Abstract: In view of the existing problems in evaluating the high temperature creep of metal by the nonlinear ultrasonic method and with referring to the use of the ratio of high frequency to low frequency energy as the definition of nonlinear parameters and to the expression of the stress wave factor, a new method of using the cumulative effect of nonlinear parameters in a certain frequency band to characterize the damage degree of the metal creep is proposed. According to the characteristics of the nonlinear energy transfer from low frequency to high frequency, the nonlinear parameters of the ratio of the high frequency energy to the low frequency band at a certain excitation frequency are obtained. On the basis of the change of the fundamental wave energy and the high harmonic energy with the frequency, the integral of the nonlinear parameters obtained in a certain frequency range is then integrated. The summation of cumulative nonlinear responses is obtained. This method is used to detect the creep degree of the P91 steel main steam pipe intercepted by the actual field. The pipe was operated at temperature of 550℃ with operating pressure of 3 MPa, and it had run for about 30 thousand hours. The results are compared with the residual stress measured by the more accurate X ray diffraction method and the traditional nonlinear coefficient method. The results show that the cumulative nonlinear parameter method has high sensitivity and good stability, and it is consistent with the stress measurement results. It can follow the stress change to evaluate the creep degree of the pipeline.

Key words: cumulative nonlinear parameters, nonlinear ultrasound, P91 main steam pipe, X ray diffraction method