Plasma boriding of high strength alloy steel with nanostructured surface layer at low temperature assisted by air blast shot peening

Plasma boriding of high strength alloy steel (AISI H13) with nanostructured surface layer fabricated by air blast shot peening(ABSP) was performed at 580 °C for 4 h. A continuous coating layer with thickness of about 4 μm was composed with two phases of Fe2B and FeB, and the nanohardness of borided layer was as high as 20 GPa. These results indicate that cyclic deformation and the angle between shot jet and sample surface play a critical role in the process to produce nanocrystalline in the surface layer. By using ABSP as pre-treatment, boron diffusion and the nucleation rate of boride can be remarkably enhanced in nanostructured surface layer. In addition, the weight loss rates of borided samples in molten aluminum alloy were only as much as ~ 40% of that of untreated ones, and the main reason was that the coating layer can effectively prevent the substrate directly contacting with molten aluminum alloy.