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.