Research On The Relationship Between Depth Of Carburized Layer And Barkhausen Noise Of 18CrNiMo7-6 Steel

18CrNiMo7-6 is a typical heavy loaded gear steel used widely in the field of industrial machinery.In order to get good characteristics which has external hard and internal toughness,chromium,nickel and other alloying elements are added in the steel for improving the material's hardenability,carburizing performance,strength and toughness,etc.The gears need to form an effective hardened layer on the surface through a heat treatment process such as carburizing and quenching to improve the anti-fatigue strength of the gears.The depth of the carburized layer is one of the important indicators for evaluating the qualification of the heat treatment process.Therefore,the depth of the carburized layer needs to be measured.However,the testing method at this stage belongs to the destructive testing that can not apply to the formed component.A non-destructive testing method based on the effect of Magnetic Barkhausen noise(MBN)is proposed to detect the carburized depth of gears milled.The following research was carried out.(1)The commonly used method for deep detection of hardened layers of gear steels is described.The advantages of Barkhausen noise detection are obtained through comparison.The development of MBN technology and the research results of domestic and foreign experts and scholars in recent years are summarized.The theoretical basis of Barkhausen noise detection and the influence factors of Barkhausen noise are introduced.(2)Barkhausen nondestructive testing system was independently built.The hardware has a XD2 signal generator,a FPA1000 pre-amplifier,and a Ushaped silicon steel sheet and wound coil which were made for a magnetizer.Then,the current signals sensed by the receiving coil were to high-pass filtered and amplificated through the UFA42 filter chip.Finally,Advantech PCI-1712 L acquisition card was used for signal acquisition.The acquired signal was processed by the signal analysis software based on Lab VIEW language,and the time domain eigenvalues and frequency domain analysis of the noise signal were obtained.(3)Based on the experimental platform,the depth of the carburized layer after the milling process was tested.Firstly,carburizing treatment was performed on the 18CrNiMo7-6 gear steel,and a 3 mm thick carburized layer was obtained on the surface of the sample.By applying different amplitude excitation current,the response MBN signal was analyzed.The most suitable excitation condition for MBN detection is the excitation frequency 20 Hz and the optimal excitation voltage 3.5 V.Then,the carburized layer specimens with different depths were sequentially milled at the same feed(0.1 mm).Finally,the carcass layers with different thicknesses were tested by using the built Barkhausen noise detection platform.The results show that the RMS value and mean value of MBN signal are approximately linearly decreasing with carburized layer depth.Therefore,the root mean square or mean value of the Barkhausen noise signal can be used to detect the carburized layer depth of the gear steel specimen.