Nondestructive measurement of grain size in steel plate by X-ray diffraction

The microstructural properties of steel products, such as their grain size and texture, are controlled in the steel industry to improve their quality. The characterization of microstructures by nondestructive methods has become increasingly important in recent years. The Committee on Sensors for Microstructure was organized by the Iron and Steel Institute of Japan to study nondestructive techniques for measuring grain size. One of these techniques, the x-ray diffraction technique, is discussed in this paper. Two x-ray diffraction techniques, the Debye-Scherrer camera method and the diffractometer method, were studied as means of estimating the grain size of steel plates. Various samples with different grain sizes, different metallurgical phases, and different degrees of texture were prepared for the experiments. The experimental results suggest that the grain size in a steel plate with a single ferritic phase can be estimated with an accuracy of better than +/-1 the ASTM number when there is little texture in the material. In addition, the presence of a secondary phase of pearlite or martensite can degrade the accuracy. Furthermore the grain size can be estimated accurately regardless of the degree of texture in the material if a suitable reflection plane is employed for the measurement.