Artificial steel samples with embedded inclusions and continuous casting slab samples were investigated using a commercial acoustic microscope. An abnormal phase information was noted and used to classify the inclusions and holes in steel. The echo waveforms from steel-air (hole) steel-water and steel-resin interfaces had typical phase reverse characteristics. Water-steel, water-resin, water-alumina and water-silica interfaces had typical positive echo polarities. However, the echo waveforms from steel-alumina, steel-silica and steel-inclusion interfaces lost such characteristics, which seems to be related to the characteristics of material, and displayed positive polarities in image. This very common phenomenon is referred to as an abnormal phase and is helpful in the differentiation of defects. Inclusions larger than 50 ~m can be detected at 50 MHz. Inclusions of about 30 ~m on a sample surface can be effectively detected using a 100 MHz transducer. The influence of size, orientation, acoustic impedance and polarity of defects on detection were investigated and discussed.
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