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未服役Cr35Ni45Nb合金真空滲碳行為及相演化機理

High temperature vacuum carburization behaviors and phase evolution mechanisms of virgin as-cast Cr35Ni45Nb alloy

  • 摘要: 采用乙炔真空滲碳工藝對未服役的Cr35Ni45Nb乙烯裂解爐管進行了加速滲碳處理,并采用X射線衍射、掃描電鏡、定量電子探針等手段對滲碳前后爐管內壁的滲碳行為及相演化機理進行了系統分析.結果表明:爐管高溫滲碳過程的主要控制因素由初期的擴散控制逐漸變為擴散-表面反應綜合控制;滲碳過程屬多元多相反應擴散范疇,爐管內側橫截面隨滲碳深度的不同依次出現了表面碳化物層、亞表層貧碳化物區、片層狀碳化物層、規則幾何碳化物區、擴散區、弱影響區等六個區域,這六個區域共同組成了M7C3、M7C3-M23C6混合區和M23C6的三級垂直層狀分布.貧碳化物區的形成原因是表面碳化物層的形成造成亞表層貧Cr;片層狀碳化物的形成源于碳在高鎳鉻合金中的低滲透性以及析出物進一步的阻礙效應.

     

    Abstract: Virgin as-cast Cr35Ni45Nb tubes were carburized firstly by low-pressure vacuum carburizing at 1080℃. Then the carburization behaviors and corresponding phase evolution mechanisms in the inner wall were systematically investigated through X-ray diffraction, scanning electron microscopy, and electron probe microanalysis. It is found that the major controlling factor during the carburizing process varies from diffusion control to diffusion and surface reaction integrated control. In general, the diffusion process is accompanied by heterogeneous or multiphase reactions. Compared with the original microstructure and morphology, several newly formed zones appear including bulk carbide scale on the external surface, subsurface depleted zone, lamellar carbide zone, regular geometric carbide zone, diffusion region and weakly affected region. All of these regions display a layering distribution including M7C3, M7C3-M23C6 mixed zone and M23C6. In addition, the formation of surface carbide scale promotes Cr depletion in the subsurface, leading to subsequent depletion of chromium carbides; the cause of lamellar carbides is the low permeability of carbon in the high-nickel-chromium alloy and the blocking effect of precipitates to diffusion.

     

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