Research On Carburized Layer Characteristics And Eddy Current Testing System Of Carburized Materials

Carburizing is one of the most widely used chemical heat treatment in mechanical industry. Carburized materials possess relatively high surface hardness, good fatigue strength and enough toughness. The case depth is the most important quality index of carburized materials, and it is usually checked by selective examination. This traditional method is inefficient and low reliability. In addition to, the sample will be destroyed in the checking of the case depth. So people make great efforts to inspect the carburized materials in NDT perfectly. In this thesis, the characteristics of carburized layer and ECT system were studied by numeric simulation and finite element method. All these research afford an important theory basis to the NDT of the carburized layer depth and the quality evaluation of surface treatment materials.Three typical carburized materials of 20Cr, 20CrMnTi and 18Cr2Ni4WA were studied in the thesis. The carbon concentration distribution model was established by experiments at different carburizing process. The result shows that carbon concentration distribution depends on carburizing time and carbon potential. The curve of carbon distribution will be gentle when carburizing time is prolonged, and becomes steep when carbon potential is raised. In this paper, the relationship among case depth, surface carbon concentration and carburizing process factors has been built. This relationship model can be used to control the case depth and carbon distribution, it may also be used to design the carburizing process factors and control the carburizing automatically.In this thesis a mathematical model referring to the relationship between the case depth and carbon distribution by using a nonlinear regression. The electromagnetic characteristics in carburized materials were discussed. Results show that inductivity and magnetic inductivity depend on the carbon concentration in carburized layer. A relationship model between the electric, magnetic inductivity and carbon concentration was built. So the relationship between case depth and the characteristics of carburized layer was established. All the results above can afford a theory and practice basis for the NDT ofcarburized layer depth.In this paper, the eddy current electromagnetic field model of axis-symmetric was built. The testing device contains two coils, that is, exciting coil and testing coil. A cylindrical carburization sample was placed in the coils. The case was separated into a lot of small piece and finite element method was used to solve the electromagnetic field. Eddy current testing system of carburized materials was development by using ANSYS software as second development base. The system includes a stock of material characteristics. Finite element preprocessor, eddy current electromagnetic field solution, the postprocessor and the visualization processor are integrated together in the software system, and good graphic user interface is also offered.In order to describe visually the influence of carburized layer characteristics on the eddy current electromagnetic field, color images were generated to simulate the electromagnetic parameters based on the calculation result. Results show that, the voltage signal of testing coil is corresponding to the case depth. The voltage signal will decrease with the case depth increasing. According to this relationship, we can get the case depth by the practical value of voltage. The result also shows that the calculation depth is in keeping with the measurement one. The average error is about 10%, according with the demands of the industry application, so the eddy current testing system of carburized materials is available to the actual situation.