Development of an ultrasonic inspection technique for cast stainless steel

Cast stainless steel is widely used for reactor coolant piping in pressurized water reactors (PWRs). Its complex crystal structure hinders accurate ultrasonic inspection. This is because the large attenuation and scattering noise due to the coarse grain structure interfere with defect detection, and the ultrasonic wave velocity variations and beam skewing resulting from the anisotropic crystal structure make the determination of defect locations inaccurate. In this study, a large-aperture twin-crystal transducer with a local water immersing holder was developed in order to enhance the defect delectability for cast stainless steel. The defect detectability was improved by combining the transducer with an automated scanning device to create images of the inspection data. This automated ultrasonic inspection system was applied to a weld test assembly of the same scale as actual PWR primary coolant piping, and was compared with other inspection techniques. The results demonstrated that the defect detectability of this technique was superior to other techniques. However, the improved performance of the new transducer may simply be due to its being focused at the correct depth.