A. "Nishimura

T." Kakeshita

National Institute for Fusion Science (NIFS-Repository)

§ 11. Effect of Long Term Exposure to HighMagnetic Field at Cryogen icTemperature on MartensiticTransformation of SUS316Nishimura, A., Kakeshita, T. (Dept. Mater. Sci. andEng., Osaka Univ.)The cryogenic support structure in the LHD, whichsustains the poloidal and helical coils, are constructed withaustenitic stainless steel, SUS316, and is exposed to highmagnetic field of around 3 T at 4.5 K for a long time. TheSUS316 plate of which thickness is up to 100 mm was hotbent, machined and welded during the construction. To holddown the residual deformation, the post weld heat treatmentwas not performed and the carbon content was not reducedto keep rather high yield stress and ultimate tensile strength.SUS316 has high nickel equivalent and a stableaustenitic phase that is non-ferromagnetic. However,stability of the heat-treated and hardly machined austeniticphase has not been clarified under a long time exposure tohigh magnetic field at cryogenic temperature.In this paper, solution heat-treated and sensitizedsamples were prepared and exposed to high magnetic fieldat 4.5 K during the LHD operation. Before and after theexposure the magnetization was measured by a squid systemand the martensitic transformation was discussed.The tested material was machined out from center ofthe left-plate for the helical coil can of which thickness was70 mm. Sample #1 was machined to 4 mm x 4 mm x 3 mmand solution heat-treated. Sample #2 was 4 mm x 4 mm x 4mm and sensitized after solution heat treatment to generatechromium carbide on the grain boundary. For the solutionheat treatment, the sample was kept at 1323 K for 30minutes in vacuum and furnace cooled. The sensitized heattreatment was carried out at 973 K for 100 hours in vacuumand furnace cooled. The heat treatment was performed usinggold imaging furnace, so the cooling rate to below 500 Kwas rather higher. The samples were polished with #1000emery paper and then heat-treated. The microstructure ofSample #2 is shown in Fig. 1. The chromium carbides areformed on the grain boundaries.After the heat treatment, the magnetization wasmeasured using a squid system (Quantum Design; ModelMPMS-XL) and the samples were attached to helical coilcover with an aluminum tape. The 4th cycle coolingoperation of the LHD started on August 28, 2000 andfinished on September 24, then the plasma and the deviceexperiments were carried out until February 15, 2001 andthe warming operation was followed. All operation of the4th cycle ended on March 15,2001. The samples were keptat 4.5 K for almost half year and about 2.5 T was expectedto act on the samples repeatedly during the experiments.After the exposure, the magnetization was measured again.The results of the magnetization measurements areshown in Fig. 2. The sensitized sample was magnetizedduring the first measurement, before the 4th cycle. Since thechromium carbide is ferromagnetic, the chromium carbidewas magnetized resulting in increase of the magnetization ofthe sample. At the same time, very small amount of alphaprim martensite (bcc) was expected to be generated on thegrain boundaries. However, Sample #1 did not show suchmagnetization and it is noted that chromium rich regionwould be transformed easily. On the other hand, there wasno clear hysteresis after the 4th cycle. From the results, it isconcluded that the austenitic phase of SUS316 would bevery stable even for a long time exposure to high magneticfield at cryogenic temperature.Fig. 1 Microstructure of solution heat-treated samplefollowed by sensitized heat-treatment.40 :- - -:' - - - - - - - - - - :: : -: : + sensitized :'.. ':f \ After 4th cycle30 :J-- ------ ---':""10,: Solution heat-treatedo 50 100 150 200 250 300Temperature (K)Fig. 2 Magnetization results of two samples before and afterthe 4th cycle of LHD operation. (Upper two data sets areobtained from the sensitized sample after solutionheat-treated. Lower two data sets are measured on thesolution heat-treated sample.)103

Effect of Long Term Exposure to High Magnetic Field at Cryogenic Temperature on Martensitic Transformation of SUS316

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