The Sensitivity of the Focused Ultrasonic Method Used in Inclusion Testing of the Thick Steel Specimen

Abstract

For the task of evaluating the inclusions in the steel specimen under high efficiency testing condition, it is important to make it clear that what testing sensitivity the ultrasonic method can reach for a specimen of large size, especially a thick one. In this paper, this problem is researched and discussed. Before experimental researching, we simulated the focused ultrasonic field formed in the steel specimen by DPSM (Distributed Point Source Method), the frequency ranges from 7.5MHz to 15MHz. DPSM analysis reveals that the ultrasonic field of certain strength can be formed in the thick steel specimen of coarse grain, and the focused transducer of 15MHz is suitable for the inclusion evaluation of thick steel specimens. Then, with the help of a precise micro-step C-scan equipment and the 15Mhz transducer (13mm Diameter, 255mm focal length), the ultrasonic test sensitivity research work was carried out on the thick steel specimen, by the way of testing the inclusions which's size were as tiny as possible, and located as far from the testing surface as possible, and at last, the ultrasonic detection results were testified by mechanical dissection. The dissected inclusions which can be tested are grouped into 8 different grades by its size, it is demonstrated that the deepest inclusion can be tested in the specimen is about 80mm beneath the testing surface by ultrasonic method, and reflection area is about 0.02mm2, equivalent to a circle inclusion of diameter 0.16mm. In addition, there are some semi-quantitative relationships can be drawn among the grade of inclusion, the location depth of inclusion in the specimen, and the reflected ultrasonic signal strength from the inclusion, this give a way of evaluating the inclusion size in the thick steel specimen by the focused ultrasonic method.