A. Kohyama

H. "Sakasegawa

T. Hasegawa

T. Hirose

T. Suzuki

T." Muroga

Y. Katoh

National Institute for Fusion Science (NIFS-Repository)

§3. Correlation of Microstructure of Ferriticl Martensitic Steel with Mechanical Properties at Elevated Temperatures Sakasegawa, H., Hirose, T., Suzuki, T., Kohyama, A., Katoh, Y. (Kyoto Univ.), Hasegawa, T. (Nippon Steel Corporation), Muroga, T. Introduction Reduced activation ferritic steels (RAFs) JLF-l (9Cr-2W-V,Ta steel) is one of the reference materials of the lEA RAF R&D activity for fusion blanket structural material. JLF -1 was. reported to keep excellent mechanical properties and microstructural stability after heavy neutron irradiation up to 1 OOdpa I). Recently, JLS-series (9Cr-xW-V,Ta steels; x=2.5, 3 .0 and 3.5) have been produced as options of RAFs for use at higher temperature in advanced blanket systems. In this series, improved creep and tensile properties at elevated temperatures were intended by increased tungsten contents. In this present, conelation between microstructure of RAF and mechanical propeliies at elevated temperature. Experimental JLS-series were named as JLS-l, JLS-2 and JLS-3 with the increase of tungsten content. To obtain fundamental properties of RAFs at elevated temperature, creep rupture test was performed. Microstructural obselvation was performed by TEM to investigate correlation with creep propeliy. TEM speCImens were prepared from creep-ruptured specimen in order to investigate phase stability under creep condition. TEM specimens were cut from tested specimens close to fracture surface Results and discussion Figure 1 shows the creep properties of JLF-l and JLS-series. According to the result, creep properties improved with tungsten addition. Microstructures in tested specimens are shown in figure 2. With the increase of tungsten content, maliensite lath width became narrower and dislocation density increased. Precipitates in JLS-2 were smaller than in JLF-l . Like this precipitation morphology improve mechanical property by inhibiting movement of dislocation. 2 ) Generally, degradation of creep propeliies is caused by coarsening precipitates on grain boundaries and within grains such M23 C(, type carbides and Laves phase, diminishing dislocation density and evolution grains. 3),4) From making comparison with microstructures, lLS-2 had better microstructural features, from the VIew of improvement of creep property, for example very fine precipitates within grain, high dislocation density in martensite structure. In addition to that, JLS-2 tungsten content is more than in JLF -I, so it can be considered that JLS-2 tungsten content in matrix was more than JLF-l too. Solutes in matrix inhibit climbing of dislocation and iron self-diffusion, these contributed to the improvement of creep propelty with the increase of tungsten content. 200 I • ; : ' '' '. ' c> i , , , ' ":' ,',' ! ~ _ '_ ~ ~- , I c> .IIS-3 ~ 100 ~-l ~-~--+----+-• flY-I o JL'>-I • .IL~-2 il 90 -r==:::±::::==±===±~--j---~~:OII-~----l 180 J1F- I : Y~-913 . llog(P)~3211 ~~'~C!; :.-6--70 -Lar.;on -l\ tiller pamnl!ter P=T(logt +J()YlOOO(K h) Figure 1 - Creep properties summarized by Larson-Miller parameter Reference JLF-l JLS-2 Figure 2 - Microstructure of crept specinlen @ 700C 1) Y. Kohno, A. Kohyama, T. Hirose, M . L. Hamilton and M. Narui, Journal of Nuclear Materials, 258-263 (1998) 145 . 2) T. Shibayama, A. Kimura, H. Kayano, "The effect of small addition I elements on the precipitatetion of reduced activation Fe-9Cr-2W steels", Journal of Nuclear Materials 233-23~, 1996, p.270-275 3) Wendel BJones, "Effects of Mechanical Cycling on the Substructure of Modified 9Cr-l Mo Ferritic Steel", ASTM Metals Park, Ohio 44073, 1983, p.22l-235 4) F. Abe, H. Araki, T. Noda, "The effect of tungsten on Dislocation Recovery and Precipitation Behavior of Low-Activation Martensitic 9Cr Steels", METALLURGICAL TRANSACTIONS A, 22A, 1991, p.2225'2235 183 

Correlation of Microstructure of Ferritic/Martensitic Steel with Mechanical Properties at Elevated Temperatures

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