Ultrasonic absorption in ultra-low carbon steel

The laser-based reverberant technique is used to measure ultrasonic absorption spectra in the 2 to 45 MHz frequency range. This technique, being contactless, allows measurements at high temperature. The absorption spectra of ultra-low carbon steel samples are studied at room temperature in a magnetic field (in order to suppress the magnetoelastic contribution) and in a high temperature furnace $(20–1200 °C)$ without magnetic field. Small steel samples (about $10×10×1 mm3)$ are used. At room temperature, two main contributions to the ultrasonic absorption are identified: microeddy currents (magnetoelastic contribution) and absorption caused by dislocations (deformation contribution). A typical microeddy current peak is observed and yields a reasonable estimate of the magnetic domain size. Above 10 MHz, the nonmagnetic contribution to the total absorption follows the classical vibrating string model. However, other phenomena also contribute to the absorption spectra. Below 10 MHz, an amplitude-independent damping background is observed. In addition, a small frequency-independent contribution to the absorption is observed at room temperature and is attributed to a thin surface layer. The absorption at high temperature is dominated below the Curie point by the magnetoelastic contribution. Two internal friction peaks are also detected. The first one, at $100 °C,$ is related to the dislocation kink motion. The second one, measured at $330 °C$ and 10 MHz, is attributed to the Snoek relaxation of carbon and/or nitrogen in α-iron. The Curie transition as well as the ferrite-austenite transition strongly affect the internal friction spectra.