Abstract
Recently ultra-supercritical steam power plants operate at <TEX>$1000^{\circ}F$</TEX> (<TEX>$538^{\circ}C$</TEX>) and 3500 psi (24.1 MPa). Thermal efficiency of power plant will be increased about 2% if steam temperature increases from <TEX>$1000^{\circ}F$</TEX> to <TEX>$1150^{\circ}F$</TEX> (<TEX>$621^{\circ}C$</TEX>). In this study valve materials Incoloy901 (IC901) and Inconel718 (IN718) were nitrided to improve the surface hardness and solid lubrication function of the valve materials. The hardness of both IC901 and IN718 increased about two times by ion nitriding. IC901, IN718 and their nitrided specimens were corroded under ultra super-critical condition (USC) of <TEX>$621^{\circ}C$</TEX>. and 3600 psi (24.8 MPa) for 2000 hours. Oxidations of both IC901 and IN718 were very small due to the formation of protective oxide layer on the surface. But the corrosion resistance of both nitrided specimens decreased because of the formation of non-protective nitride layer of <TEX>$Fe_{4}N$</TEX>, <TEX>$Fe_{2}N$</TEX> and CrN on the surface layer. The hardness of both nitrided IC901 and IN718 at <TEX>$20{\mu}m$</TEX> depth from the surface decreased about 30% and 20% respectively by USC 2000 hours.
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