Abstract. The fine-grained rolling steels NFG600 and the conventional usual rolling steels SM490 were processed by sand paper polishing and mechanical grinding to compare the residual stress generated after processing. The average grain size of NFG600 and SM490 is 3 µm and 15µm respectively. Therefore improvement of mechanical properties for such fine-grained steels is expected, it is important to understand the residual stress state of new fine-grained materials with processing. In this study, multi axial stresses of two kinds of specimens after polishing and grinding were measured by three kinds of analysis methods including cos-ψ method. As a result, as for σ 33 , the stress of NFG was compression, though that of SM490 was tension.

This paper describes a fundamental study on the possibility of a new type of the X-ray stress measurement, in which stresses are obtained from analyzing whole part of one diffraction ring detected with a two-dimensional X-ray detector called image plate (IP). The theory of the stress determination proposed by Taira and Tanaka (1978) was shown first, and then newly modified to be able to determine all plane stress components from a single-exposure experiment. An experiment was performed for the examination of the method, and the result of showed good agreement with stresses that were applied to the specimen mechanically.

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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.