ULTRASONIC TRANSDUCERS FOR CONTINUOUS-CAST BILLETS

In order to detect internal defects larger than 20 mm in continuous-ca st steel billets, our team investigated the frequency and beam width o f ultrasonic transducers. Internal defects larger than 20 mm cannot be detected by measuring defect echoes because large defects absorb ultr asonic wave energy. So, large defects should be detected by means of t he energy loss. The energy loss is related to the intensity of the bac k echo. If a steel billet has large defects, it absorbs ultrasonic wav e energy and the intensity of the back echo decreases. This means that the energy loss is available for detecting the large defects. Therefo re, in order to detect a large defect by back echo, the back echo thro ugh a defect-free billet should be of greater intensity than a set val ue. Because the average density of continuous-cast steel billets is lo wer than that of hot-rolled billets, the back echoes through the cast billets are weaker than those through the rolled ones. For detecting l arge 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 te am changed the center frequencies of the transducers from 1 to 4 MHz b y changing the thickness of the piezoelectric ceramic plates. 2.25 MHz transducers having 9 mm wide piezoelectric ceramics show the best cha racteristics for detecting large defects (> 20 mm) in continuous-cast steel billets.