Formation of a nanocrystalline surface layer on steels by air blast shot peening

A nanostructured surface layer was fabricated on two kinds of steels by means of air blast shot peening. The nanolayer shows a sharp boundary to the underlying work-hardened area and good thermal stability up to 873 K. It has much higher hardness than the work hardened region in both the as-treated and annealed states. When using small shot sizes, the nano area can be formed in very short treatment times, and the thickness and continuity of the nanolayer is enhanced. On the contrary, the nanocrystalline region is more difficult to synthesize when using large shot particles, even though the deformed area is much thicker. The effect of particle diameter is attributed to the different collision time and different strain rate of the treated materials.