Response of Phase Transformation Inducing Heat Treatments on Microstructure and Mechanical Properties of Reduced Activat
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TRODUCTION
THE 9Cr-W-Ta Reduced Activation Ferritic-Martensitic (RAFM) steels are considered as structural material for plasma-facing blanket module of fusion reactor. The blanket module is expected to get expose to the simultaneous mechanical and thermo-mechanical stresses generated by pulsating nature of plasma operation and by flowing coolant and to the intense neutron irradiation-induced damage at high temperatures.[1–3] Ferritic-martensitic steels are considered for blanket module because of their inherent void swelling resistance K.S. CHANDRAVATHI, Scientific Officer F, M. NANDAGOPAL, Scientific Officer E, and M.D. MATHEW, Head, are with the Mechanical Metallurgy Division, IGCAR, Kalpakkam 603 102, India. KINKAR LAHA, Head, is with the Creep Studies Section, Mechanical Metallurgy Division, IGCAR. Contact e-mail: [email protected] C.S. SASMAL, Engineer - SC, E. RAJENDRA KUMAR, Scientist-SF, and H.M. TAILOR, Scientific Assistant-B, are with the Institute of Plasma Research, Gandhi Nagar 382 428, India. P. PARAMESWARAN, Scientific Officer G, is with IGCAR. T. JAYAKUMAR, Director, is with the Metallurgy and Materials Group, IGCAR. Manuscript submitted July 15, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
coupled with lower thermal expansion coefficient and higher thermal conductivity as compared to austenitic steels. The material is also needed to have lower residual radioactivity capability to minimize the environment impact for the disposal of the irradiated steel after service exposure. Hence, it is mandatory to have tight control in ppm levels of the long-lived radio-active transmutation nuclides elements like Mo, Nb, Ni, N, B, Cu, Co, Ti, and also the elements which are promoting irradiation-induced embrittlement like S, P, As, Sb, Sn, Zr, and O.[4,5] These have prompted researchers worldwide to develop RAFM steels with elements W, Ta, Mn, V etc. having relatively short-lived radio-active transmutation nuclides to facilitate easy handling of the disposed blanket module at the end of service.[4,5] The ferritic-martensitic RAFM steels have been developed by replacing highly residual radioactivity-inducing elements Mo and Nb in the traditional creep resistant 9 to 12 wt pct chromium power plant steels with low radioactivity-inducing elements W and Ta, respectively, and by keeping strict control on impurity level in the steel. In most of the internationally developed RAFM steels, the chromium content has been selected at around 9 wt pct
because the fully martensitic structure in the 9Cr steel imparts low DBTT,[6] high creep strength,[6] and lower shift in DBTT on neutron irradiation.[7] The RAFM steels are generally used in the normalized and tempered condition which results in tempered martensitic structure. The tempered martensitic microstructure in RAFM steel consists of laths, blocks, and packets inside the prior-austenite grain. The lath, block, and packet contain high density of dislocations introduced by the martensite transformation and fine intragranular MX type (V, Ta) (C, N) precipitates, whereas chromium-ric
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