Koichiro NAMBU

This chapter thoroughly assessed the magnetic properties of the annealed 1-μm-thick steel, where the excitation frequency is varied from f = 10, 50, 100, 200 kHz and up to 1 MHz. First, the core loss density Pcm and relative magnetic permeability μFe,r of the non-annealed and annealed steels made of the same material at a fixed peak flux density of BFe,m = 0.3 T were examined and compared. The measured results illustrated that Pcm of the annealed 1-μm-thick steel is around 48.2% smaller than that of the non-annealed steel, and μFe,r of the annealed steel is around 185.6% larger than that of the non-annealed one. After that, many additional tests under various BFe,m and f also confirmed the much lower Pcm of the annealed steel than that of the other. Moreover, the results of BFe,m, μFe,r and Pcm obtained with the presented measurement and calculation methods in numerous experiments are almost similar, which helps evaluate and verify the reliability of the results. Thus, the 1-μm-thick steel fabricated by the rolling process and then processed in the appropriate annealing, which has noticeably better magnetic characteristics than that of the non-annealed one, is potentially used for inductor cores in power electronics applications at high frequencies. In future work, the annealing temperature will be suitably increased to 700 °C or 800 °C to significantly improve the magnetic properties of the considered 1-μm-thick steel. Furthermore, an analysis model of the 1-μm-thick steel using the finite element method will be studied and developed to validate the measured findings.

Fine Particle peening is a method to obtain surface modification effects, such as fatigue strength improvement, by bombarding the work material with particles at high velocity. However, there are many factors that affect the surface modification effect, making it difficult to select the optimum conditions. The particle velocity and particle flight behavior have not been clarified due to the large number of flying particles in addition to the extremely high particle velocity. Therefore, in this study, in addition to air flow analysis inside and outside the nozzle, particle velocity analysis using the particle method was conducted. ANSYS was used for the airflow analysis, and Particle Works was used for the particle method. The nozzle diameter and nozzle-to-work distance were varied. The nozzle diameter was varied from 3 to 10 mm. The nozzle-to-work distance was 50, 100, and 150 mm. The pressure at the nozzle entrance was set to 0.2 MPa, and air flow analysis was performed under incompressible fluid conditions. The particle method used iron-based particles with a particle diameter of 100 μm as a model for analysis. The results of the airflow analysis showed that the potential core area increases as the nozzle diameter increases. This was attributed to the shear layer caused by the wall resistance inside the nozzle. Next, particle velocity analysis showed that particle velocity tended to increase with increasing nozzle diameter. In addition, it was found that the particle velocity increased with increasing nozzle-to-work distance. Next, the particle flight behavior was analyzed, and it was found that the particles accelerated most at the parallel part of the nozzle and continued to accelerate after the nozzle exit. Finally, to verify the validity of the analysis, the particle velocities were compared with those measured by a high-speed camera. Although the geometry of the nozzle was slightly different, the measured and calculated velocities showed similar trends, suggesting that the present method is valid.

Shot peening (SP) and fine particle peening (FPP) were performed as post-treatments on induction-hardened and tempered AISI 4140 steels. Furthermore, the combined effects of surface characteristics including changes in surface morphology, compressive residual stress, and hardness on the fatigue limit were quantitatively examined. The surface characteristics of the prepared specimens were investigated using a laser and stereomicroscope, a micro-Vickers hardness tester, and an X-ray device for residual stress measurements. The rotating bending fatigue properties were also examined. FPP increased the fatigue limit of induction-hardened and tempered steel samples by introducing compressive residual stress and increasing their surface hardness. Conversely, the fatigue limit of the steel treated with SP was lower than that of the electrochemically polished sample owing to the formation of large dents. The fatigue limits of post-peened induction-hardened and tempered steels can be estimated using a modified Goodman diagram considering changes in surface properties.