A new technique for the quantitative real-time monitoring of austenite grain growth in steel

Abstract

A new technique to measure quantitatively grain growth kinetics at high temperatures was presented in this article. This technique, based on the generation and detection of ultrasound using lasers, called laser-ultrasonics, measures the scattering of ultrasonic echoes propagating in a sample to evaluate grain size. A quantitative evaluation was obtained by comparing ultrasonic attenuation with quantitative metallographic size measurements of austenite grains. Results obtained using this new technique during the austenitization of A36, DQSK, and IF steel samples were presented. The results presented showed that this new technique provides the austenite grain growth kinetics at a given austenitization temperature in a single experiment with a time resolution far superior to the resolution obtained using lengthy conventional metallographic techniques. In the present study, very few metallographic measurements of austenite grain size smaller than 100 {micro}m were available for comparison with laser-ultrasonic measurements. This small number of points is essentially due to abnormal growth of the austenite grains in the A36 steel which is associated with the dissolution of pinning A1N particles. The laser-ultrasonic technique should therefore be validated for grain sizes smaller than 100 {micro}m using materials and thermal treatments showing a low tendency for abnormal grain growth. Furthermore, systematic testsmore » should be performed to delineate the sensitivity of the technique to steel chemistry, and ultrasonic scattering models should be improved to include the effects of grain shape and of grain size distribution in the calibration.« less