Comprehensive microstructural characterization in modified 9Cr-1Mo ferritic steel by ultrasonic measurements

Modified 9Cr-1Mo ferritic steel (T91/P91) has been subjected to a series of heat treatments consisting of soaking for 5 minutes at the selected temperatures, starting from the a-phase region (1073 K) to the gamma + delta-phase region (1623 K), followed by oil quenching. Hardness measurements, microstructural features, and grain-size measurements by the linear-intercept method have been used for correlating them with the ultrasonic parameters. Ultrasonic velocity and attenuation measurements, and spectral analysis of the first backwall echo have been used for characterization of the micro structures obtained by various heat treatments. As the soaking temperature increased above Ac-1, the ultrasonic velocity decreased because of the increase in the volume fraction of martensite in the structure. There were sharp changes in the ultrasonic velocities corresponding to the two critical temperatures., Ac-1 and Ac-3. Ultrasonic longitudinal- and shear-wave velocities were found to be useful in identifying the Ac-1 and Ac-3 temperatures and for the determination of hardness in the intercritical region. However, ultrasonic attenuation and spectral analysis of the first backwall echo were found to be useful to characterize the variation in the prior-austenitic grain size and formation of 6 ferrite above the Ac-4 temperature. The scattering coefficients have been experimentally determined for various microstructures and compared with the theoretically calculated value of the scattering coefficients for iron reported in literature.