver technique, s'6'7 A wide variety of longitudinal wave angle probes is available on the market. One difficulty in the practical application of these special probes is presented by the shear waves which accompany longitudinal waves at an oblique angle.
NDT I N T E R N A T I O N A L . JUNE 1981
I
20mm
I
"N---X-OAC through parent metal - 0 - - 0 DAC through weld metal Probe : RTD,TRL 4 5 ° 4 MHz f-30mm Fig. 5 Distance-amplitude-correction (DAC) curve f r o m side-drilled holes on the transition line between parent and weld metal
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An example of a spurious indication is shown in Fig. 6. Here, during examination of a weld with 45 ° longitudinal waves, a part of the waves is deflected at the transition between the stainless steel parent metal and the lnconel weld and is incident perpendicular to the opposite surface. This effect can be confirmed by dabbing the spot in quetion on the opposite surface with contact medium, so that the indication on the screen is affected. As soon as the beam exit point of the probe is moved over the transition line, this indication disappears.
Submerged-arc weld
Crack
Material X2 CrNIMoI8 12
Test results achieved with special probes The following two examples show the possibilities of the application of angle probes using longitudinal waves.
Magnification
Fig. 7 shows a section through an austenitic submerged arc weld in which is a 3 mm high longitudinal crack, together with the corresponding ultrasonic indication on the screen. The echo heights of reference holes, 2 mm in diameter, with the beam passing through the weld have been marked on the screen. For this type of defect, the most suitable probe is a small transmitter/receiver longitudinal wave 70 ° angle probe at 4 MHz. Such probes can also be used for the examination of cracks in or under cladding. Even surface cracks can be found in this way.
Crack
Probe Krautkramer WSY 70°TRL,4MHz
The next example is shown in Fig. 8 where a comparison is made among three different types of probes on a 45 mm thick automatically welded TIG Inconel weld. The investigation was made with longitudinal wave 45 ° probes from different manufacturers. Of course, each probe produces a different DAC curve from the 2 mm diameter side-drilled holes (7, 17, 27 and 37 mm from the surface on the weld/parent metal transition) with the beam passing through the weld. As noted previously, the photographs of the screen have been exposed successively, showing in each case all four indications. The probes were then set to find the 1 mm high lack of fusion defect shown, using the individual DAC settings. The resulting indication is shown for each probe. Due to the orientation of this defect, it is best found and gives the
Indications : X Reference holes 2ram
dia I
Flaw indication long.
Z Spurious indications trans,
I Fig. 7
2
Detection of crack in submerged-arc weld
strongest indication through the weld. Differences in the relationship between the DAC curve and the indication height for each probe are obviously due to the difference in probe characteristics. 20 mm
k
I
Apparent flaw
Stolnless steel
I B = Inconel butter weld IW = Inconel weld
As can be seen, some good results have been achieved by the application of special techniques at the stage of fabrication. But a general solution of the problem is still far away. Further experience must be gained in the very near future. Above all, a reasonable sensitivity setting must be determined for the various probes in relation to the flaws to be found. It is therefore important to make as many sections as possible through defects found and to follow closely and record any grinding-out of defects during repair. Information about the position, size and nature of defects thus investigate( can then be compared with the ultrasonic examination results. By a critical analysis of test results, we can make a step forward to a successful testing procedure. This requires, however, effective collaboration between manufacturer, customer and acceptance authorities.
In-service inspection Fig. 6 Spurious indication w i t h 4 5 ° longitudinal waves at the transit i o n between stainless steel parent metal and |nconel weld
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An in-service inspection is often performed under tess than
NDT I N T E R N A T I O N A L . JUNE 1981
Lack of fusion
Magnification IOx
DAC curves
Flaw indications
Sonatest,45°L, 2.25 MHz Fig. 8
Comparison of indications of defect in TIG Inconel weld from three different probes
ideal circumstances. Apart from the ambient conditions such as radiation and temperature, the components to be tested have not been designed and made to facilitate testing. For instance, the actual metallurgical structure may not be accurately known.
