Change in Microstructure and Mechanical Properties of Steel Components Surface Layer by Severe Plastic Deformation

The change in microstructure and mechanical properties of machined steel surface was reviewed. Nanocrystalline ferrite structure layer can be produced by high energy shot peening. Nanocrystalline layer consists of equiaxed grains of about 20 nm and distinguished from the adjacent heavily deformed region with a clear boundary. Nanocrsrystallne ferrite structure showed extremely high hardness and substantially slow grain growth. White etching layer can be produced by drilling, hard-turning and sliding wear. The white etching layer thus produced was reaustenitized region due to the deformation heat. White etching layer has clear boundary (transfrmation boundary) between the matrix. White etching layer consist of submicron grained martensite which is much finer than conventional martensite. The white etching layer produced by drilling and sliding contains nanocrystalline layer at the top surface. The thickness of nanocrystalline layer is about 20% of whole white etching layer. The thickness of white etching layer and also nanocrystalline layer increases with the increase in drilling or cutting speed and feed rate. The formation mechanism of white etching layer and nanocrystalline layer were discussed. The thermal and mechanical influence for such layer formation was presented. The influence of white etching layer on the mechanical properties of machined components was introduced.