| The thermal power generation is the main way of generating power in our country at present and large capacity and high parameters of ultra supercritical thermal power units is an important development direction of current thermal power. Corresponding to the requirements on the property of the key material which used in unit equipment is also getting higher and higher, so it seemed to be necessary to develop new high temperature heat resistant materials.In this dissertation,five groups of austenitic heat resistant alloys based on Fe—18Cr—9.5Ni are designed by different content of Nb, N and a small amount Al added. The forming condition of composite oxide film is analyed by thermodynamic calculation.The high-temperature oxidation behavior of the five groups of austenitic alloys were systematically investigated by combining results from discontinuous oxidation weight increasing method, optical microscope, X-ray diffraction,scanning electron microscope and energy disperse spectroscopy. The results are as follows:Oxidation film peeling did not appear after 100 h oxidatized at 700℃,800℃ and 900℃.It shows that oxide film does not fail at high temperature. The oxidation weight fitting curves accorded with parabolic law △m=atb. And the fitting effect was remarkable. The average oxidation rates of five groups of alloys were less than 0.1g/m2.h which were all belongs to complete oxidation resistance level. The average oxidation rates of five groups of allo ys after 100 h are relatively close at 700℃ and 800℃, but it was obvious different at 900℃. The average oxidation rate of 5#（0.44Nb0.119N） alloy is 0.032 g/m2.h which is lower than the other four groups of alloys. The oxidation activation energy of the 5# alloy was the highest in the five groups of heat-resisting alloys. Which means that the surface oxidation reaction rate of 5# alloy was the slowest and the antioxidant performance was the best.The 1#（0.32Nb0.096N）, 3#（0.61Nb0.098N）, 4#（0.47Nb0.057N） and 5#（0.44Nb0.119N）alloys were formed a relatively dense oxidation film of Al2O3 at 900℃. The composite oxidation films formed on the surface of alloys are mainly Al2O3, Mn3O4 and（Fe0.6Cr0.4）2O3 mixed oxide. Alloy 1# also formed a small amount of MnO2 oxide in the process of oxidation which reduced the continuity of Al2O3 oxidation film. The dense internal oxidation film of Al2O3 was not formed in 2# alloy at 900℃ due to itself does not contain Al. The formed composite oxidation film is mainly（Fe0.6Cr0.4）2O3 and the spinel oxide FeCr2O4. The surface morphology of oxide film of 5#（0.44Nb0.119N） alloy after oxidation is more smooth and uniform than that of the other four groups of alloys. Its oxide particles also have the minimum size.The mechanical properties experimental results show that the mechanical properties of five groups of austenitic heat resistant alloys are not lower than those of Super304 H. |
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