Direct measurements of grain size in low-carbon steels using the laser ultrasonic technique
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I. INTRODUCTION
II. EXPERIMENT
CHARACTERIZATION of material, during production as well as development, is usually carried out using time-consuming and costly methods such as microscopy and mechanical tests. Evidently, a characterization technique that allows for continuous and nondestructive measurements during processing of the material would be advantageous. One promising method, which recently has gained much interest due, for example, to its ability to obtain noncontact measurements at elevated temperatures, is microstructural characterization by laser-generated and detected ultrasound, called laser ultrasonics (LUS). With this method, the properties of the ultrasonic wave traveling through the specimen (such as attenuation and velocity) are used to extract information about the microstructural properties, e.g., texture, grain size, precipitates, etc., of the material being investigated. Various types of measurements have been performed, such as the study of phase transformation for ferrite-austenite[1] and martensitic transformations,[2] microstructure development in aluminium and stainless steel,[3,4] recrystallization in copper and steel,[2,5] graphite precipitates,[6] position of solidification fronts,[7] thickness estimation of steel and aluminium sheets,[8] and stainless steel foils.[9] One of the most useful applications of laser ultrasonic characterization is perhaps for grain-size analyses. In these measurements, the attenuation of the ultrasonic signal is correlated to the grain size of the sample[2,10,11] at room or elevated temperatures. The combined results of the studies presented in the literature have firmly shown that the attenuation is related to grain size and, consequently, can be used as a measure of this. However, all these studies depend on calibration curves between the grain size and attenuation in order to obtain information about the absolute grain size of a specific sample. Such calibration must be individually obtained from metallography for each type of sample and/ or the specific frequency range. The purpose of this study is to take this one step further and show that it is possible to actually obtain the absolute grain size directly from the ultrasonic trace, without the use of any calibration curves.
The samples inspected were low-carbon steels that all contained more than 99.7 pct Fe. The samples are commercial materials and were taken from coils directly after production. Grain size is an important property of these hotrolled strips, and it is known that variations occur, depending on the steel chemistry and position along the bands. Five different materials have been inspected, and samples have been extracted from the center as well as the front end of the sheets; in total, this gave us ten different samples. The grain size of each sample has been determined from image analyses measured as the mean linear intercepts. This is advantageous for two reasons: first, this method is quite common and easy to apply, and, second, the linear intercepts correspond closely to the measu
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