Mapping of 1-MHz, 45° Longitudinal-Wave Fields in Centrifugally Cast Stainless Steel

The distortion incurred by an ultrasonic field when propagating through coarse-microstructured materials was of interest. To perform an effective and reliable ultrasonic inspection, the ultrasonic field should be both spatially coherent (i.e., the field is not partitioned into multiple wave fronts traveling to different locations) and stable (i.e., field parameters such as effective refracted angle and field position do not vary sufficiently to make an inspection unreliable). Previous work indicated that the sound field emitted by a 1-MHz, 45°, longitudinal-wave probe with a 38-mm diameter transducer maintained spatial coherency while propagating through the pure microstructural forms of centrifugally cast stainless steel (CCSS) [1,2]. This analysis was extended to the mixed microstructural modes of CCSS. Furthermore, the variation of field distortion incurred by propagating through a selected microstructure was investigated by acquiring field maps from different material volumes of the same microstructural classification. To accurately map the ultrasonic field, an improved technique was used so that receiver directivity maintained a ± 1 dB sensitivity over a broad angular range centered about 45°