未服役Cr35Ni45Nb合金真空渗碳行为及相演化机理

采用乙炔真空渗碳工艺对未服役的Cr35Ni45Nb乙烯裂解炉管进行了加速渗碳处理，并采用X射线衍射、扫描电镜、定量电子探针等手段对渗碳前后炉管内壁的渗碳行为及相演化机理进行了系统分析.结果表明：炉管高温渗碳过程的主要控制因素由初期的扩散控制逐渐变为扩散-表面反应综合控制；渗碳过程属多元多相反应扩散范畴，炉管内侧横截面随渗碳深度的不同依次出现了表面碳化物层、亚表层贫碳化物区、片层状碳化物层、规则几何碳化物区、扩散区、弱影响区等六个区域，这六个区域共同组成了M₇C₃、M₇C₃-M₂₃C₆混合区和M₂₃C₆的三级垂直层状分布.贫碳化物区的形成原因是表面碳化物层的形成造成亚表层贫Cr；片层状碳化物的形成源于碳在高镍铬合金中的低渗透性以及析出物进一步的阻碍效应.

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

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.