タフトライド処理鋼の疲労破壊に関する研究 : 残留応力の挙動について

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Recently, tufftrided steel is widely used as the structural material of machine parts since it has many advantageous properties such as high fatigue-resistance and wearing-resistance and, further, shortening the treatment time as compared with that of the conventional nitriding. It is generally known that the improvement of fatigue property depends on the cause of the residual compressive stresses the intermetallic compound and the diffusion layers near the surface and the increase of hardness. However, the degree of contribution of these factors to fatigue property is not clear. From this point of view, as a part of studies in series on the fatigue fracture of the tufftrided steel, the authors have carried out in this present study several investigations on the residual stresses in the tufftrided steel using the improved X-ray diffraction technique. The specimen used in this investigation was a S15C low carbon steel, and as was the case with all specimens, was annealed after forming and then tufftrided. The composition of salt bath far tufftriding was composed of 46.8%-KCN, 45.5%-KCNO and 0.05%-Na4Fe(CN)6. The temperature of this salt bath was kept at 570°C and the specimen was water-quenched after soaking in this salt bath for 90 minutes. By this treatment, the intermetallic compound layer composed of ε-Fe3N with thickness of about 7μ and the diffusion layer of 0.8mm thickness was formed on the surface layer and its neighbouring layer of the specimen. As the examination process, the residual stress distribution was first measured by means of X-rays as well as mechanically for the purpose of observing the nature of the residual stress in the tufftrided steel. Then the change in residual stresses due to alternating stressing was observed and the contribution of residual stresses to the improvement of fatigue limit was discussed. The results obtained in this study indicate interesting features as follows: (1) The strength of steel, especially in fatigue property, has been remarkably improved by trufftriding and quenching. However, it decreases if nitrided materials have been precipitated in the diffusion layers by tempering. For this reason, the water-quenching should be performed as quickly as possible after tufftriding in order to prevent their precipitation. (2) During the measurement of residual stresses in tufftrided steel by means of X-rays, the macroscopic residual stress itself has been observed. Almost all residual stress in tufftrided steel is influenced by the thermal stress by water-quenching and the residual stress induced by nitriding is small. (3) No change in residual stress of surface layer has been observed during the fatigue process. It is considered that this non-fading property is a character of tufftrided steel. (4) The degree of contribution of residual stresses due to cyclic stressing is about 30%. It is considered that the factors such as diffusion layers and the increase in hardness give more effective contributions.
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タフトライド処理鋼の疲労破壊に関する研究 : 残留応力の挙動について

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