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MANUGULA is with the School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad 500046, India and also with the Department of Metallurgical and Materials Engineering, Mahatma Gandhi Institute of Technology, Hyderabad 500075, India. KOTESWARARAO V. RAJULAPATI is with the School of Engineering Sciences and Technology, University of Hyderabad. Contact e-mails: [email protected], kvrse.uoh@ gmail.com G. MADHUSUDHAN REDDY is with the Defence Metallurgical Research Laboratory, Hyderabad 500058, India. R. MYTHILI is with the Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India. K. BHANU SANKARA RAO is with the Mahatma Gandhi Institute of Technology, Hyderabad, 500075, India. Manuscript submitted July 24, 2016.

METALLURGICAL AND MATERIALS TRANSACTIONS A

INTRODUCTION

REDUCED activation ferritic-martensitic steels (RAFM’s) are the primary candidate materials for the test blanket modules (TBMs) to be examined by various countries in the International Thermonuclear Experimental Reactor (ITER) and future fusion power systems.[1] These steels are derivatives of high creep-resistant Mod.9Cr-1Mo steel.[2] However, the Mod.9Cr-1Mo steel is challenged by very high induced radioactivity over a very long period in high energy-high flux environment that prevails in nuclear fusion reactors due to the presence of Mo and Nb in the steel; the alloy also reveals pronounced shift in the ductile-to-brittle

transition temperature after irradiation.[1,3,4] The RAFM steels were designed by altering the chemical composition of the conventional Mod.9Cr-1Mo steel, with the substitution of highly radioactive Mo by W and Nb by Ta in order to achieve rapid decay of induced radioactivity after irradiation in a fusion reactor environment. Further, strict control has also been exercised on the radioactive tramp elements (Mo, Nb, B, Cu, Ni, Al, Co, Ti) and on the elements that promote embrittlement (S, P, As, Sb, Sn, Zr, O). These elements have been restricted to ppm levels. Tantalum promotes the occurrence of very fine MC type of precipitates in the grain interiors and restricts grain growth during normalizing treatments. The international interest in the fusion program resulted in the development of Eurofer-97 (Europe), F82H (Japan), CLAM (China), 9Cr2WVTa (ORNL, USA), and INRAFM (India Specific) steels.[5–14] Although these steels differ to a certain extent from each other with respect to their chemical composition, all of them were recommended for use in the normalized and tempered condition. The manufacturing processes that are being considered for fabrication of TBM channels include fusion welding, hot isostatic pressing, and micromachining. The fusion welding processes explored for fabrication of TBM structure comprise tungsten inert gas welding (TIG), narrow gap-TIG, laser welding, and electron beam welding (EBW). In general, fusion welding processes develop a wider heat-affected zone (HAZ) in ferritic-martensitic steels and generate an inhomogeneous microstructure in the HAZ, r