UBC Theses and Dissertations
Evaluation of in-situ laser ultrasonics for abnormal grain growth in a plain carbon steel Tran, Brian
The control of grain size distribution is an important factor governing the homogeneity of mechanical properties for steels and other alloys used in a wide range of applications. In the present work, the evolution of austenite grain size is in-situ monitored in an A36 plain carbon steel during isothermal holding between 900°C and 1150°C using laser ultrasonics. In this technique, the attenuation of laser generated ultrasound is related to a representative mean grain size in the material. In A36 steel, the coarsening and dissolution of AlN precipitates at 1000°C leads to a stage of abnormal austenite grain growth with the development of a bimodal grain size distribution. This stage corresponds to a period of high grain growth rate when measured by laser ultrasonics, suggesting that the technique is sensitive to the progression of abnormal grain growth stages. For abnormal grain growth scenarios, the laser ultrasonic measurements are compared with ex-situ metallographic measurements of large grain area fraction on quenched samples. Experimental correlation between laser ultrasonic grain size measurement and area fraction of large grains is validated with a finite element analysis of ultrasound wave propagation in an anisotropic polycrystalline aggregate of controlled grain size distribution. The numerical analysis provides important insight into the scattering of ultrasound waves in a material of heterogeneous grain size. The range of applicability for the laser ultrasonic technique is evaluated using grain size calculations from simulated attenuation for selected grain size distributions.
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