Real-Time Monitoring Of Austenite Grain Growth In Steels Using Laser-Ultrasonics

Laser-ultrasonics, a technique based on the generation of ultrasonic waves by a pulsed laser and on their detection by a laser interferometer, was used to monitor ultrasonic attenuation in steels at temperatures corresponding to hot rolling. For most metals, ultrasonic attenuation is mainly caused by scattering when the adequate frequency range is selected and is therefore sensitive to grain size. A calibration based on the metallographic evaluation of austenite grain sizes on quenched carbon steel samples was obtained to quantitatively relate ultrasonic attenuation to austenite grain sizes. The carbon concentration of the steel samples, ranging between 0.05 and 0.72%, did not seem to affect the relationship between ultrasonic attenuation and austenite grain size. This apparent insensitivity to the carbon concentration is consistent with the fact that at austenitization temperatures, these steels are single-phased and that the carbon concentrations involved are too small to significantly affect the elastic constants of austenite. This relationship was therefore used to evaluate austenite grain growth in an interstitial fi ee steel for which austenite grain sizes cannot be determined by conventional metallographic techniques. An important advantage of this new technique is to provide an in-situ information with a time resolution that may be as low as 50 ms. Furthermore, the average microstructure over the whole thickness is measured and little or no sample preparation is required. The results presented in this paper show that laser-ultrasonics is a powerful laboratory tool to study microstructural evolution at hot rolling temperatures.