Characterization of graphite nodules in nodular cast iron via multimodal ultrasonic scattering measurements using a rosette configuration

Nodular cast iron's complex microstructure is characterized by spherical graphite nodules embedded within a polycrystalline matrix, presenting unique opportunities for ultrasonic scattering measurements due to microstructural heterogeneity. This work focuses on two grades of cast iron (65-45-12 and 80-55-06) tested in an ultrasonic immersion setup using a custom 3-D-printed “rosette” fixture. Four spherically focused 10 MHz transducers were placed to collect different scattering modes, utilizing longitudinal and transverse waves in both in-plane and out-of-plane configurations. Cylindrical specimens (diameter 52 mm and thickness 12.6 mm) were raster scanned in 0.25 mm steps with the ultrasonic rosette, followed by analysis of time-dependent spatial variance over the entire sample area and within local regions. Microscopy provided quantitative mapping of nodule properties on top sample surfaces for correlation with scattered wave measurements. Bulk analysis showed the spatial variance of scattered signals revealed significant sensitivity to microstructural variations between grades despite subtle variations in nodule properties. Local analysis demonstrated a spatial correlation between ultrasonic scattering and nodule size distribution, with locations containing larger nodules and wider size distribution having higher spatial variance in ultrasonic scattering. These results demonstrate the potential of multimodal ultrasonic measurements for quantitative microstructural characterization at both bulk and local scales.