Austenitic Weld Inspection with EMAT Phased Array.pdf

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temate PA Austenitic Weld Inspection with EMAT PA

Austenitic Weld Inspection Page 1 of 10 Innerspec Technologies

Austenitic Weld Inspection with EMAT Phased ArrayHuidong Gao, Syed Ali, and Borja Lopez

Innerspec Technologies, Inc4004 Murray Place, VA, 24501

ABSTRACT . Austenitic welds are widely used in nuclear, petrochemical, and process industries. Thestrong material anisotropy and coarse grain structure in the dendritic weld zone makes these welds verydifficult to inspect with conventional techniques. It is well known that the Shear Horizontal (SH) wave isvery well suited for this inspection, and that EMAT is the best technique for generating this wave mode, butlack of equipment has prevented its use outside of the research laboratory. In this paper we present thedevelopment of a novel phased-array EMAT instrument and tandem EMAT sensor, and the results obtainedon thick stainless steel weldments. The results corroborate the validity of the technique and the capabilitiesof the equipment, which open up new possibilities both for research and field use.

Keywords: Ultrasonic, Austenitic Weld, EMAT, Phased Array, Tandem Test

1. Introduction

Austenitic stainless steel is widely used in the nuclear and petrochemical industries as well as other high-temperature processes due to its superior resistance to corrosion. Of the many types of austenitic weld

joints, the most common are stainless to stainless steel, stainless to carbon steel with inconel butter, and300 series stainless to inconel. Austenitic welds have a strongly textured columnar grain structure thatscatter ultrasonic energy and skew the waves, which make them very difficult to inspect using traditionalultrasonic inspection systems [1].

The most common wave mode for weld inspection, Shear Vertical, suffers the most skewing due to theanisotropy of austenitic crystal structures. Longitudinal waves skew significantly less than Shear Verticalon austenitic materials, but still experience strong mode conversion at structural and weld boundaries andrequire access to both sides of the weld. Early research in the 1980s showed that the Shear Horizontal

wave does not present mode conversion at structure boundaries, so it was recognized as a potentialsolution for inspection of these welds [2].

Notwithstanding this, Shear energy does not travel through low-density couplants and the horizontalpolarization cannot be easily excited through mode conversion with a wedge, so it is very difficult togenerate with conventional piezoelectric transducers and it is impractical in field use. Electromagnetic

Acoustic Transducer (EMAT) on the other hand is an effective alternative to generate SH waves inultrasonic testing. The mechanisms for sound generation using EMAT are Lorentz force andmagnetostriction [3, 4]. Lorentz force is the dominant mechanism for sound generation in typical austeniticsteel since it has very weak or no magnetism.

Figure 2 provides a schematic of EMAT sound generation using Lorentz force and a comparison with atypical piezoelectric transducer. A typical EMAT sensor consists of two parts, a magnet and a coil. When

the coil is excited with AC current, eddy currents will be generated in the austenitic steel located below thecoil. The interaction between the eddy current and a biased magnetic field generates Lorentz force. Thisforce generates ultrasonic waves that propagate into the austenitic material. In a reciprocal process, theinteraction of elastic waves in the presence of a magnetic field induces currents in the receiving EMAT coilcircuit.


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Figure 1: Principle of Lorentz force EMAT and a comparisonto Piezoelectric transducer for generation of normal beam

The idea of using EMAT generated SH waves for austenitic weld inspection has been around for nearly twodecades. In their 1985 report "The ultrasonic inspection of austenitic materials -State of the art report",Hudgell and Gray [1] already concluded that "The development of EMAT should be encouraged". However,the development of EMAT for this application had to overcome two severe problems:

Austenitic materials have very low conductivity which affects the ability to generate eddy currents,hence sound, with an EMAT. Compared to aluminum, another non-ferromagnetic material whereEMAT generation in strictly through Lorentz forces, stainless can be 10-15 times more resistive withproportional effects on signal-to-noise.

The design of an EMAT probe for SH wave generation at an angle with Lorentz forces, require the useof alternating magnetic poles. Even though the capabilities and availability of pulsed and permanentmagnets has increased significantly in the last few years, there is a practical limitation on the size of themagnet below which the magnetic field becomes too weak, thus constraining the design options.Moreover, because of ringing effects in the magnets and the difficulty in isolating them from the coil, SHwaves at an angle can only be practically generated in a pitch-catch arrangement in which thetransmitter and receivers are different elements separated physically.

