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Research on the Scuffing Resistance of High-temperature Bolt by Chromizing Process

Author: HaoWeiXun
Tutor: ChenZhaoYun
School: Harbin Engineering University
Course: Materials Science
Keywords: 1Cr11MoNiW1VNbN Salt bath chromizing Microhardness Wear resistance Corrosion resistance High temperature oxidation resistance Killed
CLC: TG156.88
Type: Master's thesis
Year: 2010
Downloads: 8
Quote: 0
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Abstract


The heat-resistant steel having a high temperature strength and good processing properties, widely used in the thermal power generating units of the member. Order to improve unit efficiency, the steam parameters in the continuous improvement, therefore, the use of materials performance requirements are also rising. For example: cylinder bolts for the ultra-supercritical steam turbine group, not only requires a sufficiently high temperature strength and toughness, and require good resistance to corrosion and wear resistance. These requirements are often conflicting and improve the high temperature strength and resistance to corrosion and wear resistance is necessary to reduce the thermal power plants continue to occur because of these reasons, the turbine group phenomenon cylinder bolt killed. Therefore it is very urgent and necessary research cylinder bolts anti bitten to death by a surface protection layer. This selection is widely used in supercritical and ultra-supercritical steam turbine unit cylinder bolt 1Cr11MoNiW1VNbN heat-resistant steel as the base material, the method of high temperature salt bath chromizing, improve wear, corrosion and oxidation resistance of the steel to expect to extend high temperature bolts life, to prevent the bolt caused the death of the phenomenon in an overhaul period. Through theoretical analysis and experimental research, selected to meet the requirements the salt bath Chromizing temperature and holding time by metallographic observation, the computation of the phase diagram analysis, microhardness testing, nanoindentation tests, X-ray diffraction analysis, scanning electron microscopy and EDS, the friction and wear performance testing, dynamic potentiodynamic polarization curve test, salt water immersion test, high temperature oxidation resistance test, the salt-bath of chromium on 1Cr11MoNiW1VNbN base metal microstructure, hardness, wear resistance, resistance corrosion resistance and high temperature oxidation resistance. The research results show that the diffusion layer thickness to meet the requirements chromizing process to: 910 ° C insulation 6h. Chromizing after heat treatment (910 ℃ quenching, heat for half an hour; 650 ℃ tempering, heat three hours), matrix martensite. Nitrided layer is composed mainly by the Fe-Cr solid solution and Cr23C6 the Cr concentration of the diffusion layer reached 20%. Although the nitrided layer is ferritic structure, the coarse grains in the the microhardness and NANOINDENTATION test low hardness, but has a high toughness. Nitrided layer has a low coefficient of friction, to better prevent adhesive wear, has been greatly improved wear resistance, the wear rate of the base metal is 19.4 times the nitrided layer. Potentiodynamic polarization curve test, the nitrided layer showed passivation behavior exhibited good corrosion resistance in salt water immersion test. A high-temperature oxidation process, the oxidation of the nitrided layer weight gain slow, the oxide film is a dense, have excellent anti-oxidation properties. It can be concluded that: the salt bath Chromizing significantly improved the corrosion resistance of the base material and adhesive wear performance, high temperature to prevent bolt killed phenomenon.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Heat treatment > Heat treatment process > Chemical treatment > Diffusion chromizing
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