Laser ultrasonics for process control in the metal industry

Laser ultrasonic (LUS) techniques show great promise for online measurement of material structures and properties during the processing of steels and other metals due to their high information content combined with their ability to operate in difficult environments. Laboratory-scale experiments are presented together with theoretical background showing how LUS can provide insight into the phenomena of recrystallisation, grain growth and phase transformation based on the measurements of velocities for different wave types and on their attenuation. This technique is unique in allowing dynamic observations of such phenomena at elevated temperatures. As microstructure is the most important factor governing the strength of metals, it follows that LUS signals are closely correlated with strength properties in many cases. The LUS equipment has been applied in several industrial trials to successfully evaluate mechanical properties of the product materials. It is also shown how absolute measurements of dimensions such as plate thickness can be made without physical contact and without the need for any calibration factors.