In this study, the peening behavior of shot particles in a fine particle peening (FPP) process such as velocity and impact angles were analyzed by using a high-speed-camera. Results showed that the velocity of shot particles depended on a peening pressure; the higher the peening pressure, the higher the particle velocity. The particle velocity measured in this study was approximately 120 m/s; this was much higher than that of the conventional shot peening (SP) process. This was because the air resistance of shot particles in the FPP process was higher than that of shot particles in the SP process. In order to discuss the surface modification effect of the FPP process, commercial-grade pure iron treated by the FPP process was characterized by micro-Vickers hardness tester and scanning electron microscope (SEM). Thickness of hardened layer treated with higher peening pressure was much higher than that of the lower pressure treated one. The unique microstructure with stratification patterns, which was harder than that of the other part, was observed near the specimen surface. The reason for the microstructural changes by the FPP treatment was discussed based on the kinetic energy of shot particles.
In order to determine the fine particle peening (FPP) treatment conditions that generate a high compressive residual stress, the effect of specimen hardness and shot particle hardness on residual stress was investigated. The results showed that the higher the generated specimen hardness, the higher the compressive residual stress at the surface. Additionally, in the case of high hardness specimens, the shot particle hardness affected the residual stress. This was because shot particle was deformed during the FPP treatment process. Furthermore, in order to clarify the effect of FPP treatment on fatigue properties of SCM435H steel with different hardnesses, fatigue tests were performed at room temperature using a rotational bending fatigue testing machine. In the case of higher hardness substrate, the fatigue strength was dominantly increased by FPP treatment ; however, in the case of lower hardness substrate, the increasing level was very small. This was because compressive residual stress generated on the surface of higher hardness specimen was much larger than that generated on lower hardness specimen and the higher compressive residual stress remained on the surface of higher hardness specimen under the application of cyclic loading. These results indicate that the FPP treatment is highly significant to improve the fatigue strength for higher hardness substrate.
Investigation of microstructural changes in pure commercial-grade iron caused by fine particle peening (FPP) treatment was undertaken by detailed observation using field emission-scanning electron microscopy (FE-SEM) and measurement of the X-ray diffraction peak full width at half maximum (FWHM). The effect of pre-FPP treatment on the oxygen diffusion process within iron is also discussed. FPP treatment produced stratification patterns with many dislocations and grain boundaries on the treated surface. This unique microstructure strongly affected the diffusion capacity, so that thicker oxygen-concentrated layers were observed on pre-FPP/oxygen diffusion-treated surfaces.
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