Laser-Ultrasonic Monitoring of Metallurgical Transformations in Advanced Ultra-High Strength Steels

This paper discusses the use of laser-ultrasonics to monitor the metallurgical transformations of advanced high-strength steels. In this technique, acoustic waves are generated and detected in the material by use of lasers. The ultrasonic velocity can sense the austenite formation/decomposition in real time. Unlike dilatometric measurements the ultrasonic velocity measurements show little influence of the carbon concentration in solid solution and measurements can be made on-line. Also, they can be used to determine the fraction of retained austenite during and after cooling. The ultrasonic response of iron carbides is also investigated and discussed. We present examples such as monitoring the austenite fraction on transformation-induced plasticity (TRIP) steels during the bainitic transformation and at room temperature. Comparison with other sensing techniques and on-line implementation is discussed.