When applying longitudinal waves - which are able to cross the weld metal - special care has to be taken. For examination with the corner effect under 45 ° for flaws at the inner surface, some effects have to be taken into consideration. Fig. 10 shows a comparison of the amplitudes of reflections
Furthermore, geometric conditions at the inner and outer surface can bring problems. Often one has to decide during an inspection how to proceed because actual circumstances do not correspond exactly to those prescribed in the drawing. Particular difficulties are caused by the special type of expected flaws which can differ dramatically from those met during fabrication of a component.
Surface
l
The surface area can be covered by special transmitter/ receiver longitudinal (TRL) angle probes with an incidence angle of 70 ° or greater. With these probes it is often possible to detect cracks starting at the outer surface. It might also be possible to find cracks which had started at the inner surface, subsequently progressed but not yet penetrated to the outer surface. For the inner surface a 45 ° shear wave probe with a suitable frequency can be applied because the service-induced flaws occur mostly beside the root of the weld.
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oleo
Notch
(
70 ° dual search unit Ionqitudinal waves
An in-service inspection should - in the author's experience be concentrated on the areas near the inner and outer surfaces; Fig. 9 shows the technique applied with the help of a reference block. Suitable reference reflectors are notches at the inner and outer surface and side-drilled holes (SDH)in the fusion line between parent and weld metal.
INTERNATIONAL
and near surface
Side drilled holes
Principles of in-service inspection
NDT
Krautkr~Jmer VRY, 45°TRL~2MHz
RTD,45*TRL,f = 30,4MHz
1981
Root oleo incl.
volume
Side drilled holes
45 ° search unit shear : through parent metal long,: through weld metal
Fig. 9
Illustration of in-service inspection technique, using a
reference block
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Grain size
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Depth of notch ( r a m ) Fig. 10 C o m p a r i s o n of amplitudes of reflections from reference notches of different depths in a test block, for shear and longitudinal waves
from notches of different depths in a test block 50 mm thick. A 2.25 MHz crystal, size 0.5 x 0.75 in (13 x 19 mm), with two wedges producing shear waves and longitudinal waves at an incidence angle of 45 ° in steel is used. The results using the shear wave probe show an increase in amplitude with increasing notch depth. The longitudinal (compressional) angle probe produces indications which do not even allow dimensioning, by means of amplitudes of notches, let alone cracks. A very important fact for application of shear wave probes is the region of saturation which begins in this case at a notch depth of about 3 mm. These results are very important for the sensitivity setting for an in-service inspection. The behaviour of notches in the sound field of a probe has to be taken into strict consideration. If this is not the case, large errors in the test result can be the consequence. The problem of saturation is highlighted if we consider a notch depth of 10% of sample thickness, according to ASME Code Section XI. In our case this is a notch depth of 5 mm, which is in the saturation region as far as the applied probe is concerned. This means that the 5 mm deep notch gives the same ultrasonic indication as an infinitely deep notch - equivalent to a through-wall flaw! I ntergranular
stress corrosion cracking (IGSCC)
Ultrasonic testing for IGSCCidentification still creates great problems. The effects considered in all of the previous paragraphs are unfortunately valid also for components with IGSCC. Fig. 11 shows the most important factors which limit testability and have great influence on the test result. One meets different grain sizes again from component to component and even within one component, with corresponding influence on attenuation. The crack may have unfavourable shape and orientation, which makes it a bad reflector. In the limit, the occurrence of a crack network
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Shape
of crack
Fig. 11 Factors i n f l u e n c i n g t h e results of tests f o r i n t e r g r a n u l a r stress c o r r o s i o n c r a c k i n g
may, together with grain disintegration, prevent any reflection because the grain structure is no longer capable of transferring ultrasonic energy. For correct ultrasonic testing one should have a reference block as similar as possible to the object to be tested. One cannot use an arbitrary piece of austenitic steel with reference reflectors. Furthermore, a comparison of ultrasound attenuation from the reference block to different locations on the object to be tested has to be performed. Such a comparison can be made with two identical shear wave probes set in V-path (Fig. 12), both on the reference block and on the test object. If the different sound transparency is not corrected, the examination result can be worthless. Another point which has to be taken into account is the often less than ideal geometry. The objects to be tested seldom have machined inner and outer surfaces. Thickness can change, counterbores, excessive penetration at the root of the weld and the weld bead surface limit the possibilities of ultrasonic testing. In many cases these factors are identified at the beginning of an actual examination only. Each weld must be investigated as an individual problem, which demands a lot of the testing personnel. In the author's company a special training programme has been established. Another fact has to be kept in mind: even today it is not possible to give a reliable value for crack depth by ultrasonic means because a crack can be a very bad reflector due to its form, which is not known in advance!
