Surface Nanocrystallization By Shot Peening In Commercial Shotblasting Machine

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. The surface nanocrystallization is known as one of the important methods to modify surface physical, chemical and mechanical properties, and to prolong using life of machinery parts. The preparation technique of nanostructured surface layer in metals and alloys is with most important significance for the industry application.The surface nanocrystallization of low carbon steel and commercial pure titanium was studied by using shot peening equipment commonly used in industry. X-ray diffraction(XRD),optical microscope, scanning, electronic microscope(SEM), transmission electronic microscope(TEM), microhardness testing machine were used to characterize the surface layer microstructure and performance of treated specimens.The surface nanostructures of low carbon steel and commercial pure titanium was realized by modifying shot peening equipment commonly used in industry and the parameters, the finest grain size in the surface layer of low carbon steel is about 28nm, the finest grain size of commercial pure titanium is about 40nm. The longer the peening time, the thicker the deformation layers; and the deformation layer presents the gradient distribution along thickness direction. The deformation depth of low carbon steel was about 130um after 240min peening; the deformation depth of commercial pure titanium was about 283um, and the severe plastic deformation layer of finest grain was about 93u,m, the grains were equaixed shape grain about 40-150nm analyzed by TEM in this SPD layer. The hardness of the nanostructured surface layers is enhanced significantly after shot peening compared with that of the initial simple.A grain refinement mechanism of commercial pure titanium was summed up as follow:At the beginning of deformation, lots of dislocation intersect formed stress field, when the stress reach the resolved shear stress inducing mechanical twinning, one-system twin form and develop gradually to multi-system twins, twin twin intersections reduce the grain size. With increase of the twin systems and enhancement of the twin intersection activities, grains are further refined. Finally nano-scale grain with equaixed shape and random crystallographic orientation forms upon the much larger strain, high strain rate and multidirectional repeated loads.