Application of the acoustic resonance method to evaluate the grain size of low carbon steels

The velocities and attenuations of ultrasonic waves in low carbon steels were measured precisely and automatically by the acoustic resonance method using a Lorentz-type EMAT. The attenuations were measured from the decay of a signal from the resonant vibration after stopping the external excitation at the predetermined resonant frequency, while the velocities were calculated directly from the resonant frequencies and material thicknesses. It was not possible to predict the yield strength from the velocity measurements. There was a close relationship between grain size and attenuation in the specimens which consist of ferrite and pearlite. The attenuations measured at a frequency of about 5 MHz showed a good correlation with the average grain size and yield strength. The yield strength could be evaluated within the accuracy of ±50 MPa by the acoustic resonance method. The results would be used for on-line evaluation of the grain size and the mechanical strength of steels in industry.