Papers by Author: Setsuo Takaki

Abstract: The multiple precipitation behavior of NbC and Cu particles in martensitic structure was investigated by using 0.05C-0.46Nb-2Cu-1.5Mn steel (NbC-Cu steel). Additionally, 0.05C-0.45Nb-2Mn steel (NbC steel) and 2Cu-5Mn steel (Cu steel) were also prepared to examine the respective precipitation behaviors of NbC and Cu. Aging treatment at 873K after quenching revealed that these steels exhibit typical age hardening. Comparing the NbC steel and Cu steel in the precipitation rate, the Cu precipitated much faster than the NbC. On the other hand, the peak hardness in NbC-Cu steel is higher than that by the respective precipitations in NbC steel and Cu steel. Besides, the aging time for the peak hardness in NbC-Cu steel was between those in NbC steel and Cu steel. This suggests that the NbC and Cu particles were separately precipitated within martensite matrix and each of them contributed to the hardening in NbC-Cu steel. As a result of TEM investigation for crystallographic characteristics of the precipitates, the NbC and Cu particles had different crystallographic orientation relationship with tempered martensite matrix: Baker-Nutting relationship for NbC particle and Kurdjumov-Sachs relationship for Cu particle.

Abstract: It is well known that the ultra grain refinement can be achieved by sever cold rolling, followed by reversion treatment in metastable austenitic stainless steel plate. In this study, the cold rolling was replaced by cold drawing. This procedure was applied to a metastable austenitic steel (Fe-16Cr-10Ni alloy) thin wire, and then the microstructure development during cold drawing and annealing was investigated. The austenite phase transformed to martensite during the drawing. Vickers hardness of the wire markedly increased with increasing the drawing strain. When the drawing strain reached about 4.5, the wire exhibited martensite single structure and had high hardness of Hv4.4GPa. Annealing of the heavily drawn wire at around 900K for 0.6ks leads to the formation of reversed austenite with the diffusional reversion mechanism. As a result, ultra fine-grained austenitic single structure with the grain size of about 0.6μm was obtained. It was also found that the wire has an excellent combination of a strength and ductility.

Abstract: Solution nitriding and aging treatment were applied to Ti-4mass%Cr alloy in order to fabricate a ductile high-nitrogen titanium alloy with fine (α + β) structure. The solution-nitrided specimen with
α’ martensitic structure was significantly hardened by solid solution strengthening by the absorbed nitrogen. During the aging treatment, fine β grains with a size of 1 microns in thickness precipitated along the martensite-plate boundaries. Although the specimen was softened to some extent after the aging treatment, the hardness is kept much higher than that of the aged Ti-4mass%Cr alloy without solution nitriding. This indicates that the nitrogen is still in solid solution of α phase even after the aging treatment, and contributes to strengthening of the fine-structured Ti-4mass%Cr-N alloy.

Abstract: The work hardening behavior by cold rolling was investigated in ultralow carbon and low carbon martensitic steels containing 12%Cr or 18%Ni, and then the effect of carbon on the work hardening behavior was discussed in terms of the change in dislocation density and the microstructure development during deformation. In the ultralow carbon steel, the hardness is almost constant irrespective of the reduction ratio. On the other hand, the low carbon steel exhibits marked work hardening. The dislocation density of these specimens was confirmed to be never increased by cold rolling. It was also found that cold rolling gives no significant influence on the morphology of martensite packet and block structure. TEM images of the cold-rolled steels revealed that the martensite laths in the ultralow carbon steel are partially vanished, while those in the carbon bearing steel are stably remained. These results indicate that the solute carbon retards the movement of dislocations, which results in the high work hardening rate through the formation of fine dislocation substructure within laths.

Abstract: The effect of Cr2N precipitation on deformed microstructure in high nitrogen austenitic Fe-18Cr-18Mn-2Mo-0.9N steel was investigated with a particular emphasis on deformation twinning. Based on the crystallographic analysis in the stereographic projection, the orientation relationship between austenite (γ) matrix and Cr2N was determined to be Cr2N [110]γ //[1100] and Cr2N (111) γ //(0001) . The deformation twinning had {111} < 112 > crystallographic component similar to that of cellular Cr2N. The cellular Cr2N precipitates caused a different orientation dependence of deformation twinning: only one twinning system in the <111 > grain was activated almost parallel to the growth direction of Cr2N.

