Characterization of the nano-scale surface layer of a tempered martensitic steel synthesized by ultrasonic nanocrystalline surface modification treatment

The microstructural evolution of a tempered martensitic ferritic steel X20CrMoV12.1 subjected to ultrasonic nanocrystalline surface modification (UNSM) treatment was investigated by electron backscattered diffraction (EBSD) and transmission electron microcopy (TEM). EBSD results clearly show a grain refinement up to a depth of ∼210 μm from the treated surface, with the activities of formation of low angle grain boundary transforming to high angle grain boundary with misorientation. The mechanism of grain refinement involves the formation of dislocations within the tempered lath structure, transforming to a dense dislocation wall (DDW) which is always vertical to the lath boundary, dividing the original lath into a block and a subgrain structure, contributing to loss of lath structure. With increasing strain (top layer), the blocks and subgrains were further refined by dislocation slipping, resulting in the formation of nano-sized grains in the top surface. Refinement of second phase precipitation (M₂₃C₆) is observed up to a depth of ∼50 μm from the treated surface due to the dislocation cutting induced by plastic deformation. In addition, an increase in number density and volume fraction of precipitates is observed with decreasing depth from the matrix, which may be due to compressed microstructure formed as the result of UNSM treatment. Further, no evidence of carbide dissolution was observed. Consequently, a nano-sized grain and dense precipitate layer are produced by UNSM treatment.