Papers by Author: Harushige Tsubakino

Abstract: Precipitation phenomena in an austenitic stainless steel, SUS316L cold-rolled with various reduction rates were studied by transmission electron microscopy and synchrotron radiation diffractmetry. After the aging at 573 K for 15000 h, two of precipitates were observed, which were identified as M7C3 and M23C6 by SR diffraction and electro diffraction measurements. The precipitates M7C3 were formed at both innergranular and grain boundary, while the precipitate M23C6 was formed at innergranular. The precipitation was promoted with increasing cold rolling reduction. Also segregation of phosphorous was detected along grain boundaries. Besides, the residual stresses were measured with side inclination method using a synchrotron radiation facility, SPring-8. The residual stresses were increased with increasing the cold rolling reduction rate.

Abstract: A low impurity magnesium alloy has bean prepared. Deformation behavior in cold-rolling and corrosion behavior of the alloy were compared with those of a commercial alloy. The specimens were cold-rolled at room temperature with reduction rates of 0~80 %. Transmission electron microscopic observations on the cold-rolled specimens were carried out. In the case of the low impurity magnesium alloy, recrystallization easily occurred. Analyses of microstructures in the deformed specimens were carried out by means of EBSP, and the recrystallization phenomena have been discussed. Another effect of lowering the impurities is to improve a corrosion resistance with changing the corrosion morphology.

Abstract: Microstructures in a bearing steel, JIS SUJ2, have been observed and analyzed in detail by means of transmission electron microscope and synchrotron radiation diffraction in SPring-8. Spherodized carbides in the steel are generally recognized as spherical cementite particles, but some of them have been clearly shown to be M23C6 type of carbide in this study. The shapes and sizes of these two types of carbides are similar to each other. On the martensitic matrix of the steel, it is believed to be stable at relatively low temperature, but it is also shown to be decomposed to form cementite by prolonged aging at 383 K, which provides a reason for decrease in hardness in standard hardness blocks, previously reported by the authors.

Abstract: Micro-resistance spot weldability of nickel free stainless steel sheet was studied to fabricate medical implants. Weld lobe was established and the effect of welding parameters on joint strength was investigated. Weld nugget represented a rapidly quenched austenitic cellular structure whose cell size is several μm with a little ferrite. The amount of δ-ferrite in weld nugget is smaller than that predicted by some Schaefller diagrams due to rapid solidification during micro-resistance spot welding.

Abstract: The aluminum alloys AA5052, AA5083 and Al-7%Si (AC4C) were nitrided by electron beam excited plasma (EBEP) technique to improve wear resistance and hardness. The specimens were characterized with respect to the following properties: wear resistance, hardness of nitrided layer (AlN layer), surface and cross sectional microstructures. The friction coefficient of nitrided AA5052 decreased to 0.2 and that of Al-7%Si was improved to 0.1. The AlN layers were relatively uniformly formed on the AA5052 and AC4C alloys with a thickness of 4.5 μm and 2.5 μm during 2hrs and 5.4 ks, respectively. On the surface of nitrided AA5083, a uniform AlN layer was not formed due to the high nitriding temperature. The Si3N4 was found in the nitrided AC4C alloy.

Abstract: The life limit of the standard hardness block of HRC64 made from JIS-SUJ2 steel was evaluated. The HRC64 specimens were annealed at temperatures ranging from 55 to 200
C for about 104 h in maximum. The micro-Vickeres hardness decreased with the annealing time at all temperatures after some incubation periods. Transmission electron microscopy (TEM) showed that the microstructure of the specimen before annealing was composed of primary cementite particles, carbide and martensite matrix. After annealing, fine cementite particles of about few tens of nanometers in size were observed. The decrease in hardness during annealing was caused by the precipitation of the cementite. The life limit keeping the standard hardness value at ambient temperature was estimated to be about 3.5-4 years based on extrapolation of the annealing curves at various temperatures.

Abstract: In this study, we’ve used carburized-quenched-tempered steels that received shot-peening process with different shot grain sizes and evaluated variation of surface property of the steels. As a result of study, it is demonstrated that the fatigue life of carburized steels can be prolonged significantly by increasing surface hardness and compressive residual stress and by reducing surface roughness and abnormal carburized case depth at superficial layer, which we’ve investigated in the study.

Abstract: In this study, the microstructure of two kinds of diamond-like carbon films was studied. These films were produced during the deposition of two kinds of hydrocarbon gas plasma by a newly developed technique called plasma based ion implantation (PBII). Microstructural analysis was performed by means of high resolution transmission electron microscopy (HRTEM). An amorphous DLC film was obtained by using C2H2 plasma. The crystal structure of carbon ( (C 168H ) was generated inside the DLC film when C5H6CH3 plasma was used. The effect of the precursor gases used in the present experiment on the microstructure of the DLC film was also studied.

Abstract: A new technique has been developed for improving corrosion resistance on magnesium alloys. Specimens of AZ31 magnesium alloy were dipped into molten salt of NaBF4 at 723 K for various times, and then cooled, rinsed with water, and dried in air. Corrosion resistance in the surface treated specimens was evaluated by salt immersion test using 1 % NaCl solution as a time for occurring filiform corrosion. On an un-treated AZ31 alloy, the time for starting the filiform corrosion was about 1.2 ks, while on the surface treated specimen, the time was prolonged into about 1300 ks. Moreover, the surface treated specimen showed corrosion resistance in low pH solutions, such as 1 % HNO3 and HCl solutions.