Ultrasonic backscattering measurement of hardness gradient distribution in polycrystalline materials.

It is crucial to obtain the internal hardness distribution in polycrystalline materials to evaluate the mechanical performance of components and monitor their service life. Current methods, however, fail to meet the non-destructive evaluation needs for materials with hardness gradient distributions. This paper, based on the principle of grain boundary scattering of ultrasound in polycrystalline materials, combined with the Transverse-to-Transverse Singly-Scattered Response (T-T SSR) theory, proposes an ultrasonic SSR model adapted to hardness gradient distributions. The model elucidates the influence of hardness gradient variations and grain dispersion on ultrasonic scattering. Using DREAM.3D, seven different-scale polycrystalline volumes were constructed to assess the relevance of volume-weighted average grain size and spatial correlation of hardness gradient materials. Finally, induction quenching was applied to 40Cr to induce a gradient hardness distribution internally, followed by ultrasonic backscatter experiments. The results indicate that the theoretical model and the spatial variance of measured signals align well over a relatively long time window. For the specimen with minor curvature, the theoretical hardness distribution obtained by the model is accurate, with an average error of 2.55 % compared to destructive testing data. However, the results for the larger curvature reveal limitations in the model.