Research On Pulse Eddy Current Detection Technology For Quenching Hardened Depth

For many applications in the machinery industry,the important mechanical parts will be treated by means of surface hardening technology in order to improve their quality and service life.The depth of hardened layer is an important index to evaluate the quality of workpiece,and therefore it is particularly significant to measure case hardened depth on workpiece.Nowadays,there are two ways to realize the measurement of case hardened depth consisting of the destructive technology and the non-destructive technology.The former has to do sampling test and also destroys the mechanical parts during the measurement process,which leads to low efficiency.And the latter belongs to the nondestructive testing with the help of ultrasonic,magnetic coercive force,Barkhausen effect,eddy current and so on.Among them,the eddy current method has an outstanding advantage over others such as high detection speed and low cost.Generally,the eddy current used to measure the case hardened depth of hardened parts with the eddy current mainly focuses on the traditional single frequency,and few research on the pulsed eddy current(PEC)has been reported.However,there still exist some problems regarding the eddy current testing with single frequency.For example,it is affected by eddy current skin effect and cannot be analyzed in frequency domain.Thus,the nondestructive testing with the PEC is proposed to overcome the above drawbacks,and some relevant research have been done are given as follows:First,the development of PEC detection technology and the detection methods of hardened depth are summarized.Based on it,a nondestructive method to detect the hardened depth with the PEC is introduced by analyzing the variation of electromagnetic characteristics between the hardened layers of the part with surface quenching and its core structure.And the basic principle of nondestructive testing with PEC is described as well.Then the theoretical analytic model of three-layer is established according to the change of electromagnetic characteristics of the parts after surface quenching.Moreover,its analytical solution of the response signal in time domain is solved step by step,and the effect on the response signal in time domain generated by different parameters in the model is analyzed as well.Second,based on the theoretical analytic model,a simulation model of PEC testing for different hardened layers is established.And by simulation,the validity of the theoretical analytic model is verified.Furthermore,the simulation results are analyzed and processed.And a new method of detecting the depth of hardened layer based on the zero-crossing frequency of frequency-domain differential signal in frequency domain is proposed.This method is compared with the traditional method based on time-domain differential signal,and the advantages of the method proposed in this paper are obtained.Finally,according to the simulation model,a platform of nondestructive testing with PEC is set up.It mainly consists of software and hardware.The hardware part includes the selection of necessary devices,the design of excitation coil and detection coil,and the software part includes the signal acquisition and post-processing.By use of the platform,the induction hardening parts with different hardening depth are tested,and the test results are achieved.Through comparing the actual results with the simulation ones,the feasibility of the characteristic quantity proposed in the simulation is verified,and the related inclusions are given.