Abstract: Deformation behavior of high nitrogen austenitic Fe-18Cr-18Mn-2Mo-0.9N stainless steel was investigated utilizing electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). During deformation, the <110> grains rotated and the trace of these grains moved towards into the dodecahedral plane (the line connecting (001) and (111) planes). Misorientation mapping in EBSD showed that the special boundaries (almost Σ3) gradually diminished whereas the low-angle boundaries were developed. TEM observation showed that (i) the low-angle boundaries developed corresponded to the deformation twinning with {111}<112> component, (ii) the deformation twinning showed the strong orientation dependence relative to tensile axis, and (iii) the deformed microstructure was characterized by extended stacking faults, planar dislocation array in low strain regime, and by well-developed deformation twinning in high strain region, respectively.

Abstract: The nickel-free austenitic stainless steel produced by solution nitriding (Fe-25%Cr-1%N alloy) was subjected to isothermal heat treatment, and then the microstructure formed through the decomposition of austenite was investigated in terms of the morphology of eutectoid structure and the size of eutectoid block. On the isothermal heat treatment at 873K~1223K for the solution-nitrided steel, the austenite decomposed to eutectoid structure composed of ferrite and Cr2N nitride. This transformation could be completely finished after long time heat treatment in the above temperature range. The nose temperature of T.T.T. curve was around 1173K, and the time to start the eutectoid transformation was only 100~200s. The eutectoid structure was formed mainly along austenite grain boundaries and then grew into the untransformed austenite region. Finally, the austenite was completely decomposed into ferrite and Cr2N nitride. As a result of OIM observation for the specimen after isothermal heat treatment, the eutectoid structure was found to be divided into small-sized ferrite blocks, in which lamellar Cr2N plates were finely distributed. The block size and the mean ferrite path of eutectoid structure were decreased with lowering the heat treatment temperature. In the 873K heat-treated material, these values were estimated at 20 microns and 0.1 microns, respectively.

Abstract: It is well known that microstructural changes occur in a steel bearing, when the bearing is operated under conditions involving high cyclic stresses. When combined with relatively high temperatures, such microstructural changes result in the flaking of the bearing raceway. In this paper, microstructural changes that occurred during rolling contact fatigue were investigated, and the relationship between these changes and fatigue life are discussed in association with the recrystallization behavior of martensite. Conventional bearing steel SUJ2 (SAE52100) was subjected to partial solution treatment at 1133K for 2.4ks followed by oil quenching. The quenched material with a martensitic structure was tempered at 443K for 7.2ks, and then subjected to rolling contact fatigue testing. The testing was performed at temperatures ranging from 373K to 443K and surface pressures of 4.6GPa or 5.5GPa. During testing at 373K, flaking occurred from the surface of the raceway due to non-metallic inclusion and without any marked microstructural changes. On the other hand, in the case of testing at 403K or more, flaking occurs after obvious microstructural changes. Firstly, dark etching constituent (DEC) formed around the area of maximum shear stress, which was followed by the formation of white etching constituent (WEC) within the DEC at 80 and 30 degrees to the rolling direction. TEM observations showed the change from martensite lath to dislocation cell structure within the DEC, and also the existence of fine ferrite grains of 20nm through 100nm within the WEC. Arrhenius plots for the fatigue life indicated that the activation energy of the fatigue process corresponded to that of carbon diffusion in bcc ferrite. These results suggest that rolling contact fatigue originated from the WEC is controlled by the diffusion of carbon in the ferrite matrix.

Abstract: The behavior of work hardening by cold rolling and tensile deformation was investigated in an ultralow carbon and carbon bearing martensitic steels, and then the effect of carbon on the work hardening behavior was discussed in terms of the change in dislocation density and the microstructure development during deformation. In the ultralow carbon 18%Ni steel (20ppmC), the hardness is almost constant irrespective of the reduction ratio. On the other hand, the carbon bearing 18%Ni steel (890ppmC) exhibits marked work hardening. The dislocation density of these specimens was confirmed to be never increased by cold rolling. It was also found that 10% cold rolling gives no significant influence on the morphology of martensite packet and block structure. TEM images of the 10% cold-rolled steels revealed that the martensite laths in the ultralow carbon steel are partially vanished, while those in the carbon bearing steel are stably remained. These results indicate that the solute carbon retards the movement of dislocations, which results in the high work hardening rate through the formation of fine dislocation substructure within laths.