Observation of Ultrasonic Wave Propagation in Centrifugally Cast Stainless Steel

Cast stainless steel (CSS) is widely used in nuclear power plants, particularly in PWR primary coolant systems. The JSME Fitness-for-Service Code requires periodic in-service inspection of welds in such systems by ultrasonic testing (UT). but anisotropy and heterogeneity of CSS make flaw detection and sizing difficult. Phased array ultrasonic technique (PAUT) is considered the most promising technique to solve such difficulties. In order to take advantage of PAUT to CSS, it is crucial to deeply understand how waves propagate in CSS. In this study, a 2-MHz linear array transducer is used to generate waves which propagate within a specimen made of centrifugally cast stainless steel (CCSS) with columnar grain structure at different directions, and a normal beam longitudinal transducer is used to receive waves propagating in the specimen. The observation results show that wave propagation direction is different from the specified direction and the difference strongly depends on the specified direction. The measured values of phase velocity and skew angle are consistent with theoretical values based on the hexagonal system.