Research On The Scuffing Resistance Of High-temperature Bolt By Chromizing Process

Heat-resistant steel has been widely used as components for thermal generator sets due to its high temperature strength and good processability. The steam parameters should keep promoting in order to agree with the improvement of unit efficiency, so the functional properties of these materials are also rising. For example, the steam cylinder bolt for ultra-supercritical turbine requires not only enhanced high temperature strength and ductility, but also good corrosive and wearing resistance. However, these requirements are always contradictory, such as increase in high temperature strength normally decreases corrosive and wearing resistances at the same time. For these reasons, the bolt scuffing of steam turbines often occurs in fuel-burning power plant. Thus it is urgent to study the surface protection layer for the bolt material to avoid the bolt scuffing of turbines. In this work, in order to prolong the service life of the bolts worked at high temperature, and prevent the bolt scuffing, 1Cr11MoNiW1VNbN heat-resistant steel was used as matrix material, which has been widely used in supercritical and ultra supercritical steam cylinder bolt. The wearing, erosive, and high temperature oxidation resistances have been improved by high temperature salt bath chromizing.By experimental investigation and theoretical analysis, the salt bath chromizing process for the bolts worked at high temperature was optimized. The microscopic structure, hardness, wearing resistance, corrosion resistance, high temperature oxidation resistance of the 1Cr11MoNiW1VNbN matrix and the chromized case were investigated by metallographic observations, calculated phase graph analysis, micro hardness test, nanoindentation testing technique, X-Ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy, friction and wearing resistance test, electrokinetic potential polarozation curve measurement, salt immersion test experiments, high temperature oxidation tests.The results show that the process of chromizing to obtain the required layer thickness should be controlled at 910℃for 6h. The matrix structure is tempered martensite by heat treatment (held 30min at 910℃and then tempered 3h at 650℃)after chromizing. The chromizing layer is mainly composed of Fe-Cr solid solution and Cr23C6.The concentration of Cr in the chromizing layer is up to 20%. The chromizing layer is ferrite, grains are coarse, and it shows low hardness by micro hardness and nanoindentation test with a higher ductility. The low friction coefficient in chromizing layer can effective against adhesive wear, which improve the wearing resistance obviously, and the wear rate of base metal is 19.4 times higher than that of the chromizing layer. During electrokinetic measurement and salt immersion test, the chromizing layer shows passive behavior and good corrosive resistance. At high temperature oxidation process, the chromizing layer reveals outstanding oxidation resistance due to the slow mass gain and the dense oxidation film.The following conclusions can be drawn form the experimental results collected in the present work:the corrosion resistance and adhesive wearing resistance of base metal can be improved by salt bath-chromizing, which can prevent the bolt scuffing availably at high temperature.