Experimental Study On The Evaluation Of The Creep State Of Steel Used In Typical High Temperature Equipment By Reflective Nonlinear Ultrasound

Today,with the rapid development of the industry,especially in the fields of energy power,petrochemical,aerospace and many other fields,many metal components are used for a long time under the conditions of high temperature and high pressure.The mechanical equipment in this environment not only has to withstand high temperature,but also Withstand a certain amount of pressure,this will inevitably lead to creep damage.This creep phenomenon restricts the operation of the equipment and threatens the safety of the equipment.Once an accident occurs,it will cause major personnel and property damage.Therefore,it is extremely important to evaluate the creep state of the equipment and predict the remaining life of the material.The research on nonlinear ultrasonic characterization of creep is still in the laboratory stage.It is an urgent problem to be solved to explore nonlinear ultrasonic parameter detection methods and devices that are more in line with industrial production sites.The problem that needs to be solved urgently.In order to solve these problems,a method for acquiring and separating the reflected signal of the bottom wave of the wave is proposed as a method for acquiring nonlinear ultrasonic parameters and a set of reflective nonlinear ultrasonic detecting device is designed.The following work is done in this paper:(1)The domestic and international techniques for characterizing the creep state of materials are summarized,the basic principle of ultrasonic testing and the causes of higher harmonics in the material are analyzed and theoretical derivation of the expression formula of nonlinear ultrasonic parameters under amplitude detection.The expression formula is theoretically deduced.Based on the ultrasonic nonlinear parameters to characterize the creep state of materials,the common ultrasonic nonlinear detection methods such as plate wave,lamb wave,surface wave and longitudinal wave are summarized.In the case of the limitations of the existing detection methods,it is proposed to acquire and separate the ultrasonic detection of the bottom wave reflected wave as a method for obtaining the ultrasonic nonlinear parameters for evaluating the creep state of the material;(2)Based on the transmissive detection method,a reflective nonlinear ultrasonic testing platform was designed and built,and compared with the transmission nonlinear ultrasonic testing platform,and Theoretical analysis of the specimens required for reflective nonlinear ultrasonic testing and calculated the thickness requirements that the time needs to meet under laboratory conditions,On the basis of the test conditions,three P91 steel specimens with weld structure at 700? high temperature,50 MPa uniaxial tensile stress and creep time of 0h,120 h and 250 h were provided for the test.The fracture time of the test piece is 350 h,and the fracture location is the heat affected zone.;(3)According to the designed and constructed reflective nonlinear ultrasonic testing test platform,the creep specimens of P91 steel with creep time of 0h,120 h and250h and welded structure were tested.The results show that: the ultrasonic second-harmonic nonlinear parameters showed a relative change of 2.7% and 17.4%in the base material area after the creep time of the P91 steel test piece after 120 h and250h,the relative change of the weld zone was 2%,23.6%,and the heat affected zone was relatively the change is 5.6%,34%,using the reflective nonlinear ultrasonic testing test system and method for detecting can effectively distinguish the test blocks with different creep time,and the test results are very different from the ultrasonic nonlinear parameters and hardness values under the transmission test,and the reflective nonlinear ultrasonic testing system and detection method were further verified by testing the steam pipeline with a service time of more than 20,000 hours.