Ultrasonic NDT of steel: effect of the grain size on the ultrasonic propagation and attenuation

Non-destructive techniques (NDT), such as Ultrasound Testing (UT), have been industrially exploited for decades within the material field, for a wide range of possibilities. Ultrasonic non-destructive evaluation relies on exciting the sample under test with an ultrasonic beam and on measur- ing the beam reflected, transmitted or scattered from discontinuities within the material such as grains, fractures, voids or the microstructure itself. In this paper, the effect of the grain size and other several microstructural parameters on the ultrasonic propagation have been evaluated for an experimental low alloyed carbon steel. Main results are: at high frequencies the attenuation coefficient follows the Rayleigh law for scattering and shows a maximum at a frequency value that increases as the grain size decreases; the value of frequency corresponding to the attenuation maximum, converted in wavelength, can be approximately compared as 3 - 4 times the average grain size.