Acoustic Measurement of Microstructures in Steels

The measurement of acoustic properties can be used for the nondestructive characterization of the microstructure of materials. We have measured the changes in longitudinal acoustic wave velocity and acoustic attenuation in steel specimens whose microstructure and properties differ widely because of differing compositions and heat treatment. The spatial variation of the relative acoustic velocity in standard Jominy end-quench hardenability test specimens was found to correlate very well with Rockwell C hardness scans, indicating a potentially practical method for measuring the hardening response of heat treated steel. Absolute velocity measurements on steel specimens were found to be subject to random scatter related to minor compositional variations; this limits the utility of absolute velocity measurements for microstructural NDE. Attenuation measurements have also been performed on steel samples with different microstructures. The measurement utilized broadband acoustic pulses corrected for transducer response, liquid buffer/solid specimen reflection, and diffraction effects. Attenuation coefficients were seen to be proportional to frequency squared for martensite and to the fourth power of frequency for pearlite. Higher attenuation was observed for pearlitic than for martensitic microstructures