Issue 7

S. Bagheri Fard et alii, Frattura ed Integrità Strutturale, 7 (2009) 3-16 ; DOI: 10.3221/IGF-ESIS.07.01 13 Discussion It is verified experimentally that the NC regions produced by all SP methods have the following characteristics however in different extents: - Equiaxed grains of around 20nm - Extremely high hardness - Separated from adjacent deformed structure regions with sharp boundaries - No recrystallization and substantially slow grain growth by annealing - Dissolution of cementite when it exists - Surface compressive residual stresses and also work hardening of the surface layer. Accordingly it seems that the main properties of the NC structure are independent of the SP techniques used in the experiment [18]. In order to obtain the desired NC region via any of the mentioned SP methods, a proper combination of different parameters attributable to the enhancement of kinetic energy of the shots shall be chosen. It has been reported that the increase in the kinetic energy per one shot such as the increase in the shot velocity and/or the shot size is the most effective parameter to increase the thickness of NC layer. It is also found that there is a certain critical initial hardness of specimens to produce the NC structure by SP: the NC structure forms when the specimen hardness is lower than the shot hardness [18]. Another important parameter in the formation of NC layers is the coverage technically defined as: the area fraction of specimen surface deformed by shot bombarding. It is revealed that the NC thickness tends to saturate with coverage irrespective of the shot size. On the other hand, the increasing in the coverage is ineffective to increase the maximum thickness of NC layer, the thickness of NC layer is initially increased with coverage but tends to saturate. It is possible to produce the surface NC layers with several 10 μm thick by SP when the kinetic energy (shot size and/or shot velocity) and the coverage are properly controlled [18]. Actually so far apart from assessment of some particular characteristics, no comprehensive comparison between different methods of NC creating SP methods and also no detailed comparison between them and the conventional SP process is available in literature. Just some researchers have studied the effect of a number of parameters and have found some results about the contribution of each to the whole process. C ONCLUSION AND SUGGESTIONS he formation of NC surface by means of some SP processes is a promising way to improve mechanical properties of metal alloys and in recent years has been the subject of increasing scientific and technological interests. . Initial work has been performed to prove the possibility of obtaining NC surfaces by these methods and also to assess the microstructural characteristics of the obtained layers. More recently, Researches have been done to evaluate the mechanical properties of NC surfaces obtained by SP processes. The results of SP experiments demonstrate that these methods are so efficient and undemanding to produce nanocrystalline surfaces and have potential application in various fields of industry. The experiments show that a remarkable improvement can be achieved as regards wear, corrosion and hardness. Fewer investigations are performed on fatigue but also in this case all the results demonstrate an improved behavior after formation of NC layers. The enhanced material properties of NC materials demonstrate the technological significance of nanomaterials in improving traditional processing techniques even if a clear relation between the modified characteristics and the process parameters is not identified. Therefore it seems commendable to perform more comprehensive investigations on SP methods which provide a new approach for selective surface reactions. Moreover, it can be concluded that NC layers may not be induced by SP if the impacted energy of small balls is not large enough. Therefore, to fully utilize the SP processes to improve the behavior of the material with a nanostructured surface layer there is an optimized processing condition, which shall be investigated in conjunction with microstructural analysis in future. Finally SP methods seem to have the potentiality to improve many other material properties not studied in detail up to now such as different surface chemical treatments that are controlled by the diffusion of foreign atoms and many other treatments which may take effect from the grain size. This fact can open new fields of application for shot peening processes, which today are mostly used for enhancing fatigue and fatigue related damage processes. Accordingly more investigations shall be planned in future to improve the performance of engineering materials used in industry. T