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MODELING FOR THE IMPROVEMENT OF DISSIMILAR WELD INSPECTION

A. Erhard, V. Munikoti, G. Brekow, U. Tessaro and D. TscharntkeBAM-Berlin, Berlin, Germany

Abstract: Ultrasonic inspection of dissimilar welds is still a challenge due to the different acoustic impedances across the interfaces between parent metals, welded, butter or cladded regions. Typically parent metals are fine grained carbon/ferritic/20MnMoNi55 steel, which show isotropic character. Whereas weld/butter/cladded metals are INCONEL/ austenitic steel, which exhibit anisotropic character, attributed to their columnar grained texture resulting from the high thermal inputs during welding process. The ultrasound wave propagation through such anisotropic materials is not straightforward due to the skewing of energy flow direction with respect to the direction of phase velocity. Therefore the analysis of the experimental data necessitates many years of experience.

A simple theoretical model based on ray-tracing concepts can help to optimize the ultrasonic experimental procedures for the detection of transverse defects (defects with their orientation perpendicular to the welding direction) at the inner surface of the pipe in the circumferential weld. In this presentation a comparison between theoretical and experimental results will be described Introduction:
Weld joining dissimilar materials with different chemical and mechanical properties is a challenge for nondestructive testing due to the variation of physical properties in the examination region. If the viewpoint is limited only to the application of ultrasonic testing the change in the acoustical impedance along the sound path is the most important influence. Fig 1 shows an example of a dissimilar weld at a nozzle. In nuclear power plants such welds are employed among other things as austenitic and/or inconel joints between austenitic pipes (material: 1.4550 or 1.4551) and the ferritic cladded nozzle of a reactor pressure vessels. The motivation for the present investigations is due to the fact that such welds are sensitive to stress corrosion cracking. In some nuclear power stations abroad, cracks were detected on the pipe inner surface of the dissimilar weld especially if inconel material was in contact with the cooling water. These cracks due to their orientation perpendicular to the weld direction (weld di rection circumferential) are designated as transverse cracks or axial oriented cracks. A typical microstructure of such materials combination is illustrated in Fig. 1 [1-3]. Clearly visible in the micrograph are the interface areas between the different materials where a sound interaction must be assumed. In some earlier publications by this author the examination of dissimilar welds are described where longitudinal cracks are in the focus of interest. Due to recent evidence on such newer defects on dissimilar welds or inconel

welds we have focussed our activities on the detection of transverse cracks [4]. Therefore, in the present contribution only such oriented cracks are under examination. Optimisation of Inspection Metho The difference between the inspection of longitudinal cracks as written in some papers [5- 10] and those of transverse cracks is mainly given by the surface condition of the weld region [11-

Fig 1: Dissimilar weld - joints and interface structures 14]. By the inspection of longitudinal cracks starting from the interior surface, the probe position is at a offset distance from the weld e.g. a proper coupling of the probe is guaranteed. Otherwise the examination for transverse cracks regards the coupling direct on the weld cover but it is well known that in this region the sur-

face has a waviness due to the shrinkage between the weld and the base metal. Good coupling conditions for the ultrasonic transducers are found only in the vicinity of the weld and thus on the base metal surface. In the following the basic steps for the optimisation of the ultrasonic examination method regarding the above mentioned conditions is described.

For the detection of transverse cracks in dissimilar welds different probe arrangements and types are applicable under the utilisation of longitudinal and shear waves. Conventional probes as well as phased array probes for pulse echo or transmitter receiver operation are, in general, applicable. Due to the fact that such welds are often used to weld pipes together the probes must be adapted to the curvature of the pipe. It is understandable that for

Fig. 2: Specimen with dissimilar weld (Tk-E nozzle) the development and optimisation of a new technique on a number of test specimens the experimental investigation time in addition to expenditure increases. In order to reduce the experimental time a complimentary theoretical model based on the ray tracing principle can be used to confirm the influence of some inspection parameters on sound propagation in the anisotropic region of dissimilar weld. Test specimen for the Experiments A number of test specimens with transverse cracks in dissimilar welds are available for the optimisation of an ultrasonic technique.

One such test piece is shown in Fig. 2 with an inconel 182 weld. The buttering has a thickness of approximately 10 mm. and the interface between butter and carbon steel is perpendicularly orientated to the inner surface, similar to the micrograph in Fig. 1. The defect situation is pictured in the schematic

drawing of Fig. 2. As shown in

the figure the spark eroded notches have a depth extension between 2 mm and 8 mm and the position is in the weld and butter as well as in the austenitic or ferritic base metal. The extension in length is10, 20 and 26mm.

