| Large castings and forgings are the key basic parts necessary for the construction of national major technical equipment and major projects such as metallurgy,aerospace and military industry,etc.Meanwhile,the manufacturing industry of large castings and forgings is also an indispensable strategic industry for national security and national economic lifeline.In the iron and steel industry,with the rapid development of continuous casting billet hot delivery and hot charging and direct continuous rolling,continuous casting technology has been greatly improved.However,in the process of industrial continuous casting,the surface and internal defects of continuous casting billets cannot be avoided due to the complex continuous casting process,especially the abnormal mold vibration frequency and excessive casting speed in the process of casting of liquid steel,which will not ensure the production of high quality billets.In the process of continuous casting and cast steel,accurately measuring the solidified shell thickness of continuous casting slab can avoid the crack or pore defect at the end of liquid core caused by inaccurate roll pressure due to too fast or slow solidification speed of continuous casting,which can greatly improve the yield of continuous casting slab and reduce the economic cost.In the process of high-temperature forging,large forgings are usually free forging.The manufacturing process is carried out in extreme environments such as high temperature and large plastic deformation,which inevitably leads to problems such as cracks,inclusions and abnormal changes of workpiece thickness.Provided that online defect detection and thickness measurement can be implemented in the process of high-temperature forging,the waste of raw materials caused by large allowance and excessive defects can be effectively avoided.Electromagnetic acoustic transducer(EMAT)is suitable for high temperature,high speed online,rough surface casting and forging inspection due to its non-contact,easy to excite various types of ultrasonic waves and high temperature resistance.However,electromagnetic ultrasound excites and receives ultrasound through electromagnetic coupling,and the energy conversion efficiency is low.At the same time,the reduction of energy conversion efficiency at high temperature,the increase of ultrasonic attenuation coefficient and high-temperature structural noise lead to the further decline of echo signal-to-noise ratio(SNR),which will greatly limit the rapid on-line detection ability and technology application of electromagnetic ultrasonic technology in high-temperature casting and forging process.In order to solve the above problems,focusing on the thickness or defect electromagnetic ultrasonic testing of high-temperature castings and forgings based on pulse compression technology,this paper discusses the application of pulse compression technology in high-temperature continuous casting electromagnetic ultrasonic thickness measurement,the application and comparison of linear frequency modulation(LFM)and pulse compression technology of coded excitation signal in spiral coil EMAT,and the application of composite coded pulse compression technology in high-temperature spiral coil EMAT defect detection.Firstly,aiming at the problems of poor SNR,spatial resolution and low detection efficiency of high-temperature continuous casting EMAT detection echo signal,linear frequency modulation and phase coded pulse compression technology are applied to continuous casting thickness measurement.By establishing the racetrack coil finite element model of electromagnetic ultrasonic testing process and combined with the thickness measurement experiment of continuous casting slab at room temperature,the effects of excitation signal time width,bandwidth and carrier cycle number on echo SNR and main lobe width after pulse compression are studied,and the selection criteria of excitation signal parameters are given.The thickness measurement experiment of high temperature continuous casting slab at 600℃～750℃ was carried out with water-cooled racetrack coil EMAT probe,which verified the unique advantages of pulse compression technology in the thickness measurement of high temperature EMAT continuous casting slab.Secondly,in order to realize the accurate thickness measurement and defect detection of high-temperature forgings and improve the detection resolution,it is necessary to further improve the ultrasonic frequency.However,the ultrasonic attenuation coefficient increases sharply with the increase of temperature.Therefore,in order to solve the contradiction between high resolution and high SNR in the detection of high-temperature forgings,based on the basic principle and implementation process of pulse compression technology,the modulation signal with broadband product greater than 1 is selected as the excitation signal,and the matching process is matched with the mismatch filter to improve the SNR and spatial resolution of the detection echo.LFM,13 bit Barker code and 3×13 bit combined Barker code signals are selected as excitation signals respectively.The finite element model of carbon steel electromagnetic ultrasonic testing process is established.Combined with the thickness measurement experiment of forgings at room temperature,the influence of excitation signal time width for pulse compression technology on pulse compression effect is discussed.The thickness measurement experiment of high temperature forging at 500℃～710℃ was carried out with high temperature spiral coil EMAT probe.The application effect of pulse compression excitation signal after parameter optimization in thickness measurement of high temperature forging was verified and compared.Finally,aiming at the mismatch between the spectrum characteristics of electromagnetic ultrasonic testing system and the bandwidth of modulation signal,the finite element model of spiral coil EMAT field circuit coupling analysis of LFM signal,Barker code signal,LFM+Barker composite modulation signal and AMLFM+Barker composite modulation signal based on amplitude weighting is established.The effects of wire diameter,impedance matching type and matching parameters of spiral coil on pulse compression effect are studied.Using high temperature spiral coil EMAT probe,the detection of 8 mm flat bottom holes in high temperature forgings at 450℃～650℃ is realized,and the SNR is not less than 15.8d B at 650℃.In this paper,the mature pulse compression technology in the fields of radar and medicine is applied to the on-line thickness or defect detection of high-temperature castings and forgings.The results show that this method can effectively improve the detection echo SNR,spatial resolution and detection speed of high-temperature castings and forgings,and can provide technical reference and theoretical guidance for the on-line fast detection of high-temperature castings and forgings. |
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