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Microstructural variability from manufacturing and post-fabrication activities have been attributed to a significant portion of failure in grade 91 steels. For example, heat treatment from welding can change the microstructure locally, which can change the hardness, phase content and grain size. This article explores the use of ultrasonic coda waves to detect microstructural changes in martensitic grade 91 steel. Tube samples with eight different microstructural conditions were simulated using different heat treatments and were tested using coda waves. Six different differential features were extracted from the coda wave signals. Only three features: Loss of correlation, differential curve length and coda wave interferometry showed accuracy > 50%. Further, clustered analysis to test the ability of the features to classify good, mid-level and bad microstructures showed that only loss of correlation had good accuracy. Overall, the study suggests that coda waves can be used to detect subtle changes in microstructure. [ABSTRACT FROM AUTHOR]

This article reports on the characterization of the "as-fabricated" state of Inconel 718 samples fabricated using laser directed energy deposition (DED). Laser-DED is known to produce complex metastable microstructures that can significantly influence the baseline ultrasonic response compared to conventional processing methods. The present work uses three parameters to characterize the samples: (a) ultrasonic velocity, (b) an attenuation coefficient, and (c) a backscatter coefficient. The baseline ultrasonic response from the DED sample was compared against the ultrasonic properties of conventional IN718 samples reported in the literature. The results suggest that strong grain boundary scattering from large macrograins can lead to attenuation and backscatter values that are significantly higher than conventional samples. Additionally, the results including velocities, attenuation and backscatter coefficients were found to be dependent on the fabrication direction, with the build direction being different from the transverse directions. Finally, destructive analysis was used to develop conjectures to explain the experimentally observed ultrasonic response. [ABSTRACT FROM AUTHOR]