Enhanced chromizing kinetics of tool steel by means of surface mechanical attrition treatment

A nanostructured surface layer of about 20 μm in thickness was fabricated on a hot-working tool steel (AISI H13) plate by means of surface mechanical attrition treatment (SMAT). Chromizing behaviors of the SMAT sample were investigated in comparison with its coarse-grained counterpart. Relative to the coarse-grained form, the Cr-diffusion depth was obviously enhanced by SMAT at various chromizing temperatures, with a maximum increment at 600 °C. A duplex chromizing treatment at 600 °C followed by at a higher temperature led to the formation of a much thicker chromized surface layer (up to 30 μm in thickness) on the SMAT sample. This might be attributed to the formation of a large amount of nano-sized chromium compounds in the surface layer at 600 °C, which retarded growth of nano-sized ferrite grains and improved the Cr diffusion at higher temperatures. Gradient distributions of Cr concentration, microstructure and hardness were formed in the chromized surface layer in the SMAT steel after the duplex chromizing treatment, leading to an obvious enhancement in the wear resistance.