Papers by Author: Yo Tomota

Abstract: High temperature nitriding was applied to a low carbon steel, and then the transformation microstructure formed from the high nitrogen austenite was observed. The microstructure was greatly influenced by cooling rate after the nitriding. The austenite transformed to martensite when the cooling rate was fast (water-cooling) though a certain amount of austenite was retained in the martensite. On the other hand, a diffusional transformation product such as (ferrite + Fe4N) eutectoid structure was formed when the cooling rate was slow (gas- or furnace-cooling). The hardness profile obtained in these specimen were related with microstructure depending on the nitrogen concentration profile and cooling rate.

Abstract: To understand the reason of high work hardening in high nitrogen steel, a scale-bridging analysis consisting of in situ neutron diffraction, in situ electron backscattering diffraction measurement during tensile deformation and weak beam transmission electron microscopy was performed with particular attention to stress partitioning. This study demonstrates the contribution of stress partitioning, back stress due to dislocation pile-up at grain boundaries and dislocation interaction to work hardening at each strain level.

Abstract: This study highlites deformation behavior of austenitic and pearlitic steels by in-situ neutron diffraction and 3D/4D EBSD measurement with a particular attention to their hierarchy.In particular stress partitioning in these microstructures is examined from macroscopic as well as microscopic scale length levels, and they are correlated to each other.

Abstract: The warm-compression microstructures of two nickel-containing tempered martensite steels with and without carbon were investigated. The EBSD measurement and TEM observation/Kikuchi pattern analysis confirm that the ultrafine microstructures contain ferriteaustenite dual phases with more than 18% austenite. The difference in the austenite amount before and after warm compression proves that the dynamic austenite precipitation occurs during warm deformation and the carbon addition is beneficial to the austenite precipitation. It is also found that the ultrafine austenite grains as a hard phase promote the dynamic recrystallization of ferrite during warm deformation.

Abstract: The low carbon steel, SM490 was austenized at different temperatures followed by quenching into water to obtain martensite microstructures with different grain sizes. Then specimens were heated up to 600°C followed by warm-compression at έ=1.7x10-3 s-1 (strain rate) to investigate the dynamic recrystallization behavior. The influence of pre-tempering before compression was also investigated. The microstructure observations were performed with FE-SEM and orientation imaging analysis with EBSD. It is confirmed that the dynamic recrystallization occurred in the tempered martensite as well as the as-quenched marteniste, resulting in fine grained ferrite microstructure with about 2μm. The dynamic recrystallization grain size is hardly dependent on the block size of initial martensite.

Abstract: This study aims at examining thermomechanical controlled process to realize ultrafine TRIP-aided multi-phase microstructures in low carbon steels. Heavy deformation at a supercooled austenite region was found to lead the formation of 2 μm ferrite as well as retained austenite with high volume fraction. The morphology of retained austenite was changed from film-like shape to granular shape with lowering finish rolling temperature in austenite field. This ultrafine TRIP-aided multi-phase steel showed good balance of tensile strength with total elongation, ie. 1080MPa and 26.9%. A novel in-situ neutron diffraction measurement demonstrated that the retained granular austenite transformed to martensite at a relatively large strain compared with the retained film austenite. The therein-underlying mechanism of the good mechanical properties was discussed from the view points of the morphological and thermodynamical stabilization of retained austenite.

Abstract: Dislocation density and crystallite size of steel wires with various carbon concentrations and drawing strains were determined by profile analyses for neutron diffraction profiles. The density is found to increase while the size decreases with increasing of carbon concentration and/or drawing strain. Both of the Bailey-Hirsch relation and Hall-Petch relation hold for the present results to suggest that these two are not independent., i.e., indicating an identical strengthening mechanism from a different point of view.

Abstract: In order to develop the SUS304L housing by powder metallurgy for an optical device useful for the FTTH communication system, the optimum compacting pressure and sintering temperature were investigated using granulated powder as the material to satisfy high air-tightness and high laser-weldability. Then the laser-welding test of specimen made under the optimum condition was carried out to observe welding sputters.

Abstract: Neutron diffraction technique was employed to measure stress distribution in an interior region of a SUS304 rotating mechanical component with a fringe after solution treatment and after subsequently conducted stress-relief annealing. A coupon with 3x3x3 mm was prepared by spark cutting to determine stress-free spacing d0. The elastic strain was determined by using the measured (111) spacing d and d0 by (d - d0)/d0. Then the obtained strains were input into the general Hooke’s law to calculate stresses. The residual hoop stress near the surface is compressive which is lowered in the inner part. The residual stresses near the center of the component are hydrostatic tensile stresses. These residual stresses were not relaxed by the annealing treatment recommended in a handbook. Hence, more appropriate stress relief treatment has to be developed. In case of a cast specimen, coarse grains with a strong texture made it difficult to determine residual stresses precisely.