【摘要】 采用光学显微镜、扫描电镜、X射线衍射仪和振动样品磁强计分析了Cr25Ni35Nb和Cr35Ni45Nb两类合金长期服役后的氧化行为以及其对磁性的影响。结果表明:长期服役后形成的氧化层主要由Cr2O3、SiO₂、Fe2O3组成,并且氧化膜发生了破裂,外壁和内壁氧化膜的破裂分别是Cr2O3在高温下的不稳定性和周期性的结焦/清焦所造成的;裂解炉管内壁和外壁均出现了氧化层、贫碳化物区、碳化物区,外表层为连续的Cr2O3氧化层,SiO₂则在贫碳化物区沿晶界分布;氧化前两种奥氏体耐热合金均为顺磁性结构,经氧化后形成的贫碳化物区中,基体的Cr含量较低,因而转变为铁磁性结构,其饱和磁化强度均超过20 emu/g。因此,氧化也是乙烯炉管长期服役后转变为铁磁性结构的重要因素。更多还原

【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 continuous Cr₂O₃ oxide layer,and SiO₂ disperses along the grain boundaries at the carbide-depleted zone. The original two kinds of austenitic heatresistant 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.更多还原