Recently, the improvement of the high strength of fine grain steels has been investigated actively. Fine grained steels have high yield stress, as expected by the Hall-Petch relationship. Therefore, these materials are considered for use as structural material. Welding is one of the most effective methods for connecting structural components. Nevertheless, the negative influence of tensile residual stresses and coarse grains due to the welding process must be taken into consideration. It has been proved that the shot peening process can effectively overcome these problems. In this study, samples prepared with various mean grain sizes were processed by shot peening. The triaxial residual stress distribution after shot peening was measured by X-ray diffraction. Moreover, the distribution of the hardening effect and nanocrystalline layer near the shot peened surface was observed. In this paper, the relationships between the effects of triaxial residual stress, structure, fatigue and hardness are discussed. As a result, thin nanocrystalline layer was formed on the surface layer of over 90 percent of surface area. Therefore, hardness of the surface, fatigue limit and fatigue life improved. The surface of plastic flow layer became the starting point of the crack initiation. Moreover, compressive residual stress by shot peening processing was confirmed and the depth of the nanocrystalline layer and the plastic flow layer correlated with depth of changing point of the triaxial residual stress distribution.
