Laser-Ultrasonics as a Technique to Study Recrystallisation and Grain Growth

Abstract

Laser-Ultrasonics (LUS) provides a means of obtaining microstructure information continuously and non-destructively both in the laboratory and for quality control on-line in industry. Ultrasound is both generated and recorded using lasers which permits remote, non-contact operation with fast sampling and also the capability of working at high temperatures or at moving surfaces, for example during industrial continuous annealing. Examples of dynamic heating trials will be presented for samples of cold rolled steel sheets where primary recrystallisation and ferrite austenite transformation are monitored in-situ as a function of temperature. Examples are also presented where the grain size of low carbon steels have been quantitatively analysed and show very good agreement with microscopy methods.

Evolution of Austenite Recrystallization and Grain Growth Using Laser Ultrasonics

“…[17] Thus, in the absence of porosity, the attenuation data can be directly correlated to the austenite grain size. [14][15][16]18,19] Further, the change in the relative velocity of the ultrasound wave during its propagation through the material is related to the evolution of texture and can be utilized to monitor recrystallization in situ. [16,[20][21][22] Laser ultrasonics is a remote, continuous, and nondestructive technique that can be operated online at high temperatures for bulk observation.…”

“…Ultrasonic attenuation, i.e., the decay of an ultrasonic wave as it propagates through the material, is sensitive to material parameters such as grain size [14][15][16] and porosity. [17] Thus, in the absence of porosity, the attenuation data can be directly correlated to the austenite grain size.…”

Laser-ultrasonic monitoring of ferrite recovery in ultra low carbon steel

“…Due to the interaction of the microstructure with the ultrasonic waves, various phenomena can be studied. The literature on ultrasonics has mostly focused on: grain size measurement [1][2][3], monitoring of phase transformations [4,5], recrystallization [3,[6][7][8], magnetic domain effects [9,10] and dislocation behaviour [10][11][12][13][14].…”

Ultrasonic Wave Generation by Lasers on Different Metal Surfaces

“…The laser-ultrasonic technique has been widely used for inspection of metallic materials, both in laboratory [1][2] and on-line in industry [3]. The efficiency of lasers to generate ultrasonic waves in the thermoelastic regime on metal surfaces is very low due to the very shallow penetration of light.…”