Ultrasonic transducers for continuous-cast billets

Abstract

Abstract In order to detect internal defects larger than 20 mm in continuous-cast steel billets, our team investigated the frequency and beam width of ultrasonic transducers. Internal defects larger than 20 mm cannot be detected by measuring defect echoes because large defects absorb ultrasonic wave energy. So, large defects should be detected by means of the energy loss. The energy loss is related to the intensity of the back echo. If a steel billet has large defects, it absorbs ultrasonic wave energy and the intensity of the back echo decreases. This means that the energy loss is available for detecting the large defects. Therefore, in order to detect a large defect by back echo, the back echo through a defect-free billet should be of greater intensity than a set value. Because the average density of continuous-cast steel billets is lower than that of hot-rolled billets, the back echoes through the cast billets are weaker than those through the rolled ones. For detecting large flaws in cast billets, the back echoes through defect-free zones should be higher than a set level, such as 50 dB in our system. Our team changed the center frequencies of the transducers from 1 to 4 MHz by changing the thickness of the piezoelectric ceramic plates. 2.25 MHz transducers having 9 mm wide piezoelectric ceramics show the best characteristics for detecting large defects (20 mm) in continuous-cast steel billets.