Real Time In-Situ Monitoring of Metallurgical Phenomena with Laser Ultrasonics

Abstract. The laser based ultrasonic technology allows assessing checking and measuring the properties of several materials without contact and in harsh conditions (high temperature, high speed...). In Arcelor Research, we integrated a laser-based ultrasonic equipment to an annealing simulation pilot in order to monitor in real time and in-situ the evolution of the microstructure of steel samples during thermal cycles. The digitalisation of ultrasonic signal at 1 Ghz combined with specific signal processing allow determining steel grade microstructure with great precision in particular: Phase transformation, recrystallisation temperature and recrystallised fraction,.. Moreover, a growth of austenitic grain size can be monitored in real time. The robustness, the reliability and the simplicity of this approach have been proved through many trials and a correlation with classical methods (metallographic examination, modelling,...) This equipment appears as a new tool, which should ease the work of the metallurgists by measuring automatically grain size or evaluating the temperature of phase transformation and thus improve the knowledge of the properties of new steels.