Grain size evaluation of structural materials in nuclear power plant using a thickness independent ultrasonic method

It is important to accurately and nondestructively evaluate the grain size of structural materials used in nuclear power plants. The current ultrasonic non-destructive methods are so dependent on the thickness measurement of a square pipe that it reduces their practicality and reliability. In this paper, a novel method using the coefficient of ultrasonic attenuation rate is developed by using the transmission and reflection coefficients. As a result, the mean grain size of the pipe can be nondestructively evaluated without measuring its thickness. Moreover, the signal preprocessing is studied to improve the stability and accuracy of evaluation results. The experimental results show that the dependence of the attenuation rate on grain sizes is much higher than that of the ultrasonic velocity. The relative error of the attenuation rate method is lower than that of the backscatter method if the thickness of the sample is less than 5 mm. When evaluating a TP304 stainless steel square pipe whose thickness is not convenient to measure, the mean grain sizes are measured 103.5 ± 2.6 μm, 96.9 ± 3.5 μm and 94.0 ± 1.7 μm by the attenuation method, the attenuation rate method and the electron backscattering diffraction method, respectively. The result verifies that the presented method works better than the attenuation method due to the fact that the error of the thickness measurement has no effect on the ultrasonic attenuation rate.