NDT I N T E R N A T I O N A L . JUNE 1981
Gain setting
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Fig. 1 2 Arrangement of shear wave probes to establish sensitivity correction for different ultrasound attenuation in reference block and test piece
Thick wall cast components
A special problem is the ultrasonic testing of cast components. In Westinghouse type PWRs the main cooling pipes are made of austenitic steel in a thickness range of 60-85 mm. At the Beznau power plants I and II an in-service inspection of austenitic welds of the primary coolant system was performed in 1979. Cast elbows welded together with different types of austenitic pipe materials were encountered. A report about this problem has been published. 4 Here, the most important results will be mentioned only. Fig. 13 shows the attenuation for different frequencies in a 40 mm thick piece of austenitic casting - which is typical for elbows. It is obvious that 1 MHz is the most suitable frequency. But great differences in attenuation may occur in the various components. Fig. 14 shows the reference block with reference reflectors, side-drilled holes and notches, for cast components welded together. Examination of the surface and near surface region is done with a special 70 ° TRL-probe. A 2 mm deep reference notch in the cast material at the surface and the 5 mm diameter SDH at a depth of 15 mm produce indications with a good signal to noise ratio for sound waves incident through the weld metal (Fig. 15). This probe has been developed for the detection of subcladding cracks. 8 The volume and the root area are examined by a special transmitter/receiver probe with longitudinal waves of 45 ° at a frequency of 1 MHz. 9 The indications of the 5 mm dia. SDHs at 15, 30 and 45 mm depth for sound waves incident through weld and parent metal can be seen in Fig. 16. In this case the sound transparency through the cast parent metal is worse than through the weld metal. This demonstrates that there exist cases where the problems in the parent metal are more pronounced than in the weld metal. The applied probe was the best available for the cast material in the above mentioned elbows. But even this probe is not sufficient for a thickness of 60 mm as can be seen in Fig. 17. The upper screen trace shows the noise level and the lower trace shows an indication which could be associated with the 4 mm deep notch. The notches of 1,2 and 3 mm, and in
NDT INTERNATIONAL
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Austenitic costing ASTM A551 CF8M thickness 40 mm
~ ~
......
20 mm I
I
Fig. 13 Ultrasound attenuation for different frequencies in an austenitic casting
actual testing also the 4 mm notch, would not be found. With shear waves there is no possiblity of detecting either the side drilled holes or the notches through the cast parent
Ce~ting G-X5CrNIMo 1810 A- ~kSIGr CF8M
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Reference reflectors
Notches Fig. 14 Reference block for austenitic weld of cast components in main cooling system of Pressurised Water Reactor
131
Through parent metal
Through weld metal
B2
Indication of the 2mm deep surface notch through cast parent metal B3
Vin~otte probe V 4 5 ° L SE 85F IMHz Fig. 16 Indications from testing of volume and root area of reference block (shown in Fig. 14) with 45 ° TRL probe
Notch (4ram deep) at inside surface Indication of the 5ram dia SDH at 15mm depth through weld metal Probe: RTD BAM, 70°SEL, subcladding crack probe Fig. 15 Indications f r o m testing of near surface area of reference block (shown in Fig. 14) with 70 ° TRL probe
metal. These facts together with those"demonstrated in Fig. 10 (notches as reference reflectors) forced us to concentrate the imservice inspection on those components which have a better sound transparency. Such components with a much better sound transparency than that of the reference blocks of Figs. 13 and 14 were found during in-service inspection and were carefully investigated.
