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76 巻, 11 号
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論文
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金児 紘征, 中川 時子, 平泉 浩成2012 年 76 巻 11 号 p. 615-623
発行日: 2012年
公開日: 2012/11/01
ジャーナル フリーFollowing the accident during 2011 at the Fukushima Daiichi nuclear plant, sea water for cooling and boric acid for maintaining a non-critical condition, both corrosive liquids, were injected into nuclear pressure vessels. In order to estimate corrosive characteristics of the pressure vessels an experimental study was undertaken to provide an accelerated corrosion test on SA533B low alloy steel and Inconel 600, materials used in the construction of the pressure vessels. In a typical experiment, samples of these materials were immersed in saturated NaCl and concentrated H3BO3 aqueous solutions at a temperature of 423 K. SA533B suffered little or no corrosion in saturated NaCl solution, significant corrosion in concentrated H3BO3 and substantial corrosion in the binary saturated NaCl-concentrated H3BO3 solution. Galvanic corrosion of SA533B was examined when Inconel 600 was also immersed in the same solution and the two samples were electrically connected either externally by a wire lead or internally by a screw made of Inconel 600 or both. Corrosion rate in the initial stage was 0.07 mm/h. The corrosion product on SA533B was porous and easily detachable, indicating corrosion to be progressive without producing a stable protective corrosion layer. The validity of the extreme experimental condition for accelerated corrosion tests is discussed and experimental programs for further investigation are proposed.
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松崎 隆, 杉浦 隆次, 南雲 佳子, 横堀 壽光2012 年 76 巻 11 号 p. 624-633
発行日: 2012年
公開日: 2012/11/01
ジャーナル フリーCrack growth characteristic under the conditions of high temperature creep and fatigue interaction is dominated by cyclic dependent mechanism due to fatigue and time dependent mechanism due to the time of load application (creep). For many cases, this characteristic changes from the cyclic dependent to the time dependent mechanisms through an unstable transition region induced by creep and fatigue competitive mechanisms. To understand the physical mechanism of the interactive effects of creep and fatigue, it is important to clarify the damage mechanics around the crack. In the present study, the experiments of creep-fatigue crack growth tests and the quantitative analysis of damage by measuring Vickers hardness were conducted to understand the interactive effects of creep and fatigue on the crack growth characteristic. Additionally, by observing the material microstructure using EBSD, damage mechanisms were clarified. As a result, creep effect contributed to the expansion of damage region and fatigue effect increased the degree of damage. Interaction of these effects resulted in the occurrence of unstable transition region on the characteristic of creep-fatigue life.
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中里 直史, 香山 晃, 幸野 豊2012 年 76 巻 11 号 p. 634-638
発行日: 2012年
公開日: 2012/11/01
ジャーナル フリーAs one of the most attractive methods for fabricating SiC and SiC composite materials, NITE (Nano-Infiltration and Transient Eutectic-phase) method has been investigated. This paper concerns densification behavior of SiC preform where effect of SiC nano-powders' surface characteristics is emphasized. In this study, two types of SiC nano-powders, the one with high purity surface and the other with oxygen rich surface (SiO2 layer) were used. The densification behavior of preforms during hot-pressing is analyzed and is divided into two stages. In the stage I, the lower temperature side, with and without rapid densification is interpreted to be responsible to surface property of the SiC nano-powders. Main difference in surface property of the SiC nano-powders is surface SiO2 layer. To clarify SiO2 mass effect on rapid densification, SiO2 powder blending in preform is conducted and no direct evidence for elucidation of the rapid densification mechanism is obtained. In the stage II, densification behaviors of preforms with and without SiO2 powder blending are nearly identical and SiO2 mass effect on the stage is not detected. The utilization of oxygen rich surface powder provides high density SiC with uniform microstructure, and its flexural strength is higher than that utilizing high purity surface powder.
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吉田 卓, 奥村 晃央, 北 裕史, 高橋 淳, 潮田 浩作2012 年 76 巻 11 号 p. 639-646
発行日: 2012年
公開日: 2012/11/01
ジャーナル フリーThe aim of this study is to clarify the high-temperature strengthening mechanism of Nb-bearing ultra-low carbon steel, which is well-known as a superior steel for high-temperature applications. Observations by 3DAP suggested that the Nb atoms are either distributed in a solid solution within the grain or segregated at the grain boundary after hot rolling. The strength at 600℃ increases significantly upon the addition of Nb, and the corresponding dominant strengthening mechanism is considered to consist of the following: the resistance for the dislocation gliding motion due to solute Nb, the retardation of the dislocation climbing-up motion due to solute Nb and Nb-C dipoles, and the resistance of the dislocation motion caused by the Nb-C(N) clusters formed when the materials are heated up to 600℃ within 10 s and then held for 600 s. Further, compared with Nb-free steel or 0.1% Nb-bearing steel, 0.3% Nb-bearing steel has considerably reduced ductility at 600℃. This is attributed to the retardation of recovery due to the Nb addition. TEM observations imply that the dynamic recovery takes place easily during the tensile deformation at 600℃ in Nb-free steel or 0.1% Nb-bearing steel, whereas the tensile stress increases significantly because of the work hardening presumably caused by the retardation of the restoration process by further addition of Nb. Hence, a rupture followed by necking is thought to occur easily. Moreover, there is a possibility that the segregated Nb at the ferrite grain boundary might affect the dislocation behavior resulting in an increase in the steel strength at a high temperature and a retardation of the recovery process. This possibility will be investigated in a future work.
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