Nanocrystalline structure formation in carbon steel introduced by high speed drilling

The microstructure of drilled hole surface on an Fe–0.56% C steel (as-quenched martensite structure) has been investigated. A several microns thick nanocrystalline structure layer with an average grain size of the order of 10 nm was observed on the topmost surface of holes drilled at high speed. The existence of retained austenite on the hole surface suggests the occurrence of dynamic phase transformation (DPT) from bcc to fcc during high speed drilling. It is proposed that the nanocrystalline structure layer on the drilled hole surface is produced by a large strain gradient together with severe plastic deformation (SPD) induced DPT.