Investigation of surface nanocrystallization of 45 steel

By means of Supersonic Fine Particles Bombarding (SFPB), a nanostructured surface layer was fabricated on a 45 Steel plate consisted of pearlite and ferrite phases. The microstructure features of various sections in the treated surface layer were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) observations, and the hardness variation along the depth was examined by means of Nanoindentation. Experimental evidence showed, after SFPB treatment, severe plastic deformation was observed and a nanocrystalline surface layer comprising ferrite and cementite phases was found; The thickness of nanostructured surface layer varies from a few to about 15μm (grain size<100nm ) depending upon the treatment duration; At the top surface layer, the mean size of equiaxed nanocrystallites with random crystallographic orientations is approximately 17nm. The grains size increases along with increment of distance from the treated surface. The equiaxed grains and cell structures with size of about 100 nm were formed at the depth of about 15μm; In the region from about 30μm depth, large numbers of roughly equiaxed and lamellar-shaped cell structures with sizes of about 200~500nm separated by dense dislocation walls were observed. Experimental evidence and analysis indicated the ferrite and cementite phases all occur grain refinement, and grain refinement can be mainly attributed to the movement of dislocation. The hardness of the surface layer in the SFPBed 45steel sample increased evidently compared with that of the matrix, which can primarily be attributed to the grain refinement.