Noise indications
Summary
There are a lot of cases where ultrasonic testing of austenititic components is possible, but a general solution is far away. Development has to be performed on a case-by-case basis. It is very important that each factor influencing the test result is taken into consideration. One has to differentiate between parent metal and weld metal. Manufacturing process, welding technique and heat treatment are very important influences on testability. Control of the grain configuration by means of measurements of the sound transparency may indicate an incorrect manufacturing process. Differentiation between fabrication and in-service inspection is most important. For both, adequate reference blocks are absolutely necessary. On these blocks the testing
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Notch indication Vin~otte probe V45°L SE 85F IMHz Fig. 17 Indications from inner surface of reference block (shown in Fig. 14) using 45 ° TRL probe
NDT INTERNATIONAL. JUNE 1981
problem can be analysed and a suitable testing technique and sensitivity laid down. In older plants the manufacturing of adequate reference blocks is a difficult task because similar materials and welding techniques to those used in the plant have to be repeated for the reference blocks. This procedure is necessary in order to produce the same ultrasonic response characteristics both in the plant and in the reference blocks. Unforeseen difficulties on the components to be tested can complicate the examination. Often, a definite technique can be set down only in course of the actual examination especially for in-service inspection - because of geometrical and metallurgical problems. These facts demand a lot of the testing personnel. Special training, to acquaint them with the problems encountered in the ultrasonic testing of austenitic welds, is therefore necessary.
References 1 2 3 4
sium on New Methods of Non-destructive Testing of Materials and their Application Especially in Nuclear Engineering, Saarbracken 17-19 September 1979 p 177
5
6
Conclusions Experience has to be gathered with the new testing techniques on actual flaws. Exact documentation and critical analysis of findings will allow a judgement of possibilities and limitations and will also help in taking corrective action in the development of testing techniques. Further research on the testing of strongly attenuating cast structures and on techniques of distinguishing false indications from real flaws is urgent. These developments must be tested in practice. Testability must be considered in the stage of fabrication of a component, in relation to geometry, accessibility, metallurgy and surface condition. Most important is a careful pre-service inspection of new components.
NDT I N T E R N A T I O N A L . JUNE 1981
Lapides, M.E. 'Improved ultrasonic inspection method for stainless steel piping' EPRI NP-1153 Special Report (August 1979) Edelmann, X. and Hornung, R., 'Erfahrungen im Prtffen von austenitischen Schweissverbindungen', Material und Technik 5 No 1 (1977) p 19 Edelmann, X., 'The application of ultrasonic testing of austenitic weld joints', Materials Evaluation 37 No 10 (1979) p 47 Edelmann, X., 'Wiederkehrende Prfffung mit UltraschaU an austenitischen Schweissverbindungen im Hauptkt/hlmittelsystem von Druckwasserreaktoren', Conf Proc International Sympo-
7 8
WUstenberg,H., Just, T., MOhrle, W., and Kutzner, J., 'Zur Bedeutung fokussierender PrtffkOpfefor die Ultraschallprafung yon Schweissn~hten mit austenitischem Gefttge', Materialprtlfung 19 No 7 (1977) p 246 Just, T., ROmer, M., Neumann, E., Matthies, K., and Kuhlow, B., 'Leistungsfa'higkeit verschiedener Ultraschallprtlftechniken an austenitischen Probeschweissungen mit nattlrlJchen Tesffehlern',Materialprttfung 19 No 12 (1977) p 488 Herberg, G., Laufer, W. et al, 'Statusbericht zur Ultraschallprdfung an austenitischen SchweissnaTtten',Materialprttfung 20 No 3 (1978) p 120 WUstenberg,H. and Schulz, E., 'Versuche zur Feststellung plattierungsnaher Reflexionstellen an Reaktorteilen mit Ultraschall' (DGZfP-Vortragstagung, Saarbrtlcken, 1972)
9
Caussin, P. and Cermak, J., 'Performances of the ultrasonic examination at austenitic steel components' Conference on Periodic Inspection o f Pressurized Components, London 8-10 May 1979 (I Mech E, 1979) C 49/79 p 207
Author Mr Edelmann is with Sulzer Brothers Limited, Dept NDT - 1513, CH-8401, Winterthur, Switzerland.
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