The end result is that despite of many attempts in the past [7, 8], EMATs are still not used in the field for

austenitic weld inspection. As the leader in EMAT technology, researchers and engineers at Innerspechave been working on this problem since 2003, and have filed several patents for equipment used in thisapplication. In a paper presented in 2010, we introduced an 8 Channel instrument with up to 20 kW ofinstant power per channel, and an angled pitch-catch SH wave phased array 2-sensor arrangement withpermanent magnet arrays [9]. In this paper, we will introduce a new tandem phased array EMAT sensor,and the results obtained with it. This equipment has been tested by several multinational companies andinstitutes specializing in nuclear power generation, and it has demonstrated the ability to reliably detect allthe defects in 6 different mockups..

Piezoelectric UT EMAT UT

EMAT CoilCircuit

Ultrasonic WaveUltrasonic Wave

Eddy Currents

LorentzForce Magnetic Field

Couplant

PZT

Magnet


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2. Inspection System Description

2.1. High-Power 8-Channel EMAT Phased Array Instrument

The traditional disadvantage of EMAT transducer is its low efficiency in sound generation and reception. Asa result, EMAT systems typically produces less signal amplitude and require powerful pulsing electronics,good noise reduction, and signal processing algorithms to enhance the signal to noise ratio. One of ourpatent-pending innovations is a new tone-burst amplifier with an H-bridge topology that enjoys the highestpower to size output, efficiency, and wider frequency response of any equipment in the market.

Figure 2 shows a picture of our temate PowerBox 8 that uses this technology to produce up to 2000Vppor 20kW of peak power per channel at 1% duty cycle in frequencies from 100kHz to 7MHz. Two X-Y built-in encoders and 12 programmable Inputs/Outputs facilitate integration with a scanner and other externalequipment. Control is performed through an external PC over an Ethernet connection. The PC includesour PowerUT software with proprietary filters and other features devised specifically for EMATapplications.

Figure 2: temate PowerBox 8 Instrument

2.2. EMAT Phased Array Probe

Phased array technology has been widely used in ultrasonic testing to achieve higher sensitivity andresolution. EMAT phased-arrays use a series of RF coils analogous to the small piezoelectric materials thatare arranged together in a piezoelectric phased array [10]. We have experimented with several designs ofEMAT phased array construction including a flexible electromagnet array (US Patent 7,165,453), a twosensor construction with arrays in angled pitch catch, and a set of low profile permanent magnet array intandem configuration. Each design has different advantages, but the tandem sensor with a footprint of25mm by 50mm is the ideal configuration for use on tubes because of its reduced dimension and simplicity

of operation.

Figure 3 shows a schematic of tandem pitch catch phased array for beam steering. The eight transmittersare excited with independent time delays so waves constructively interfere with each other around the focalspot to achieve a wave field of strong intensity. The reflected signals from the focal region arrive at eachelement at a different time, and it is delayed according to focal laws so the signal from the focal regionsums up in phase. As a result, the ultrasonic beam can be steered across a predefined range. Without theeffect of mode conversion, shear horizontal waves can be focused at any range of angle, and can be usedfor both zero degree, and angle beam steering.


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Figure 3: EMAT phased array sensor, and beam focusing principle

2.3. Inspection System Configuration

Figure 4: Basic components of EMAT inspection system

Figure 4 shows the components of our EMAT phased array inspection system. It includes a computer, thetemate PowerBox 8 , an eight-channel signal conditioning box, the sensor assembly, and interconnectingcables.

Figure 5 shows the information flow in the system. The user sets in the computer all the controllingparameters for the pulsing signal including tone-burst frequency and duration of the pulse. The instrumentreceives this input and generates a low-voltage pulse train, which is subsequently amplified to high-voltagethat is fed through the signal conditioning box to the transmitter. With the high-voltage excitation, thetransducer generates sound waves that propagate into the material. The receiver detects ultrasonic wavesand converts them into electrical signals. These signals are amplified and filtered in the signal conditioningbox and sent to the instrument where they receive further amplification and treatment. The signal is thendigitized and sent to the PC for further processing if needed, display, and storage.

