The macroscopic mechanical properties like yield-strength, ductility or hardness play an important role during the steel production and the design of new steel grades. The austenite grain size is an important parameter, which influences the final microstructure and the properties of the material. When developing grain growth evolution models, typically many samples have to be treated thermally and micrographs have to be prepared ex situ. To reduce the time expenditure of this procedure we carried out in-situ laser ultrasound measurements of austenitic grain growth.&nbsp; A thermomechanical simulator of the type LINSEIS L78 has been upgraded with a laser ultrasound measurement system, which enables the continuous and contactless determination of the austenite mean grain size during a thermal cycle.Acoustic waves propagating in a polycrystalline material, are attenuated by various physical phenomena. At ultrasonic frequencies, the major attenuation contribution stems from the scattering of the elastic waves at the boundaries between adjacent grains. This scattering induced attenuation depends on the ratio between the wavelength and mean grain size D. If the attenuation coefficient can be determined, conclusions can be drawn about D using heuristic models based on well-established mean-field attenuation theories. In classical attenuation models, three asymptotic cases are defined. For these cases, the frequency dependent longitudinal attenuation can be described by a power-law of the form α(f) = a + CD^n-1f^n-1, where n can take the values of 4 (Rayleigh regime), 2 (Stochastic regime) or 0 (Geometric regime). &nbsp;However, in ultrasonic attenuation measurements, the acoustic frequencies/wavelength often falls within a transition region and the asymptotic power-law formulas are not applicable. In particular, this may happen when grain growth is studied. Moreover, it was recently shown, that in addition to the mean grain size, also the grain size distribution has an influence on the attenuation. This may change the exponent n even for asymptotic cases.In this work, we will show the calibration workflow and grain size results by a new attenuation model based on the power-law formula for the plain carbon steel (DIN 1.1191/AISI 1045). Therefore In-situ laser ultrasound measurement data is compared with several micrographs defined at supporting points along a specified temperature program in the thermomechanical simulator. Additional in-situ laser ultrasound measurements were carried out on different temperature profiles in order to show the robustness of the application. The measured mean grain size from the laser ultrasound experiments were compared with the measured mean grain size from the analyzed micrographs.

In-Situ Laser Ultrasound Measurements of Austenitic Grain Growth in Plain Carbon Steel

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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