Application of Residual Stress Using Polished Impact Media

Author:  Tomohiro Minami, Hatsuhiko Usami, Keju Chou, Yoshikazu Yamada
Source:  ICSP-13
Doc ID:  2017117
Year of Publication:  2017
Introduction: Shot peening is a surface modification process that uses blunt-shaped particles as the impact media to apply residual stress fields near the surface, and it has been widely used to improve the fatigue properties of structural and mechanical components. Recent progress in the projection systems and fine particle production methods has made it possible to apply fine particles of less than 100 􀁐m as the impact media, and a micro shot peening process has been proposed as a further effective peening treatment. On the other hand, shot blasting is a grinding and cleaning process that uses polygonal particles such as abrasive media, and it has also been widely applied as a means of descaling cast bodies. The mechanism for the surface removal in shot blasting is mainly erosive wear, and it results in an increase in the surface roughness accompanied by a degradation of the residual stress field near the surface. As the impact particles’ diameter decreases, their geometry becomes much sharper and they act as abrasive media. Therefore, it is possible for erosive wear to occur in micro shot peening. Steel beads are frequently used as the impact media, since their geometry and hardness can be controlled. The surface of the steel beads becomes covered with a thick oxide scale because of the rapid cooling during the atomization process, and becomes rough and contains irregularities. Some beads have a distorted shape and there are concerns about their increasing the erosive wear. Therefore, improvement in the particle roughness,which will affect the reduction of the occurrence of erosive wear, is anticipated. The present study describes the effects of additional polishing of the steel beads on the mechanical properties,including the hardness, roughness and residual stress, of the shot peened surface, and the applicability of the additional polishing to the improvement of the effectiveness of the shot peening process.

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