Effect of Oxidation on Microstructure and Magnetic Properties of Cr25Ni35Nb and Cr35Ni45Nb Alloys
GUO Jingfeng1, CAO Tieshan1, CHENG Congqian1, WANG Fugang1, MENG Xianming2, ZHAO Jie1
1 School of Materials Science and Engineering, Dalian University of Technology, Dalian 116085; 2 China Automotive Technology & Research Center,Tianjin 300300
Abstract: In this work, the oxidation behavior and the magnetic properties of Cr25Ni35Nb and Cr35Ni45Nb alloys after a long-term service were investigated by employing OM, SEM, XRD and VSM analysis. The results show that the oxidation layer formed consists of Cr2O3, SiO2 and Fe2O3, and exhibits cracks and rupture. The high-temperature instability of Cr2O3 and the cyclic coking/decoking operation during long-term service are found to contribute to the rupture of the outer and inner surface oxidation layers, respectively. There could be observed three zones, i.e. oxidation layer, carbide-depleted zone and carbides zone, at both inner and outer walls of the pyrolysis furnace tubes. The oxidation film is a conti-nuous Cr2O3 oxide layer, and SiO2 disperses along the grain boundaries at the carbide-depleted zone. The original two kinds of austenitic heat-resistant alloys are paramagnetic. And due to the lower Cr content of matrix at the carbide-depleted zones, the paramagnetic structures are transformed into ferromagnetic structures, with saturation magnetizations larger than 20 emu/g. Therefore, it could be concluded that oxidation is also one of the critical factors that impel ethylene pyrolysis furnace tube to the transformation into ferromagnetic structure.
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2019, Vol. 33 