Modeling of the inspection method The theoretical model used in the present section is based on the ray tracing method. The principles of the method are described in [15/16]. Other investigations using theoretical models for the optimisation of inspection techniques and for the interactions of the sound field with the structure of dissimilar welds are described in [17-23]. The following assumptions were made for the calculations using the method described in [15/16].

*
The grain texture is simulated by an empirical formula, which is not based on the principles of solidification mechanics
* A layback angle of 10° is assumed in the welding direction * Elastic constants for austenite and Inconel 182 are taken from the literature and must be therefore not exactly in accordance with those of the testblock * Multi reflection on the grain boundaries and sound attenuation are not considered * Planar waves are further bases of model

Fig 3: "V" arrangement of two transducers

Fig. 4: Coordinate system (top view)

Fig. 5: Ultrasound propagation in an Inconel dissimilar weld Due to the fact of waviness at the surface in the welded region the surface is hand grinded and a V-arrangement of the ultrasonic probes as demonstrated in Fig. 3 were chosen. The advantage of such an arrangement is clearly visible because both of the probes are coupled on the base metal of the austenite or the carbon steel. With the definition of he azimuth angle Φ as the angle between the weld middle line and the main beam (fig. 4) the optimisation of the probe coupling position for the detection of transverse cracks will be explained. Fig 5 shows the transmitter coupled on the austenite base metal at an x-position of 20 mm (for the definition of the coordinate system see Fig. 4).

The angle of incidence for the longitudinal wave in that particular case was 40°, the azimuth angle 30° and the wall thickness of the test specimen 47 mm (OD 478 mm). The example in Fig. 5 shows that the sound field (nine incidence beams were calculated) impacts the assumed defect area. Also clearly visible is the divergence sound field reflected at the defect but the sound field direction (x and y direction) is not distinguishable from the figure. This information can be estimated as in Figure 6 where the top view is presented. Viewing in the direction of the reflected sound field the receiver must be at a position in opposite of the transmitter coupled on the carbon steel surface. To make sure that the receiver probe has the right "receiving" angle the side view presented in Fig. 7 is very helpful.

Fig. 6: Ultrasound propagation in an Inconel dissimilar weld (top view) Fig. 7: Ultrasound propagation in an Inconel dissimilar weld (side view)

With the help of the theoretical model the probe arrangement can be optimised and the experimental investigation times can be reduced. The results of the calculation gives the angle of incidence for the transmitter probe, the position of this probe as well as the receiver probe on the surface, the azimuth angle and the "receiving" angle of the receiver probe. The reliability of the calculated results because are strongly dependend on the input of the mechanical parameters into the model. In table 1 the parameters used for the optimisation of ultrasonic methods for the detection of transverse defects in dissimilar welds are listed. In the next chapter the comparison between experimental and theoretical results are described. Comparison of the results between experiments and model The testblock Tk-E nozzle was used for the comparison between the theoretical and experimental results. This specimen (see Fig. 2) has a wall thickness of 35 mm, an outer diameter in the weld region of 220 mm and four transversal defects with depth extensions of 2, 5 and 8 mm at the inner surface. The length extensions of the 2 mm deep notches are 20 mm and 26 mm whereas the location of the longer notch is between the butter and the

Fig. 8: Asymmetric probe arrangement 6 CrNi 18 11 Inconel 182 [10¹¹ N/mm²] C11 2,4110 2,78 C12 0,96920 1,15 C13 1,3803 1,3889 C44 2,4012 2,5376 C66 1,1229 1,06 10³Kg/m³ρ 7,82 8,61 Table 1:
Used parameter

austenitic base metal and the shorter notch between the weld centre line and the butter. An asymmetrical "V" form arrangement of two transducers with 35° angle of incidence is shown in a drawing in Fig. 8. An asymmetrical arrangement means that the transducers have different positions at the circumference (y- direction). In Fig. 8 the deference between the two transducers is 10.5 mm. One of the experimental results is presented in Fig. 9.

