| Electrical Discharge Hardening(EDH) is the technology that infiltrate the melted conductive electrode materials into the surface of the metal substrate, becoming high hardness, high wear resistance and erosion resisting alloy strengthening layer, which could improve the physical, chemical and mechanical properties of the substrate surface, meantime maintaining the properties of the mrtal matrix. In this thesis, On the basis of the erosion mechanisms of hydraulic flow components,aiming at the critical wear issue of hydraulic turbine components, a surface protection coatingr was made on the Cr-Ni-Mo stainless steel commonly used materials for hydraulic turbine,which made a protection. In addition, The research on properties of layer and affecting factors were carried out,which had a great significance for improving the anti-abrasion and anti-erosion property and prolonging the service life of the turbine blades.The behaviors of interface between the strengthening layer and the substrate were investigated by means of scanning electron microscopy(SEM), X-Ray Diffraction(XRD) microhardness tester. the microstructure and morphology, the structure of phase, the distribution of elements, and the microhardness distribution were alse analyzed. The results of SEM show that Sample section in the gradient direction is light layer, transition layer, substrate, and the strengthening layer is bonded metallurgically with the metal substrate. The tissue is uniform, dense, continuous.The thickness of hard coating is 50μ. The results of X-Ray Diffraction(XRD) show that physical and chemical reaction emerge between the electrode and the complex matrix elements. the element of electrode and substrate materials melt and filter each other in the form of diffusion, new phases are formed in the EHD coating, such as Fe3W3 C,Co3W3C,å'Œ W2 C. By microhardness tester,The micro hardness along the depth gradually reduce. The hardness of the white bright layer is highest, which could be 1696HV0.3 and 3.43 times of the substrateThe wear resisitance of the coating was investigated deeply by means of friction and wear test,which are analysised from three aspects systemically as the friction coefficient, abrasion volume and the worn surfaces. The experiment results indicated that The wear resistance of the strengthening layer improve significantly, which is 3.43 times of the substrate.The main wear mechanism is abrasive wear. The process parameters influencing the the microhardness and anti-abrasion were analyzed, optimum processing parameters were selected which would make the strengthened layer obtain satisfactory sur-face quality. The corrosion resistance of the coating in 3.5% Na Cl solution was evaluated by linear polarization.The results show that the corrosion potential and pitting potential increases, and the corrosion current density and maintaining passivity current density decreases, the speed of corrosion slows down, all that tell us that the corrosion resistance of EDH coating get a larger advance.The application experients show that the strengthening layer which was made on the Cr-Ni-Mo stainless steel commonly used materials for hydraulic turbine not only has high hardness,but also excellent abrasion and erosion resistance.It has board application prospects and great economic Value on hydroelectric turbine surface. |
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