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Research on Surface Modification of Power Station Valve Steel

Author: TanYanHong
Tutor: ChenYuYong
School: Harbin Institute of Technology
Course: Materials Engineering
Keywords: surface modification plasma spray plasma RE nitrocarburizing vacuum sintering nanostructured Al2O3-TiO2 Ni-WC composite coating
CLC: TG174.44
Type: Master's thesis
Year: 2011
Downloads: 12
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Abstract


Surface modification technology has been widely noticed and applied in various fields. In order to improve the surface properties of power station valve steel, three methods of surface treatment were carried out （plasma spray, plasma RE nitrocarburizing and vacuum sintering）. Meanwhile the microstructure, wear resistance and corrosion resistance of the modified coatings were investigated. The experimental analysis was employed such as SEM, XRD and so on to investigate the microstructure and phase composition of the layer 12CrMoV steel and 45 steel.Nanostructured Al2O3-TiO2 coating was prepared by plasma spray process, and the 13% content of TiO2 was adopted in this article. Results revealed that transition between substrate and coating was mechanical combinated because of no element diffusion. There were partial melted and full melted organization, and the stable phase was -Al2O3. The hardness value of nanostructured Al2O3-TiO2 coating was 5.035GPa by nano-indentation. Compared to substrate the friction coefficient was kept at a lower value below 0.45. The corrosion potential was -434.9mV, and the corrosion current density was lower.The surface hardening layers on two kinds of matrix steels were obtained by plasma RE nitrocarburizing techniques at 460 and 560 . With the temperature rised, the amount of nitrides and extent of element diffusion increased, and the proportion ofε-Fe3N and′-Fe4N phase composition changed. Results of microhardness and wear test indicated that hardness value of 12CrMoV steel presented a gradient change along the cross section, in which the highest hardness value was 884.1HV. Thus the wear resistance had a better performance, and the friction coefficient was stably kept at 0.30. According to corrosion resistance test, the 460 RE nitrocarburized layer had a higher corrosion potential in the intial stage, after that the 560 RE nitrocarburized layer performed better which retained in stable passivation area.Vacuum sintering technology was carried out on the substrates to prepare Ni-based WC composite coating which was about 2 mm thickness. Due to the large number of element diffusion, alloy trasition layer appeared between coating and substrate. Not only particles of WC hard phase existed in Ni-WC composite coating, also some new dispersal hard phases participated such as W2C, Cr1.8W3.2B3, etc, so that the hardness was up to 60HRC. In the condition of room temperature wear, the friction coefficient reduced to 0.20 or 0.25, only some increace in 500 wear test.The corrosion potential of composite coating was -392.0mV, and the passivation current density was 7.43×10-9A/cm2. Pitting corrosion slightly occurred at porosities, so Ni-WC composite coating performanced better corrosion resistance.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metal corrosion protection,metal surface treatment > Corrosion control and protection > Metal surface protection technology > Metal complex layer of protection
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