sound path 25.3 84.5 mm 143.7 notch 8 mm 5 mm 2 mm 0.0° 19.0° 37.9° 56.9° 75.8° 94.8° 151.6° 170.6° 189.5° 208.5° 227.4° 246.4° 265.3° 303.2° 322.2° 341.1° 360.0° 113.7 132.7° 284.3° n io i t pos obe pr ferrite N2 5 R 11 ⁰R 7 ^N21N20 counter clock wise direction of incidence austenite 26 x 0,5 20 x 0,5 N1 2 mm deep N1 scanning direction N2 2 mm deep 14,5° 11° 10,5 receiver transmitter 82 93 asymmetric probe arrangement 10 x 0,5 10 x 0,5 N21 8 mm deep N20 5 mm deep 35.0° beam angle 56 dB gain 0.46 mm pixel size 5780 m/s sound velocity Fig. 9: Transverse flaw inspection at the dissimilar weld of the Tk-E- nozzle specimen There are four clear indications visible in the time displacement image (TD); two of the indications -not marked - are from the disconnections (the specimen was made from two cylindrical half shells) of the test specimen. The other indications were generated from the 2, 5 and 8mm deep notches. With the help of the transducer positions on the coupling surface the physical effective angle of incidence can be estimated to 39°. While the signal to noise ratio (SNR) of the 2 mm deep notch was 3 dB (to small for a mechanized inspection of the weld), so for the 5mm and the 8 mm deep notches a SNR of 13 dB and 17 dB was meas ured. The scanning direction for this measured example was counter- clock-

wise. One of the results from the experiment carried out in the clockwise direction is shown in Fig. 10. All of the four notches could be detected whereas the signal to noise ratio was less, in the average than 4 dB. The asymmetric probe arrangement was developed from the theoretical model and could be confirmed experimentally.

sound path 25.3 84.5 mm 143.7 notch 2 mm 14,5° 2 mm 8 mm 5 mm 0.0° 22.9° 45.6° 68.4° 91.1° 159.4° 182.1° 204.8° 227.6° 250.3° 273.1° 303.2° 322.2° 341.1° 360.0° 113.9° 136.6° n i o i t s po obe pr N2 N1 ⁰R 7 5 R 11 ^N21N20 26 x 0,5 20 x 0,5 N1 2 mm deep ferrite clock wise direction of incidence scanning direction dissimilar weld N2 2 mm deep transmitter 93 82 receiver 10 x 0,5 10 x 0,5 N21 8 mm deep 11° 10,5 asymmetric probe arrangement austenite N20 5 mm deep 35.0° beam angle 56 dB gain 0.46 mm pixel size 5780 m/s sound velocity Fig. 10: Transverse flaw inspection at the dissimilar weld of the Tk- E-nozzle specimen With the aid of the Fig. 11 to 12 we will demonstrate how the theoretical model can help for the optimisation of the inspection technique. The theoretical results for two scans are presented in these figures; track # 186 and #189. Fig. 11 shows the calculated sound path for the detection of the 8 mm deep notch at the track # 189. For the calculation nine beams were chosen in such a way that eight of them are representing the ultrasonic filed and the 9^th beam the centre line of the field. The calculation results are listed in table 2 together with the parameters used for the experiments. The tracing shows that only two beams out of nine are reflected at the notch. The rest have due to the anisotropic

structure of

the dissimilar weld no chance to hit the defect. The two reflected beams have nearly the same direction from which there are coming from. Basis for the calculation were an asymmetric "V" form probe

arrangement. This result shows that with such a probe arrangement defect detection in the track #189 is impossible as demonstrated in Fig. 13. In fig. 12 the ray tracing of the track #186 for the detection of the 8 mm deep notch is presented. The results are listed in table 3 also for this particular case together with the parameter used for the measurements. The tracing shows that four out of the nine beams are reflected at the notch in direction to the receiver probe i.e. with the asymmetric probe arrangement the reflector is detectable in the scan # 186 (see also fig. 13). The comparison of the TD scan in fig. 13 shows the difference in the reflectivity using the same probe arrangement - asymmetric "V" form - but in different probe tracks. Whereas in the track #186 the 8 mm deep notch was detectable with a SNR of about 15 dB, the track #189 i.e. 3 mm difference in the probe movement, shows that the notch was undetectable. This circumstance was confirmed with the measurement results.

