Abstract: The current ultrasonic nondestructive methods for evaluating grain size require the thickness of the object to be highly accurate, and thus reduce its practicability and its reliability of the evaluation result. Based on the propagation of acoustic beams in material, the attenuation characteristics of first and second bottom-wall echo as against the front-wall echo are described, and the parameter of ultrasonic attenuation rate exclusive of the thickness is derived, both transmission coefficient and reflection coefficient are used to correct the ultrasonic attenuation rate. Taking the ultrasonic attenuation rate as the acoustic eigenvalue, an ultrasonic nondestructive evaluation model for mean grain size is presented. The proposed method is validated through the use of TP304 stainless steel frequently used in nuclear power plant, and a comparison is conducted between our method and the traditional methods such as ultrasonic velocity, attenuation and backscatter. The results show that the sensitivity of our method is notably better than that of the ultrasonic velocity method and the relative error of the developed method is greatly lower than that of the backscatter method when the thickness of the block is less than 6mm. Furthermore, the presented method effectively restrains the adverse impact on the grain size evaluation induced by the thickness, which is demonstrated by the fact that a TP304 square tube with wall thickness not convenient to measure, the mean grain sizes of measured by the attenuation method, the proposed method and the metallographic method are (100.3±2.8) m，(96.7±3.4) m and (93.1±1.8) m, respectively.

Key words: attenuation rate, grain size, thickness, ultrasonic nondestructive evaluation