This paper documents the microstructure changes of steel SUPER 304H caused by high temperature overheating (1140 °C, 15 hours). The experimental material was firstly isothermally laboratory exposed about 30,000 hours at temperature 650 °C and then subject to high temperature overheating. Main focus was given to microstructural changes and precipitation processes. Paper summarized influence of those changes to practical applicability of steel SUPER 304H for ultra-super critical (USC) coal power plants. Base material, laboratory expose state and overheated state were compared. Combination of color etching and optical microscopy method, electron scanning microscopy and energy dispersive spectroscopy analysis was used for comparison.Keywords: SUPER 304H, high temperature overheating, USC, sigma phase precipitation, microstructure changes
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