Fig. 11: Geometry and parameter used for modelling Fig. 12: Geometry and parameter used for modelling

The evaluation of TD scans is sometimes time consuming because for each ultrasonic function e.g. different angle of incidence or differences in the frequency etc and for each scan a TD scan is produced and must be evaluated. Therefore a presentation where the results of all functions and all scans are presented in one image is helpful for a fast and confident evaluation of the integrity of the component.

.3 84.5 mm 143.7 8 mm notch 85.4° 94.8° 104.2° 113.7° 123.2° 132.7° 142.1° 151.6° 317.6° 170.6° 180.0° 189.5 ition pr obe po s dB 48 46 44 42 40 38 36 34 28 1 25.3 84.5 mm 143.7 8 mm notch Asymmetric "V" arrangement track #186 track #189 scanning direction counter clock wise 85.4° 94.8° 104.2° 113.7° 123.2° 132.7° 142.1° 151.6° 317.6° 170.6° 180.0° 189.5 ition p r obe po s Fig. 13: Transverse defect detection at the Tk-E-nozzle specimen For problems discussed in this present paper where pipes are the focus of attention a so called "Echotomography" presentation can be used to advantage. The principle of the Echotomography will be explained with the help of Fig. 14. The digitised A-scan will be stored in a pixel net in such a way that the amplitude is stored in the hit element. This could be carried out in two ways mean amplitude presentation (addition of all amplitudes in the pixel element and divided by the number of hits after the measurement) and maximum amplitude presentation (the maximum amplitude hit the pixel element is stored only). For storage the mean amplitude during the measurement procedure the equation presented in Fig. 14 is used. 25

One of the advantages of these reconstruction procedures (mean or maximum amplitude presentation) is an increase of the SNR. Fig. 15 provides an example of a maximum amplitude presentation. The indications from the notches are clearly visible as well as the indication from the edges of the two half- cylindrical test piece.

Conclusions: The paper on how to overcome the inspection problem on the detectability of transversal cracks at the inner surface of a pipe's joint, together with dissimilar weld had two main topics:

* optimisation of the inspection techniques using a theoretical model * verification of the theoretical results due to experiments with the aim that experimental investigations can be reduced in the future. We learned that with the help of a theoretical model based on "ray tracing" such an optimisation is possible. This was demonstrated at an example where a "V" form probe arrangement was used for the inspection of transversal cracks at the inner surface of pipes in the dissimilar weld region. The advantage of such a probe arrangement is due to the fact that the ultrasonic probes can be coupled on surfaces in the vicinity of the weld area. With the angle of incidence, the divergence angle of the probes and the arrangement symmetrical or asymmetrical an adaptation on the geometrical conditions can be performed. Furthermore with the aid of the model demonstrated and experiments the influence of the relative position of the probes to the weld centre line could be analysed. For a reliable inspection on site a number of scans with different positions to the centre line is required.

fixed pixel coordinate system in the cross sectional image whitch is reconstructed for a circumforential scanning track rectified A-scan with digiticed echo amplitudes scanning direction of the probe angle beam probe a beam angle notch Nut y x measurement data processing in one pixel element d echo different echo heights, stored in one pixel element during the measurement period e l ix t ere p e on regi s in echo height n g resul t i 25 20 15 10 5 ⁰1 4 7 10 measurement period c ho hei ght lt i ng e res u maximum value algorithm 25 20 15 10 average value algorithm 25 20 10 5 5 measurement period 15 ⁰1 4 7 10 ⁰1 4 7 10 measurement period Fig. 14: Echotomography - principle maximum echo height, registered in one pixel element after ending of measurement period average echo height, registered in one pixel element after ending of measurement period. dynamic averaging (M = measurement value) ∑ M M n + 1 + measurement data aquisition component of tomogram rconstruction edge indication 2 mm notch 5 mm notch 8 mm notch edge indication Fig. 15: Transverse defect detection - Echotomography

Beamangle Probe position (transmitter) Azimuth angle theory experim. theory experiment theory experim. theory experim. coordinates X (mm) Y (mm) X (mm) Y (mm) Probe position (receiver) X (mm) Y (mm) 41° 39° -17 101 - -17 101 -1 +9 16° 15° Table 2: Parameter for track # 189 mm Beamangle Probe position (transmitter) Azimuth angle theory experim. theory experiment theory experim. theory experim. coordinates X (mm) Y (mm) X (mm) Y (mm) Probe position (receiver) X (mm) Y (mm) 41° 39° -21 101 - -20 101 19 +6 16° 15° Table 3: Parameter for track # 186 mm

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