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内容記述 |
Development of breeding and structural materials for blankets is the key issue of fusion reactors. Fe-Cr-W based RAFM (Reduced Activation Ferritic/Martensite) steels are widely regarded as promising blanket structural materials, because of its low activation properties, radiation resistance and industrial maturity. Blanket concepts with liquid lithium (Li) breeder/coolant provide attractive options for high tritium breeding ratio, high efficiency and simplicity of blanket system. <br /> One of the critical issues for RAFM/Li blanket is the compatibility of RAFM steel<br />with liquid Li. As to the corrosion of ferritic steels in Li, only studies on conventional<br />Fe-Cr-Mo are available. In those studies, however, investigation on microstructure and<br />micro-chemical processes are quite limited. Any data are not available for RAFM<br />(Fe-Cr-W) steels yet. <br /> The purpose of the present study is to examine the compatibility of RAFM steels<br />with liquid Li with respect to corrosion rate and the degradation of mechanical<br />properties and to classify the underlying mechanism based on the element transfer and<br />change of microstructure during the corrosion process. <br /> In this study, the compatibility of JLF-1(Fe-9Cr-2W-0.1C), a RAFM steel<br />developed in Japan, with static and flowing Li was investigated. The coupon specimens<br /> (16×4×0.25mm) were exposed in an isothermal pot for static tests and thermal<br />convection SS316 (Fe-Cr-Ni) loop for flowing tests. After exposure, the corrosion<br />characteristics were examined by fine scale weight measurement, SEM/EDS (Scanning<br />Electron Microscope/Energy Dispersive X-ray Spectrometer), TEM (Transmission<br />Electron Microscopy) and Vickers hardness test. <br /> In the static test, the weight loss of JLF-1 specimens increased with temperature. <br />After exposure at 700°C for 100h, JLF-1 specimens suffered severe corrosion and the<br />corrosion rate was 0.18mm/yr. The kinetics of weight loss at temperature of 500°C and<br />600 °C showed that the corrosion of JLF-1 became saturated with the exposure time. <br />This is possibly due to the formation of saturated layer of dissolved elements in liquid<br />lithium near the specimen surface. In the corrosion test in a thermal convection loop, the<br />corrosion rate at 500°C for 250h was significantly larger than that obtained in the static<br />test in an identical condition. After Li exposure, the phase transformation from<br />martensite to ferrite was found on the samples. This is the first time that the phase<br />change from martensite to ferrite is observed after Li attack. The chemical analysis<br />results and the depletion of carbides suggested that the phase change should be caused by the depletion of carbon. At the same time, selective depletion of other alloy elements, such as Cr and W, was detected by EDS on the surface. Vickers hardness results showed that obvious softening occurred on the surface of the specimens after Li exposure and the depth of the softened region was consistent with that of the phase transformation. <br />The flowing Li enhanced the weight loss, phase change and reduction of hardness due<br />to the mass transfer effect. <br /> The influence of alloy composition on the corrosion was investigated by<br />comparing the corrosion behavior of JLF-1, binary Fe-Cr and pure iron at 700°C for<br />100hr. The corrosion of JLF-1 is more significant than that of Fe-9Cr and pure iron at<br />973K. The selective dissolution of Cr, W and C into lithium seems to enhance the<br />corrosion. Significant phase transformation from martensite to ferrite to the depth of<br />100μm was observed on the JLF-1 specimens after exposure in Li at 700°C for 100h. <br />The phase change resulted in drastic hardness drop from 250 to 140Hv. For the Fe-9Cr, <br />the softened layer (～5μm) was found on the surface of the specimen after Li attack due<br />to the depletion of Cr and W. This was verified by the EDS line scan on the cross<br />section of specimens. No mechanical change was observed on the pure Fe specimen<br />before and after exposure. <br /> To study the influence of container materials, the coupon specimens of JLF-1 were<br />exposed in Mo, SS316 and Nb crucibles separately at 600°C for 250h. The corrosion<br />characteristics in different crucible were compared. After exposure, the specimens<br />exposed in Mo and Nb crucible lost weight, while the specimens in SS316 crucible<br />gained weight due to the precipitation of Ni dissolved from crucible materials. The<br />phase transformation was observed on cross section of specimens exposed in Nb<br />crucible (～20μm) and Mo crucible (～10μm). No phase change was found in the case of<br />SS316 container. The phase change caused a corresponding reduction of hardness on<br />JLF-1 samples in depth. It is clear that the Nb container enhanced the depletion of<br />carbon and the phase transformation by trapping C and achieving a very low C<br />concentration in Li.<br /> Analysis of the experimental results was carried out based on the thermodynamic<br />and kinetic modeling. The results showed that the driving force of corrosion is the level<br />of Fe and Cr in Li. Saturation of those elements in Li results in the suppression of<br />corrosion. However, production of compounds of N, Li and alloy elements was shown<br />to determine the level of Fe and Cr in Li. The loss of C leads to the phase change. The<br />driving force of decarburization is the level of C in Li. The trapping of C by the<br />container materials can enhance the phase transformation. The diffusion of C in the<br />martensite controls the extension of phase change region.<br /> In conclusion, expected influences of Li attack to RAFM steel are the loss of<br />materials by dissolution of the constituent elements and the degradation of mechanical<br />properties caused by phase transformation from martensite to ferrite as the result of dissolution of C. Based on the data obtained, the compatibility of RAFM steel in Li<br />seems not to be a serious issue once the level of N in Li is kept low. The phase<br />transformation will be reduced by avoiding the use of materials which has high affinity<br />with C. |
要旨・審査要旨 (336.3 kB)
