A double strengthened surface layer fabricated by nitro-chromizing on carbon steel

•A double strengthened surface with excellent properties can be fabricated on a carbon steel by a nitro-chromizing process.•The presented nitro-chromizing technique not only lowers the chromizing time but forms a thick N/C diffusion layer with adjustable microstructure and properties.•The N/C diffusion layer can be optimized to contain ultrafine martensite–bainite associated with nanometer-sized precipitates and retained austenite, leading to a surface hardness of ~ 1000 HV or higher.

The chromized layer on carbon steels demonstrates high hardness and excellent wear-resistance. However, it usually suffers from its thin thickness and the softened underneath diffusion layer owing to the decarburization of the diffusion layer. In the present work, a double strengthened surface with a chromized layer and a N/C-rich diffusion layer was fabricated on a carbon steel by a nitro-chromizing process. The effect of chromizing time and cooling method on the microstructure and property of the nitro-chromized surface layers was investigated. The phase transformations occurring in the double strengthened surface were also studied in detail. Although it is difficult to improve the chromized layer, which consists of CrN, β-Cr2N and (Cr,Fe)7C3 phases, the microstructure of the diffusion layer with a thickness of more than 500 μm can be optimized to contain ultrafine martensite–bainite associated with nanometer-sized precipitates and retained austenite, leading to surface hardness of ~ 1000 HV or higher. The microstructure in the diffusion layer plays a key role in improving the property of the double strengthened surface.