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New approach to optimise the ultrasonic testing of austenitic welds Didier Flotté - Daniel Chauveau Institut de Soudure - Zi les jonquières - 57365 Ennery - France Contact

1 - INTRODUCTION

To date, the ultrasonic inspection of austenitic alloys has formed the subject of a large number of publications describing quite a variety of techniques, but unfortunately, it was often observed that the results yielded by the actual application of a given technique were not always as good as those reported in the publications mentioned. That is why the problems associated with the selection of the adequate inspection technique for a given configuration of material / assembly / sought defect and its real efficiency are not solved yet. Lastly, there is no doubt that some progress has been made in the nuclear industry but the transfer to other industrial segments (for instance the petrochemical industry) has not taken place.

Fig 1: Drawing of a 20mm thick Specimen

Within a group sponsored project (10 partners - French, German and Canadian) led by INSTITUT DE SOUDURE with the collaboration of IZFP - Sarbrücken, the state of the art of the various techniques available for testing austenitic welds has been performed in order to determine the influence of the parent material and the type of welding process on the performances of ultrasonic testing.

To perform this programme, specimens in 316L, inconel and Duplex have been welded by several welding process. Artificial reference defects were machined in each specimen :

• Side drilled holes parallel to the weld axis (diameter = 3 mm)
• Notches (depth : 1 mm to 3 mm )
Figure 1 give a drawing for the 20 mm thick specimen.

Several techniques have been compared: manual ultrasonic techniques, phased arrays, EMAT, TOFD and various signal processing applied. For conventional ultrasonic testing, the influence of the operating parameters has been studied: frequency, type of probes, propagation through the welded zones.

The aim of this conference is to explain how to prepare a conventional ultrasonic inspection taking account of these results by:

• Selecting the probes best fitted in a database of more than 40 probes experimented on reference blocks.
• Determining by a new technique the frequency to use to obtain the best signal to noise ratio.
• Drawing the beam through the welded zone by using focused probes instead of electrodynamic probes.

2 - PROBES DATA BASE

The object of the database is to give an help to the ultrasonic operator to select the best probe for a manual testing in a given configuration. For that purpose, some measurements have been performed on the specimens with a collection of probes.

For each specimen described in figure 1 and for every probes, the following parameters have been measured:

• low and high ultrasonic path in austenitic steel giving the best results
• low and high ultrasonic path in the welded zone for which the probe offers good performance
• the gap of difference in gain Gd with WSY probe
• the signal to noise ratio
• the reserve of amplification
• the characteristics of the probe.

The list of the main probes tested to elaborate the database is given here after :

• A : KRAUTKRAMER MSWQC SW
• B : KRAUTKRAMER MSWQC LW
• C : KRAUTKRAMER MWB SW
• D : KRAUTKRAMER WSY LW
• E : KRAUTKRAMER VS dual element SW
• F1 : KARL DEUTSCH dual element ASF55 SW
• F2 : KARL DEUTSCH dual element ASF30 LW
• G1 : CHEN & LEHMANN dual element R150 LW
• G2 : CHEN & LEHMANN R100 / R75 LW
• H : SIEMENS SEL4 70° LW
• I : KRAUTKRAMER 3XW composite dual element LW
• J : KRAUTKRAMER prototype n°1 composite dual element LW
• K : KRAUTKRAMER VSY OL
These probes were compared to the WSY of KRAUTKRAMER, which is taken in reference.

A data base stores all the results obtained. This database shall be used carefully because it does not integrate the feeling of the operator.

The table below gives an example of values obtained from the database with the criteria :

• a refraction angle of 45°
• testing depth below 15 mm
• 316L steel welding by a manual process
• signal to noise ration greater then 18 dB.

Probes	Waves	Freq.(MHz)	Angles(°)	Depth (mm)	Steel	Welding process	Gain (dB)	Attenuation(dB)	Signal/ noise(dB)
B	L	7,5	45	5	316L	111	40	-8	22
C	T	4	45	5	316L	111	26	-8	30
D	L	2	45	5	316L	111	46	-10	24
D	L	2	45	10	316L	111	46	-8	24
I	L	2,25	45	10	316L	111	42	-8	28
K	L	2	45	5	316L	111	38	-4	22
K	L	2	45	10	316L	111	32	-4	24
Table 1:

In this case, two probes can be chosen, the KRAUTKRAMER MWB SW 4 MHz (C) and the KRAUTKRAMER 3XW composite dual element LW (I).

For each specimen, it is possible to select from the results stored in the data base the probes that give the best ability for testing austenitic steel. The choice must be however tempered by the feed back of the controller.

After these measurements, it was found that shear waves generated by composite probes could be a good compromise to test several types of austenitic welds.

