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 of γ＇-nitrocarbide （Fe4（C,N）） and ε-nitrocarbide lamellas that are formed during nitrocarburizing due to high nitrogen concentration at GBs. It is shown that many of {111}〈112〉 micro twins exist in the γ＇-phase lamellas and thin ε-phase slices prefer to nucleate at their twin boundaries with the orientation relationship of {0001}ε//{111}γ＇. Upon growing large γ＇-Iamellas may become faceted and thin ε-Iamellas 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] → ＋ [C,N] →ε-nitrocarbides.
