Characterization of microstructural anisotropy in pearlitic steel with mode-converted ultrasonic scattering

A mode-converted (longitudinal-to-transverse, L-T) ultrasonic scattering technique was applied to evaluate the variation of microstructural anisotropy in a railroad wheel sample. The anisotropy was examined using a pitch-catch longitudinal-to-transverse measurement configuration in two perpendicular directions. Then the whole scan area was divided into 14 sections to calculate the variance of scattered signals for each section. The variance curves were fit with the theoretical models to determine amplitudes for each section. The experimental results show the variance amplitudes of the L-T response are very similar for two perpendicular directions near the tread surface which indicates microstructural isotropy. For measurement areas near the center of the sample from the tread surface, the variance amplitudes in two perpendicular directions split and the difference increases with depth. The result indicates growing microstructural anisotropy in regions associated with the coarse duplex pearlite microstructure. The fine and coarse duplex microstructures have already been examined by optical analysis, but the microstructural anisotropy cannot be characterized from metallographic images. The mode-converted ultrasonic scattering approach presented in this paper provides a nondestructive evaluation (NDE) method for characterization of microstructural anisotropy in complex metallic materials.