The Study Of Surface Nanocrystallization Of Metallic Materials By High-energy Peening

In industry,most of failures of machine parts begin from their surface,so it is important to explore new surface-strengthening technologies in order to improve their durability. Surface nanocrystallization has been regarded as one of the most prosperous surface technologies.The surface nanocrystallization of a low carbon steel and commercial pure titanium was realized by using high-energy shot peening technique. The relationships of the buckling height as a peening intensity measurement with the peening time,the peening height,the inputting frequency,the size and the number of balls,and the variation of hardness and roughness were analyzed by means of optical microscope,scanning electronic microscope(SEM),transmission electronic microscope(TEM),X-ray diffraction(XRD),microhardness testing machine and surface roughness testing machine .Experimental results showed that the layers of severe plastic deformation with nanocrystalline structures were gotten.on low carbon steel surface and on commercial pure titanium after the high-energy shot peening treatment. The longer the peening time,the thicker the deformation layers. The surface-hardness of the samples increased greatly after the treatment,and decreased gradually from the surface to the centre. During the peening,the surface-roughness was enhanced rapidly in the beginning,and finally it would wave around certain value. The depths of plastic deformation were well coincided with the buckling heights of the plane samples,and the buckling height was influenced greatly by the peening processing parameters. The buckling height increased little by little with the increment of the peening hight in some periods and then decreased.,that is,the buckling height increased with the peening height first,and then decreased after the peening height was over a critical value. The effect of the number of inputted balls on the buckling hight was similar to the influence of the peenig height,that is,itincreased first and then decreased over a certain number. The buckling was high when the bigger balls in size and higher input frequency were employed.The microstructure analyses of surface deformation layers showed that the depth of plastic deformation increased with peening time,but the increase was fast in the starting stage and then slow during peening. After peening for 120 minutes,the depth was over 80 micron and grainsize was fined to about 20 nanometers from micron size. The metals with hep structure were nanocrystallizationed in surface layer by means of high-energy shot peening. The plastic deformation depth of commercial pure titanium would reach to over 200 micron and the grainsize would reach to 20 nanometers.