Research On High Temperature Creep Status Of P91 Steel By Nonlinear Ultrasonic Testing

Pressure-bearing equipment after a long term of running under high temperature and pressure will occur varying creep damage,especially in weld heated zone,especially in the weld heat affected zone.The traditional ultrasonic testing method has low sensitivity to the material damage caused by creep.As a new high sensitivity detection method,nonlinear ultrasound has been applied to evaluate the degree of creep damage.There are many problems in the method of representing the degree of creep by the nonlinear ultrasonic coefficient represented by the amplitude ratio method.If the fundamental and second harmonic amplitudes vary with the excitation frequency,the higher harmonics can not be separated from the different frequencies due to the pulse excitation bandwidth.In order to solve these problems,a state parameter for characterizing the creep damage of metals at high temperature is found:(1)Based on the amplitude ratio method,energy ratio method,the amplitude ratio of the integral method,proposed the concept of cumulative energy ratio method to characterize the creep damage state,which formed a kind of nonlinear ultrasonic detection method based on energy ratio integral characterization of creep state.formed a kind of nonlinear ultrasonic detection method based on energy ratio integral characterization of creep state.Firstly,the nonlinear parameter of energy ratio is defined by the concept of Parseval's relation and energy spectrum density;The cumulative effect of the second harmonics is analyzed theoretically,and the expression of the cumulative amplitude ratio is derived by referring to the expression of the stress wave;considering the change of amplitude with frequency and the nonlinear parameter defined by the ratio of energy,the ratio integral method of energy is deduced.In theory,the energy ratio integral method has more advantages than the other three methods.(2)The nonlinear ultrasonic testing platform was built and the energy ratio integral method was used to detect the base metal zone,heat affected zone and weld zone of the P91 steel creep welding standard block.The creep temperature of the creep specimen is 700?,50 MPa tensile uniaxial tension and creep time are 0h,120 h,250h,respectively.The results of the test were compared with those of the other three methods and hardness measurements.the results show that the energy ratio integral method has better stability in multi-point detection of the same creep block,and has higher sensitivity to different creep damage degree.(3)The nonlinear ultrasonic creep characterization method of energy ratio integral method is applied to evaluate the creep degree of P91 steel main steam pipeline.The running temperature of the pipeline is 550?,the running pressure is3 MPa,and the running time is about 30 thousand hours.The results show that the energy ratio integral method has the highest sensitivity,the best agreement with the hardness and stress measurement results,followed by the amplitude ratio method and the energy ratio method.for the amplitude ratio integral method,the sensitivity of creep damage response caused by different stress is not high.