| Due to good oxidation resistance, corrosion resistance, large resistivity and stable resistivity value, high temperature strength and other comprehensive performance, Cr20Ni80 electrothermal alloy is widely used in household appliances,industrial furnaces and medical, communications, nuclear power and other electric alloy components manufacturing. Oxidation temperature and oxidation time are the important factors that affect the life of Cr20Ni80 electrothermal alloy as an electrothermal component in the process of use, which affect the formation and destruction of the surface oxide film during the use of the heating element, which has a large influence on the service life of the alloy components. Cr20Ni80 electric heating alloy was studied in this paper for aluminum melting furnace in heating devices, meanwhile, the kinetics of oxidation process and film morphology and phase composition of Cr20Ni80 alloy were studied, for the mechanism of formation and growth of oxide film, two kinds of experimental methods, such as constant temperature oxidation and cyclic oxidation, were used to carry out the experiments.The oxidation temperature of the constant temperature was 700 ??1000 ?, the oxidation time was 1h?100h, the temperature of the cyclic oxidation was 1000 ? ,with the oxidation time of 4h, 10h, 25h and 50h. The oxidation experiment of 8 mm Cr20Ni80 electric heating wire was carried out by using box resistance furnace. OM,X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the structure of oxide film formed on the surface of the sample. The main results were as follows:The results showed that the oxidation kinetics curve shows the segmentation phenomenon with the oxidation time at the same oxidation temperature. Compared with the late oxidation, the initial oxidation possess faster rate of oxidation. And the oxidation rate gradually increased with the increase of temperature. During the oxidation process, the oxidation resistance of the material was always complete oxidation resistance (Oxidation resistance refers to the ability of metal materials to resist oxidation atmosphere corrosion at high temperature, which is called oxidation resistance. The average oxidation rate of the material is less than 0.1 g/m2· h.)In the initial stage of oxidation, the oxide film formed on the surface of the sample. With the increase of the oxidation time, the oxide film begin to peel off under the action of internal stress and thermal stress of the oxide film, and then new oxide film began to form in the area where the oxide film peeled off. The main oxides of the oxide film were Cr2O3, and the NiCr2O4, Cr2O3 and NiCr2O4 with spinel structure in the oxide film are interdependent to form a dense uniform oxide film, which made the material have good oxidation resistance and corrosion resistance. In the oxidation process, the diffusion driving force of the smaller conditions, the diffusion of chromium ions mainly along the grain boundary and short-circuit diffusion defects,which will lead to the formation of the surface of the material oxide film, because the initial formation of the oxide film many holes, which for the late diffusion of chromium ions to provide a rapid diffusion channel, was conducive to the formation of dense uniform oxide film, with the oxidation time, octahedral shape of N iCr2O4 spinel will gradually became a layer Shaped and rod-shaped.At the temperature of 1000 ?, the surface oxide film produced on the surface of the sample in the early stage of oxidation of Cr20Ni80 electrothermal alloy sample composed of spherical or ellipsoidal oxide (mainly composed of Cr2O3) . With the oxidation time prolonged, the spherical or ellipsoidal oxide got bigger and fructure.Compared with the constant temperature oxidation , the mass gain and the Cr2O3 in the surface oxide film of cyclic oxidation is larger. Because of the alternating effect of the hot and cold cycle, and the new oxide film formed on the place where the preformed oxide film peeled off from the base metal, and with the oxidation time prolonged, the oxide film became uniform and dense. |
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