3 - FREQUENCY ANALYSIS THROUGH PARENT METAL AND WELDED ZONE

A other important step for the optimisation of the ultrasonic testing of austenitic welds is the choice of the probe's frequency. For that, it is useful to know the attenuation versus frequency when the ultrasonic beam travel through the parent metal or welded zone.

To perform this frequency analysis, a specific procedure of measurement has been developed. The principle of this analysis is the measurement of the ultrasonic signal amplitude transmitted by reflection on the back wall of the material specimen at a given frequency like it is described in the drawing in figure 2.

Fig 2: Configuration used for frequency analysis

The generator is set as followed:

• Burst mode
• Number of alternation n_a : 5< n_a <10. This number will be chosen in order to avoid overlapping when more than one echo is observed. A number of 5 alternations is generally enough. It will not be modified during the measurement.
• Time between two bursts: from 200 µs to 1 ms
• Voltage: maximum
• Type of wave: sinusoidal.
The frequency will range between 1 MHz to 12 MHz.
The measurement of the transmitted signal is performed by an oscilloscope or by peak amplitude detector of the oscilloscope if it is existing. In that case, the length of the measurement window will be equal to the length of the burst.

A reference transfer function between 1 MHz and 11 MHz is determined on a disc of silicon nitride. The same function is measured on the material under testing. The attenuation through the material versus the frequency is obtained by calculating the ratio between these two functions.
Figures 3 give as an example the curves obtained on a 316L specimen with 45° longitudinal waves.

Fig 3: Curves Obtained with 45° Longitudinal waves on a 316L Specimen

We can see that attenuation doesn't change in a regularly way with the frequency. Minima were observed. The frequency position and amplitude of these minima depends on steel composition, welding process and especially on the position along the weld. Amplitude variations are less important for the minima in the low frequency zone (around 2 MHz).

Using probes with operating frequencies around the attenuation minima observed in the high frequency zone is embarrassing. As a matter of fact, maximum of attenuation could be very important.

For shear waves, the increasing of attenuation according frequency is more regularly than with compression waves but is very important. It not depends on too much with the position along the weld. To use this type of wave, it is recommended to work at a frequency as low as possible.

4 - BEAM DRAWING

To obtain a greater confidence in the ultrasonic testing of the austenitic weld, we must know what the ultrasonic beam shape will be when it propagate through the welded zone. To perform the corresponding beam drawing, classical technique is to work in a transmission mode with an electrodynamic probe as the receiver. But this technique is relatively complex and necessitate a specific electronic tool.

An other way is to use an ultrasonic probe, piezoelectric or EMAT, as the receiver. After comparing the two methods, we have chosen a focused beam probe like the receiver for the beam drawing. As a matter of fact, this method offers a greater resolution than the EMAT technology and, in addition, the level of ferrite in the steel does not affect it. Contrary to EMAT, it is not possible to separate the contribution of shear waves to the contribution of longitudinal waves. But in the context of this programme, the two lobes are stored for two different spatial positions and can be separate easily.

The weld cap disturbs the acquisition made through the melted metal but a great part of the beam can be observed yet and, generally, the interesting parameters have been measured.

The principle of this analysis is the measurement of the amplitude of the ultrasonic signal transmitted by the probe in study and travelling through a block or a specimen made with the specified material. These analysis can be performed with the beam propagate through the parent metal or through the welded zone (figure 4).

Fig 4: Configuration used for the beam drawing

The first step of the procedure is to research the maximum of the amplitude by moving the receiver probe in the scanning plan (the emerging point of the ultrasonic beam and the focusing point of the receiver probe merge). The gain for the receiver channel is adjusts to obtain an amplitude of 90% of the full screen.

The origin of the scanning area is fixed at X and Y co-ordinates in relation to this position, for example -30 mm in X and Y direction. The acquisition windows are adjusted to perform the measurement of the signal amplitude during the complete cycle.

The example in figure 5 has been obtained through a weld. The definition of the various angles measured on the scan is also given. In this case, the beam of longitudinal waves is not very disturbed. The example in figure 6 gives the case of a very disturbed beam.

Fig 5: Weld region drawing of beam explanation

Fig 6: An example with a very disturbed beam

5 - CONCLUSION

The techniques and tools exposed in this paper can help a controller to prepare the test of an austenitic weld. The best approach should be :
• Performing frequency analysis to optimise the compromise resolution/attenuation,
• If the tools exists, chose in the database the probe giving the best results,
• Drawing the beam through a representative welded sample to obtain an idea of the distortion of the beam.

In all cases, the ultrasonic operator must not miss that testing welds with an austenitic structure already require a particular approach and result obtained with a sample must be used with caution.

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