EMAT-Phased Array Inspection of Thick Austenitic Stainless Steel and Dissimilar Metal Welds

Abstract

AbstractNon-destructive testing (NDT) of austenitic stainless steel and dissimilar metal welds is important for inspection of main vessel in prototype fast breeder reactor (PFBR). The strong material anisotropy and coarse grain make these welds very difficult to inspect using conventional ultrasonic techniques (UT) employed with piezoelectric transducers. It is well known that the shear horizontal (SH) wave is very well suited for this inspection, and electromagnetic acoustic transducers (EMAT) are the best for generating this wave mode. In order to overcome the low-efficiency sound generation due to low conductivity and strong attenuation, an 8-channel EMAT-phased array (PA) sensor has been used in tandem mode to enhance the power level and to improve the signal-to-noise ratio. It generates bulk SH waves with almost uniform amplitude for beam angles from 0° to 90° and can cover the entire volume of the weld including the heat affected zone by scanning from one probe position. In this paper, the EMAT PA probe was used for detection of defects in thick austenitic stainless steel and dissimilar metal mock-up weldments at 600 kHz. It has been successfully demonstrated that the EMAT PA probe can detect 3 mm deep notches and 2 mm diameter side-drilled hole in 30 and 25 mm thick weld pads. The large active aperture allows the use of highly focused beams for good defect detection and high-resolution imaging of weld defects. The results obtained from the mock-up samples with artificial defects demonstrate the potential of this EMAT PA probe in laboratory settings and recommendations to transit to the actual field are also explained in this paper. Though the exciting frequency of the EMAT PA probe is very low, it offers good defect detection sensitivity in thick austenitic stainless steel and dissimilar metal welds in main vessel of PFBR.KeywordsElectromagnetic acoustic transducerPhased arrayShear horizontal waveAustenitic stainless steel weldDissimilar metal weld