Austenitic stainless steels with a bimodal harmonic structure, in which the fine grain structure (Shell) exists around the coarse grain structure (Core), are prepared by powder metallurgy to improve both strength and ductility. Herein, X‐ray diffraction contrast tomography, a 3D grain mapping technique for polycrystalline materials using ultrabright synchrotron radiation X‐rays, is used to reconstruct the grain shape and location and to evaluate the average excess dislocation density of the Core and Shell structures. This technique allows one to evaluate the excess dislocation density not only on the surface, but also inside the sample where damage occurs in tensile tests. The results show that the excess dislocation density of the Shell structure is higher than that of the Core structure. The excess dislocation density of homogeneous austenitic stainless steels with grain sizes similar to the Core structure of the harmonic structured stainless steel is higher than that at comparable stresses, indicating that the deformation of the bimodal harmonic structured alloy is localized in the fine grain structure. This is consistent with the results obtained from electron backscatter diffraction analysis, in which the surface grains are evaluated.
Mean torsional stress is considered to less affect torsional fatigue strength of steels, but several experimental results have been recently reported that mean torsional stress caused significant reduction in torsional fatigue strength in the very high cycle region for shot-peened spring steel. To investigate the effect of mean torsional stress on high strength steel, ultrasonic torsional fatigue tests with mean torsional stress were conducted for spring steel and bearing steel, which are used for mechanical components subjected to cyclic shear stress. Torsional fatigue strength up to 10 9 cycles were obtained for fully reversed torsional loading (□ = −1) to pulsating torsional loading (□ = 0). The results revealed that mean torsional stress caused reduction in fatigue strength in the very high cycle region for both spring steel and bearing steel, and applying higher mean shear stress would result in transition of the fracture origin from surface to an internal inclusion. The reduction in torsional fatigue strength was discussed from the viewpoint of the transition of fatigue origin, and applicability of a √□□□□ parameter model was discussed for predicting the reduction in torsional fatigue strength.
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