Papers by Author: Kota Sawada

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Authors: Kota Sawada, Hideaki Kushima, Kazuhiro Kimura
Abstract: The precipitation site, main metallic composition and number density of Z phase have been investigated in T91 in order to clarify the influence of Z phase formation on recovery of martensitic structure and creep strength degradation. The Z phase particles were mainly present around prior austenite grain boundaries and/or packet boundaries in the steels crept at 550oC and 600oC. The Z phase particles were found in specimens crept at 550oC to 650oC. There was no indication of Z phase formation up to about 62475.0 h at 500oC and 14106.5 h at 700oC. The Nb content of Z phase observed at 550oC was lower than that at 600oC. The number density of Z phase measured at 550oC was lower that that at 600oC, indicating that the preferential recovery of martensitic lath structure around prior austenite grain boundary is not remarkable at 550oC in contrast with 600oC.
Authors: Kazuhiro Kimura, Kota Sawada, Yoshiaki Toda, Hideaki Kushima
Abstract: Degradation mechanism and life prediction method of high chromium ferritic creep resistant steels have been investigated. In the high stress condition, easy and rapid extension of recovered soft region results in significant decrease in creep strength, however, ductility is high. In the low stress condition, extension of recovered soft region is mainly controlled by diffusion and it is slow, therefore, deformation is concentrated in the recovered soft region along grain boundaries and ductility is extremely low. Delta-ferrite produces concentration gap due to difference in equilibrium composition of austenite and ferrite phases at the normalizing temperature. It increases driving force of diffusion and promotes recovery of tempered martensite adjacent to delta-ferrite. Concentration gap may be produced also in heat affected zone (HAZ), especially in fine grain HAZ similar to dual phase steel, and it has possibilities to promote recovery and, therefore, to decrease creep strength. It has been confirmed the advantage of region splitting analysis of creep rupture strength for high chromium ferritic creep resistant steels, through a residual error analysis. It is important to avoid a generation of concentration gap in order to improve stability of microstructure and to maintain high creep strength.
Authors: Masataka Yoshino, Yoshinao Mishima, Yoshiaki Toda, Hideaki Kushima, Kota Sawada, Kazuhiro Kimura
Abstract: The precipitation behavior of MX carbonitride during a normalizing heat treatment with and without ausageing was investigated in a modified 9Cr-1Mo steel. The normalizing heat treatment was performed at 1150 oC for 1800 s. Ausageing was conducted at 765 and 500 oC for 1800 to 86400 s during the cooling from the heat treatment. The matrix of the steel was austenite single phase during normalizing and ausageing, except for that ausaged at 765 oC for 86400 s. The initial austenite grain size and hardness were not influenced by ausageing, except for the sample ausaged at 765 oC for 86400 s. Although Nb-rich MX (NbX) and cementite were observed, V-rich MX (VX) was not observed under any of the conditions investigated. The amount of NbX in the steel ausaged at 500 oC was at least twice as large as that under the other conditions, and the amount in the steel ausaged at 760 oC was slightly larger than that in the steel that did not undergo ausageing. The precipitation of NbX took place during ausageing in the austenite matrix. On the other hand, it is well known that VX precipitates during tempering. An equilibrium mole fraction of VX in the austenite matrix calculated by Thermo-Calc. was larger than that of NbX at the ausageing temperatures. It is proposed that VX is an equilibrium phase at the ausageing temperature; however, VX nucleation takes much longer in the austenite matrix. It is postulated that the precipitation of VX is more strongly influenced by the interfacial energy rather than supersaturation. It is concluded that the precipitation of MX carbonitride, especially NbX, can be controlled by ausageing during cooling after a normalizing heat treatment.
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