Use of Electromagnetic Acoustic Resonance Method to Detect Micro-Voids Via Evaluation of Ultrasonic Wave Attenuation Coefficient of SUS304 Steel Fabricated by Hot Isostatic Press

Abstract

Creep damage in SUS304 steel samples fabricated by a hot isostatic press (HIP) at 1050°C was evaluated using the electromagnetic acoustic resonance (EMAR), noise energy and ultrasonic spectroscopy (first moment) methods. The coefficients of attenuation of 1.1 to 5.4 MHz electromagnetically excited acoustic waves in the samples were investigated. By reducing diffraction loss and the loss to the electromagnetic acoustic transducer due to transmission of the ultrasonic waves via mechanical contacts, it was possible to detect shear wave attenuation coefficients as low as 2.5×10^-4/microsecond at 1.1 MHz. With specimens fabricated at pressure of 170MPa, the attenuation coefficient increased in proportion to frequency up to 5.4 MHz, whereas with specimens fabricated at pressure lower than 80 MPa the attenuation coefficient increased rapidly above 4 MHz. The void fraction was found to be greater in specimens fabricated under lower pressure, which may be responsible for the markedly higher ultrasonic attenuation in the specimens fabricated at pressure lower than 80MPa. It was apparent from the experiments that the EMAR method detects creep voids with greater sensitivity than the other methods. Accordingly we hope it will be possible for the EMAR method to be used on real facilities in the future.