An Electron Microscopy Study of Vein-like Grain Boundary Microstructure in Nitrocarburized Low Carbon Steels

The coarsened grain boundaries（GBs） with vein-like morphology frequently appear in the nitrogen diffusion layer of nitrocarburized carbon steels.The electron probe X-ray microanalysis shows that such vein-like GBs are rich in nitrogen and carbon atoms.Transmission electron microscopy and scanning electron microscopy further reveal that the coarsened GBs consist ofγ’-nitrocarbide（Fe4（C,N）） ande-nitrocarbide lamellas that are formed during nitrocarburizing due to high nitrogen concentration at GBs.It is shown that many of {111} micro twins exist in the y’-phase lamellas and thin e-phase slices prefer to nucleate at their twin boundaries with the orientation relationship of {0001}e//{111}γ’.Upon growing large y’-lamellas may become faceted and thin e-lamellas may grow thicker and become the main portions in the vein-like GBs.The microstructure evolution occurring in the vein-like GBs can be depicted as:α+[N]/[C]→>γ’ +[C,N]→e-nitrocarbides.