PULSED ELECTROMAGNET EMAT FOR HIGH TEMPERATURES

Transcription

1 PULSED ELECTROMAGNET EMAT FOR HIGH TEMPERATURES F. Hernandez-Valle 1, S. Dixon 1 1 Dept. of Physics, University of Warwick, CV4 7AL ABSTRACT. We presented recently a design of Electromagnetic Acoustic Transducer (EMAT) that uses a pulsed electromagnet to provide the required magnetic field for operation. This new EMAT exhibited a significant improvement in the generated ultrasonic signal amplitude and signal to noise ratio, for operation on mild steel samples at room temperature, compared to equivalent EMATs that use permanent magnets. Results for using the pulsed electromagnet EMAT to generate and detect shear waves in mild steel at elevated temperatures are presented here. Keywords: EMAT, Pulsed electromagnet, High temperature. PACS: R43.35.Yb, Zc, Dv, Bx, Ex. INTRODUCTION Inspections of components at high temperatures have wide use in industrial plants and processes [1-2]; the cost associated with shutdowns for unscheduled inspections or repairs is high [3]. Currently, there are a number of methods used for inspection at high temperature, including laser based interferometric methods, and eddy current based methods [4-7], amongst others. Laser based methods are usually expensive, highly dependent on surface condition and require fairly large diameter optics, even when fibre-coupled. Furthermore, for long term measurements in condition monitoring application the use of these systems can be impractical. Eddy current methods can be limited to some extent for operation on ferritic steel below the Curie point, where the steel is ferromagnetic and has a very small skin-depth, typically in the order of microns at eddy current khz test frequencies. High temperature piezoelectric materials and their potential use in NDT applications are receiving increased attention [8]. Extensive reviews on progresses and challenges on designing piezoelectric ultrasonic transducer for operation at high temperatures can be found in the literature [9]. Previous works have shown that ultrasonic measurements can be done at temperatures in excess of 1100ºC using laser-emat configuration [10-11]. However, when using EMATs, the temperature of the magnet must be kept below the maximum operating point (typically in the range of ºC), and the temperature of the coil must be sufficiently low to avoid damage to the coil s insulation. This usually requires water cooling. An alternative and promising approach is to employ an electromagnet, as was typical in the earlier generations of EMATs and is still used in various designs today [12]. Electromagnets can be used at elevated temperatures without cooling, beyond the maximum operation temperature of permanent magnets such as NdFeB or SmCo.

2 This paper shows results obtained at a range of temperatures using a prototype EMAT with a pulsed electromagnet; and a comparison between experimental measurements and a 3D simulation of the magnetic field produced by the electromagnet at room temperature. EXPERIMENTAL WORK Setup The performance of a pulsed electromagnet EMAT on low carbon mild steel sample at a range of temperatures was performed using the setup shown in fig. 1. FIGURE 1. Experimental set-up to investigate EMAT performance at different temperatures. The pulsed electromagnet is energized using a current pulse generator; a pulser-receiver system is employed to drive the EMAT coil in generation and detection [12]. The EMAT coil is covered with ceramic alumina in order to protect the wire insulation from heat. Measurements of the ultrasonic signal using the pulse echo mode were carried out from room temperature (RT) to 250 C, with 10 C increments. From RT to 250 C In order to perform the experiments at different temperatures a hot plate heater was used to heat the sample and a general purpose thermocouple type K (chromel-alumel), was employed to monitor the temperature of the sample surface. The procedure to execute the experiments is as follows; 1) The sample was heated up to the target temperature, 2) The sample is left on the heater for a certain time in order to reach a uniform temperature (the sample surface temperature is monitored with the thermocouple), 3) The pulsed electromagnet is positioned on the sample surface, and 4) The measurement is taken. This procedure was repeated from RT to 250 C, every 10 C. The shear wave signals obtained have acceptable signal to noise ratio, even for the signal recorded at 250 C. The shear wave signals recorded at RT and 250 C can be seen in figure 2.

