Ultrasonic Phased Array technique for Austenitic Weld Inspection

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Ultrasonic Phased Array technique for Austenitic Weld Inspection

A. Erhard, G. Schenk, W. Möhrle, H.-J. Montag
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Abstract

Austenitic base- and weld materials have their own laws for ultrasonic inspection due to the anisotropic behavior of these materials. Since the establishment of these materials for some components, especially in the chemical and nuclear industry, the challenge for nondestructive testing techniques is increasing. A lot of activities in the research labs and in the industry around the world were carried out, but anyway there is no ready-made solution for all problems recognizable at the horizon. For the improvement of defect detectability at such materials some sound field adaptations are described in the literature e.g. longitudinal waves instead of transverse waves, focused sound field, the application of twin crystal probes etc. whereas for the time being the transmitter-receiver technique for longitudinal waves (TRL-Probe) is the most favored and successful technique in Germany. Due to the application of TRL - probes an increase of the signal to noise ratio is achieved. Depending on the wall thickness of the welded component a number of probes are needed for the whole inspection i.e. the weld area and the adjacent material (heat affected zone). The number of probes can be reduced by the application of the phased array technique. Therefore in the ultrasonic lab at BAM a transmitter receiver phased array probe (TRPA-Probe) were developed with the advantage that only one probe is necessary for the inspection. In the present paper the experimental results received at an austenitic pipe weld as well as some characteristics of the TRPA-probe are described.

Introduction

Defect detection in welds and in the adjacent areas are influenced e.g. by the weld root shape, root sagging and the shape of the weld seam or other imperfections. Except the inspection of austenitic or other coarse-grained materials, most of the nondestructive testing methods have a high and reliable potential for defect detection. In a lot of presentations and papers the difficulties about defect detection in austenitic welds are described as well as the solution of these problems [1-3]. For the ultrasonic inspection the signal to noise ratio is the important parameter for a reliable detection. Therefore investigations were carried out to improve the signal to noise ratio. This can be done by focused sound fields i.e. the interaction between the sound field and the grains of the material will be reduced. Unfortunately, focused transducers are unwieldy for practical application. One of the reasons is the size of such probes and the other reason is the focused beam itself. Good solutions for the practice are transmitter -receiver - probes (TR-probes). Due to the crossing of the sound beam in a certain distance the interaction between the grains of the material and the sound field is reduced to the sensitivity area. The drawback is, that for a wall thickness up to 15 mm (20 mm) often more than one probe is necessary, as described in [2,4]. With a TR-phased array probe this drawback must be insignificant, because due to the angle steering of such probes the sensitivity area is adaptable to each depth. In the present paper the characteristic data of the developed TR-phased array probe as well as results received from experiments carried out in the lab are described. For the experiments EDM notches with different depth and positions were eroded in the weld area at a test specimen. The experiments carried out at the test specimen had the aim to demonstrate the detectability of crack like defects at the ID as well as at the OD by using the developed TR phased array probe. Due to the recent increase of the number of austenitic gird weld inspections, this paper also presents some results received with conventional TR-longitudinal wave probes. Finally, the main goal of the paper will demonstrate the diversity and flexibility of phased array technology.

Probe Characteristics

With the optimized parameter given by the calculation, a TR-longitudinal phased array probe was produced. The developed probe is shown in fig. 1. The size of the probe housing is 25 x 25 mm^2. This size gives the guaranty, that for a lot of pipe diameters the adaptation of the probe on the curvature is not necessary. Other characteristic data of this probe are described in the following. A number of 12 single elements for the transmitter and receiver respectively were used. The frequency is 3 MHz and the size of each element 2 x 6 mm². So we have a total dimension of 16 x 6 mm² for the transmitter as well as for the receiver. A 1-3 piezoelectric composite crystal material with epoxy matrix and PZT ceramic was used too. Figure 2 is presenting the directivity pattern of different angle of incidence. The figure shows that for a wide range of angle of incidence the shape of the pattern is nearly constant, except a little bit increase of the beam spread. Also the difference in the echo-height between the main lopes of the different angles is in an acceptable range. In the maximum a difference of 3 dB is estimated. It seams to be clear, that these differences in the echo-height must be corrected during the examination. This gives the guaranty for an objective evaluation of the measured data independent on the angle of incidence.

