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Cr35Ni45鋼高溫長期服役過程的氧化與滲碳機理

High temperature oxidation and carburizing mechanisms of Cr35Ni45 heat-resistant steel under service conditions

  • 摘要: 采用掃描電鏡、電子探針和X射線衍射等手段對不同服役時間(原始態、1.5a和6a)Cr35Ni45乙烯裂解爐管內壁的氧化與滲碳機理進行了系統分析.結果表明:高溫長時服役后爐管內壁出現了氧化層、碳化物貧化區和碳化物富集區三個區域,其氧化行為包括Cr2O3外氧化和SiO2內氧化,且服役過程中外氧化膜發生反復破壞和重建;爐管服役過程的滲碳行為主要由內表面結焦引起,外氧化膜的反復破壞可以加重滲碳,但外氧化膜在破壞后能自動修復,所以服役態兩個爐管的滲碳程度較輕;外氧化膜的反復破壞和重建使亞表層貧鉻,導致形成碳化物的臨界碳濃度增加,在內壁亞表層形成貧碳化物區,多余的碳原子在其內側析出,形成碳化物富集區.

     

    Abstract: The oxidation and carburizing mechanisms of Cr35Ni45 type pyrolysis furnace tubes serviced for different time(as-cast,1.5 a and 6 a) were systematically investigated by scanning electron microscopy(SEM),electron probe and X-ray diffraction(XRD).Compared with original uniform microstructure distribution,there are three zones including an oxidation layer,a carbide depletion zone and a carbide-rich zone at the subsurface region of the Cr35Ni45 tube inner wall after long time service. The oxidation behavior of the Cr35Ni45 tubes at high temperature consists of the external oxidation of chromium and the internal oxidation of silicon,and the outer oxidation layer is greatly affected by repeated destruction and reformation in decoking. The carburizing behavior of the Cr35Ni45 tubes mainly results from coking on the inner wall surface,and repeated destruction of the outer oxidation layer can aggravate this carburization. But due to auto-remediation of the outer oxidation layer,carburization of both the serviced tubes is at a lesser degree. Also,repeated destruction and reformation of the outer oxidation layer cause the depletion of Cr,the increase in critical concentration of C in the subsurface of the inner wall and carbide participation at the below region,leading to the formation of a carbide depletion zone and a carbide-rich zone.

     

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