3 FIGURE 2. First back-reflected shear wave signals at different temperatures (mean of 16 signals). An important consideration when performing inspections at high temperatures is the change of the acoustic velocity in the sample. This physical change can lead to large errors if this is not taken into account. The variation with temperature, from RT to 250 C of the shear wave velocity is shown in figure 3. FIGURE 3. Shear wave velocity variation with temperature (RT to 250 C) The details of the velocity variation with temperature provided by this graph will be used to modify the calibration details of the pulsed electromagnet EMAT. Magnetic field intensity Experimental measurements of the magnetic field intensity produced by the pulsed electromagnet on two different samples, low carbon mild steel and aluminium, at RT were presented in [12]. Nonetheless, a non-linear behaviour of the Hall-effect sensor (model 634SS2 by Micro Switch) for the highest magnetic field intensity made the measured value unreliable. In order to avoid this issue, an alternative suitable sensor was used (P15A by Advanced Hall Sensors LTD), and the measurements for the aluminium and low carbon mild steel samples are shown in figures 4 (a) and (b). Previous work demonstrated that the measured magnetic field intensity is

4 approximately three times stronger in low carbon mild steel when compared to aluminium [12]; which is consistent with the values obtained from 3D finite element (FE) simulations using FEMLAB in this work. A sketch of the 3D model and the results of the FE simulation in both samples are presented in figures 5, 6 (a) and (b), respectively. (a) Fig. 4. Experimentally measured magnetic field intensity on (a) Aluminium and (b) Steel.

5 Fig. 5. Sketch of 3D model used for FE simulation. (a) Fig. 6. Line plot of z component of the magnetic field intensity for (a) Aluminium and (b) Steel sample. The magnetic field strength simulations were calculated assuming the experimentally measured maximum value of DC current and do not take into account the change in

6 relative permeability of the electromagnet core. The simulated values obtained at the same position as the sensor was located, were mt and mt for low carbon mild steel and aluminium, respectively; and their corresponding experimental values were 818 ± 35mT and 255 ± 35mT. CONCLUSION A significant progress has been made with the pulsed electromagnet based EMAT towards its main goal, operation at high temperature. It has been shown that is possible to perform measurements on low carbon mild steel with surface temperatures as high as 250 C. The single shot signal to noise ratio is acceptable (10.3 db), even at that temperature. Based on the results described here, and the advantages over EMAT with permanent magnets showed in a preceding work [12]; the electromagnet based EMAT will be a valuable tool for many industrial applications, especially for those that require operation at high temperatures. FUTURE WORK At this stage there are some fundamental tasks to be accomplished; amongst these the most important is to improve the transducer design to make it capable of withstanding an operation temperature significantly higher than 300 C. In addition, more works is required regarding the FE simulation, and the variation in temperature has to be taken into account. ACKNOWLEDGEMENTS The authors would like to thank the following organizations for supporting the PhD studies of Mr. Hernandez-Valle: the Mexican National Council of Science and Technology (CONACYT), the Royal Academy of Engineering and Elster Group. REFERENCES 1. K. J. Kirk, I. Cornwell, A. McNab, A. Cochran, G. Hayward, Insight. 38, pp (1996). 2. I. Atkinson, C. Gregory, S. P. Kelly, K. J. Kirk, Ultrasmart: Developments in ultrasonic flaw detection and monitoring for high temperature plant applications, 2007 Eight International Conference on Creep and Fatigue at Elevated Temperatures, Proceedings on CREEP 8, ASME, San Antonio, TX, 2007, pp M. J. Bergander, Appl. Energ. 74, pp (2003). 4. A. McNab, K. J. Kirk, A. Cochran, IEE Proc. Sci. Meas. Technol. 145, pp (1998). 5. H. Nakano and S. Nagai, Jpn. J. Appl. Phys. 32, pp (1993). 6. X. Jian, I. Baillie and S. Dixon, J. Phys. D: Appl. Phys. 40, pp (2007). 7. C.B. Scruby, B.C. Moss, NDT&E Int. 26, pp (1993). 8. T. R. Shrout, S. J. Zhang, R. Eitel, C. Stringer and C. A. Randall, High performance, high temperature perovskite piezolectrics, 2004 IEEE International Ultrasonics, Ferroelectrics, and Frequency Control Joint 50 th Anniversary Conference. Conference proceedings, IEEE, Montreal, Canada, 2004, pp R. Kazys, A. Voleisis, B. Voleisiene, Ultragarsas (Ultrasound). 63, pp (2008).

7 10. A. Idris, C. Edwards and S. B. Palmer, Nondestr. Test. Eval. 11, pp (1994). 11. I. Baillie, P. Griffith, X. Jian and S. Dixon, Insight. 49, pp (2007). 12. F. Hernandez-Valle, S. Dixon, Preliminary tests to design an EMAT with pulsed electromagnet for high temperature in Review of Progress in Quantitative Nondestructive Evaluation, edited by D. O. Thompson and D. E. Chimenti, AIP Conference proceedings vol. 1096, American Institute of Physics, NY, 2009, pp