We performed defect detection simulation considering billets with a deep-hole or spherical defect. We conducted defect detection in a billet of duralumin with a deep-hole defect and found no discrepancy between our previous and present research results because the images obtained are similar. We also conducted defect detection in a billet of steel with a spherical defect. We obtained visualization images in multiple measurement planes. We also obtained three-dimensional visualization images by binarizing the pseudo sound velocity. From the images, we found that the three-dimensional visualization of spherical defects is possible and that the scanning pitch in the longitudinal direction is about 10 mm at maximum.

For the nondestructive inspection of steel billets, the pulse echo method is generally employed in their manufacturing process. However, for high-attenuation billets, the pulse echo method is useless owing to the low echo level, while the transmission method is expected to detect defects inside high-attenuation billets. In this study, we consider the effectiveness of ultrasonic computerized tomography (CT) using time-of-flight (TOF) measured by the transmission method compared with the pulse echo method. From the simulation and experimental results, even though it was difficult for the pulse echo method to detect defects inside high-attenuation billets, the defects could be visualized by ultrasonic CT using TOF measured by the transmission method. Therefore, ultrasonic CT using TOF measured by the transmission method is more effective for detecting defects inside high-attenuation billets than the pulse echo method.

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We have proposed a nondestructive inspection method for steel billets using ultrasonic computerized tomography by time of flight (TOF). It has been found that the proposed method may detect defects inside high-attenuation materials with the aid of sequential pulse transmission all around the surface. However, it takes a long time to measure the TOF by this method owing to large numbers of scans and transmissions. In this paper, we propose a TOF measurement method to apply our inspection method to simultaneous TOF measurement in a shape-distorted billet to speed up the inspection. In this method, besides a measurement plane and a reference plane, a standard reference plane is used to compensate not only the effect of the characteristics between a transmitter and a receiver but also the effect of the shape distortion in simultaneous measurement. In this paper, the performance of the present method was verified by numerical simulation and experiment. As a result, the defects in a shape-distorted billet were successfully visualized in simultaneous TOF measurement by the present method. Moreover, it was confirmed that the proposed method diminished the artifacts in the computerized tomography (CT) images by compensating the error caused by multiple simultaneous transmissions. It was revealed that the present method is more robust to noise than the previous method. Therefore, it is expected that the TOF measurement in the nondestructive inspection of a shape-distorted billet can be sped up by the proposed method.

In this study, an ultrasonic computerized tomography (CT) using time-of-flights (TOFs) has been used for the nondestructive inspection of steel billets with high acoustic attenuation. One of the remaining problems of this method is noise in CT images, which makes it difficult to distinguish defects from noise. Conventionally, noise is suppressed by a low-pass filter (LPF) in the process of filtered back projection (FBP). However, it has been found that there is residual noise even after filtering. To cope with this problem, in this study, the noise observed in ultrasonic testing was examined. As a result, it was found that the TOF data used for CT processing contains impulse noise, which remains in the CT image even after filtering, owing to the existence of transducer directivity. To remove impulse noise selectively, we propose a noise reduction technique for ultrasonic CT for steel billet inspection, that is, preprocessing (outlier detection and removal) of TOF data. The performance of the proposed technique was evaluated experimentally. The obtained results suggest that the proposed technique can remove impulse noise selectively and markedly improve the quality of the CT image. Hence, the proposed technique can improve the performance of ultrasonic CT for steel billet inspection.