SynopsisUsing a steam oxidation testing apparatus, operating under almost the same steam conditions as those in actual boilers, the influences of temperature and pressure (especially above critical pressure) on steam oxidation were investigated with austenitic heat-resisting steels such as SUS 347-HTB, 17-14CuMo, and the newly-developed 20Cr-25Ni and 22Cr-35Ni steels for advanced fossil steam plants.Oxidation loss in weight increased with higher steam pressure and temperature. Increase of Cr and Ni contents improved the oxidation resistance. Cr concentration in inner scale increased with Cr content of steel. This caused the higher resistance to steam oxidation. The existence of a Ni enriched layer at the scale and metal interface contributed to the improvement of steam oxidation resistance. The Ni concentration of the layer was higher in steels containing a higher Ni content.Morphological investigation of the scale cross sections showed that considerably large amounts of voids and cavities were produced in the outer scale of SUS 347HTB and 17-14CuMo steels oxidized in steam at high temperature and pressure.

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