| Development and advancement of the society is always accompanied by the demand for resources,the increasing demand of oil and gas resources,forcing our country increases the pipeline and the construction of storage vessel and use,and these special equipment tend to work in high temperature and high pressure conditions,if the leakage and explosion will do great harm to the state and society.The damage of oil and gas pipelines and pressure vessels is often caused by the accumulation of early damage that has not been found over time.Therefore,it is particularly important to carry out the detection of oil and gas pipelines and pressure vessels at regular intervals,especially at high temperature.Ultrasonic testing is one of the methods of nondestructive testing.In recent years,Electromagnetic ultrasonic testing technology is particularly suitable for detection at high temperature due to its characteristics of non-contact,no coupling agent and low accuracy on the surface of materials to be tested.However,due to the changes in the electrical,magnetic and acoustic characteristics of the material under test at high temperature,the ultrasonic signal strength and propagation sound velocity vary,which makes it difficult to use the echo equivalent flaw assessment technique in ultrasonic flaw detection,thus directly affecting the accuracy of thickness measurement and flaw detection and the effectiveness of the focusing method in high temperature phased array ultrasonic detection.This paper based on the theory of electromagnetic ultrasonic experiments have been carried out to study the basic characteristics of EMAT,including the experimental study of EMAT structure design,put forward the basic choice of high temperature components EMAT,and adopt the method of tomography to determination of EMAT sound field,based on the development of EMAT test system setup,studied 700 ? within the scope of ferromagnetic material and the key parameters of ferromagnetic materials testing,such as sound velocity,amplitude,and applied in the field of nondestructive testing provides reference for the heat.The main contents are as follows:(1)Firstly,the basic principle of electromagnetic ultrasound is analyzed,the excitation and receiving mechanisms are studied,the ultrasonic waves generated by different combinations of permanent magnets and coils are analyzed,and the theoretical formulas of ferromagnetic materials and non-ferromagnetic materials are detailed based on the three excitation mechanisms of electromagnetic ultrasound.(2)The characteristics of the electromagnetic ultrasonic transducer were studied,including the design of the electromagnetic ultrasonic transducer at high temperature,and the arrangement of the internal structure of the transducer,including the optimal distance between the transducer coil and the permanent magnet and the optimal lifting of the coil from the test block.Then impedance matching experiments were carried out on the electromagnetic ultrasonic transducer to solve the problem of low energy exchange efficiency.(3)C-scan of the sound field test method,through 20 # steel in ferromagnetic materials and non-ferromagnetic materials Al block test,high temperature reconstructed the EMAT in internal three-dimensional radiation sound field of a solid material,high temperature measured EMAT under 3.5 MHz incentive frequency of radiation sound field near field length is about 8 mm,for choosing high temperature detecting the thickness of the block to provide the reference.(4)The use of high temperature EMAT 15 CrMo of ferromagnetic material within the range of 700 ?,20 # steel Q235,and experiment Al non-ferromagnetic material,found that different materials of amplitude and velocity changing with temperature present nonlinear variation,and ferromagnetic material attenuation law of showing a "step",and the study found that the material thermal expansion through the experiment of velocity measurement is affected,can be neglected. |
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