Transmitter Receiver


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In addition to the main signal flow, the computer can also communicate with other systems for automatedoperation (start, stop and alarm).

Figure 5: Schematic of information flow in EMAT inspection system

2.4. Test Setup

For our laboratory tests, the sensor was attached to a XYZ gantry scanner with a fixture that allowed thesensor to float freely and adapt to the curvature of the samples. Figure 6 shows the gantry, fixture andsensor on a test sample.

Figure 6: Test system configuration; (a) Test Sample, (b) Mechanical fixture


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Figure 8: Sector scan of defect A: (a) Same Side 75mm, (b) Opposite side 75mm

Figure 9: Sector Scan of defect B, (a) Same Side 50mm, (b) Opposite side 100mm

Figure 10: Sector Scan of defect C, (a) Same Side 50mm, (b) Opposite side 75mm

Defect

Defect


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Figure 11: Sector scan of defect D: (a) Same Side 50mm, (b) Opposite side 100mm

Figure 12: Defect E: (a) Same Side 50mm, (b) Opposite side 75mm

Figure 13: Defect F: (a) Same Side 50mm, (b) Opposite side 75mm


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4. Experience

The test system has been tested on different mockups used in nuclear power plants. The system wascapable of detecting all artificial defects in these samples with superior signal to noise than conventionaltechniques. The pictures below show the mockups tested to date that include both stainless steel anddissimilar metal welds. The results from these tests are proprietary and cannot be released.

Figure 14: Mockups Tested with EMAT Phased Array System


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5. Conclusions

In this paper we discussed recent developments in EMAT technology for the inspection of thick austeniticwelds. Using a high-power 8-channel EMAT instrument and a novel tandem SH phased array sensor, weare able to detect defects in austenitic weld mockups from both sides of the weld with superior signal-to-noise. The results from a 50mm sample are shown in detail.

In addition to the ability to detect defects from either side, this EMAT technique permits sweeping the wavefrom 0 to 90 which provides a completely new benefit not available on traditional piezoelectric systems.Using this system, it is possible to perform normal beam inspection of the base material, angle beaminspection of the heat affected zone, the austenitic weld zone, and the heat affected area in the oppositesize of the weld, all at the same time and in one pass.

Future work will be geared towards improving sizing techniques as well as well as adding high-temperatureand submersion capabilities to the equipment.

REFERENCES1. J. Huggell and B.S. Gray, The ultrasonic inspection of austenitic materials -state of the art report ,

1985.2. M.G. Silk, "A computer model for ultrasonic propagation in complex orthotropic structures",

Ultrasonics, 19, 208-212, (1981).3. Thompson, R.B. in Physical Acoustics, Vol. 19. Edited by Thurston R.N. and Pierce A.D., Academic

Press, New York, 1990, 157-200.4. M. Hirao and H. Ogi, EMATs for Science and industry -NonContacting Ultrasonic Ultrasonic

Measurements, Kluwer Academic Publishers, 2003.5. Huidong Gao, Syed Ali, Borja Lopez, "Efficient detection of delamination in multilayered structures

using ultrasonic guided wave EMATs", NDT&E Int (2010), doi:10.1016/j.ndteint.2010.03.004,6. Huidong Gao, Syed Ali, and Borja Lopez, "Inspection of Austenitic Weld with EMATs", Review of

Progress in Quantitative Nondestructive Evaluation, 29B, 1175-1181, (2010).7. G. Hubschen, H.J. Salzburger, M. Kroning, et.al, Results and experiences of ISI of Austenitic and

dissimilar welds using SH-waves and EMUS- Probes, Elsevier Science Publishers, K. Kussmaul,

Editor, 1993.8. Ludwig von Bernus, Werner Rathgeb, Rudi Schmid, Friedrich Mohr, Michael Kroning, "Current in-service inspection of austenitic stainless steel and dissimilar metal welds in light water nuclearpower plants", Nuclear Engineering and Design, 151, 539-550, (1994).

9. Huidong Gao and Borja Lopez, Development of Single Channel and Phased ArrayElectromagnetic Acoustic Transducer for Austenitic Weld Inspection , Materials Evaluation, Vol.68.(7), 821-827, 2010.

10. Noel Dube, Introduction to Phased Array Ultrasonic Technology Applications, RDTech, 2004.

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