電磁超音波共鳴法による炭素鋼結晶粒度の測定

概要

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Grain size in carbon steel exerts a great influence on its mechanical properties. The average grain size was evaluated from the frequency dependence of ultrasonic attenuation using a noncontacting electromagnetic acoustic transducer(EMAT). The present measurement is composed two steps. First, the resonance frequencies of plate samples were determined using an EMAT to excite and receive thickness-shear oscillation. The resonance frequencies are measured at a regular interval in the 0.5-20 MHz range to be on the order of l0^6 accuracy by sweeping the operating frequency and obtaining its amplitude spectrum. The ringing signals were processed with a superheterodyne phase-sensitive detector. Second, the attenuation coefficient was determined as a function of resonance frequency. At each resonance frequency, the output signal from the detector decreases exponentially with time, and the coefficient is obtained by fitting this ring down curve to an exponential function. Using a suitably chosen scattering coefficient, the average grain size was obtainable from the fourth power term in the frequency dependence. The final results were favorably compared with the average of the 3D distribution of grain size estimated from the photomicrographs of the exposed cross sections.
1994-01-25