Fig 1: Characteristic Data of the TR - Longitudinal Phased Array Probe.

Fig 2: Directivity Pattern of the TR - Longitudinal Phased Array Probe.

Practical examination using TR longitudinal probes requires sensitivity setting at cylindrical or flat bottom holes [5] and most of the evaluation procedures are based on DAC diagrams. Therefore such diagrams must be available for TR-phased array probes with its wide range of angle of incidence. In fig. 3 the dependency of the amplitude versus the time of flight for a number of angle of incidence is shown. The shape of these curves are typical for TR-probes i.e. a maximum in the amplitude in a certain distance, the distance of the sensitivity area, except the 70° curve.

Fig 3: Distance Amplitude Correction curves for the TR-Longitudinal Phased Array Probe.

Experimental Results

Fig 4: Experimental Arrangement for the Optimization of the Examination.
The experiments were carried out at the test specimen as shown in fig. 4. In the total 16 EDM notches were made at the ID as well as at the OD. An overview about the different notches is also shown in fig. 4. The table shows that notch depth between 2 mm and 7 mm were made. The positions of the notches are in the weld region except the KTA notches. These notches were made for the calibration of the UT system. For the examination the test specimen was fixed at a rotation-table to simulate a mechanized UT inspection. During the rotation of the specimen the angle of incidence was changed in a range between 40° and 70° for longitudinal waves. In one case we changed the delay time in such a way, that a shear wave with an angle of incidence of 45° was generated. Some measured results will be presented now.

Fig. 5 is presenting the TD image (Time-Displacement) received with 45° angle of incidence of the longitudinal wave at a probe distance to the weld center line of 40 mm. The notches at the ID are detectable with a signal to noise ratio in the average of about 16 dB. This is signal to noise ratio enough for the application on site.

Fig 5: TD-Image of the ID Notches

Fig 6: TD-Image of the ID and OD Notches

In fig. 6 the results received at two different distances to the weld center line are shown. The TD image presentation gives a good overview about the detectability of the EDM notches at the ID as well as at the OD. The numbers of the notches in the TD-images are identical with those in the table shown in fig. 4. It must be mentioned, that at the given probe positions only that notches are not detectable if the sound beam comes through the weld all the others are detectable with a good signal to noise ratio. The angle of incidence of the longitudinal wave was 70° at that case. Both examples are demonstrating the flexibility of the phased array technique because with one probe at each side of the weld the examination of the weld and the adjacent material is possible. The detectability of surface defects and embedded defects can be done by changing the angle of incidence during the probe scanning between 40° and 70° .

Conclusion

The whole investigations in relation to the optimization of austenitic pipe weld inspection technique have shown the advantage by the use of the phased array technique because surface breaking cracks as well as embedded defects are detectable with one phased array probe. The inspection of the weld and the adjacent material can be done using one phased array probe at each side of the weld.

References

X. Edelmann: Application of Ultrasonic testing techniques on austenitic welds for fabrication and in-service inspection; NDT-International, June 1981, p. 125-133
Handbook on the ultrasonic examination of austenitic welds, The International Institute of Welding, Edition 1985
T. Seldis, C. Pecorari, M. Bièth, Measurement of longitudinal wave attenuation in austenitic steel; Proc. 1^stInt. Conf. On NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, 20-22 October 1998, Amsterdam Vol. II p 769-777
E. Neumann et. al; Ultraschallprüfung von austenitischen Plattierungen, Mischnähten und austenitischen Schweißnähten, Expert Verlag 1995
Guide Line UT3: Ultrasonic inspection of the probe near surface area; German Soc. for Nondestructive Testing; May 1999.