Mechanical Properties and Microstructure of Dissimilar Friction Stir Welds of 11Cr-Ferritic/Martensitic Steel to 316 Sta
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INTRODUCTION
THE superior swelling resistance of ferritic/martensitic (F/M) steels makes them ideally suited for use as long-life core materials in fast reactors. The Japan Atomic Energy Agency (JAEA) has developed a F/M steel containing 11 pct Cr, denoted as PNC-FMS, which is intended for use as the fuel subassembly wrapper tubes of a demonstration fast reactor, specifically, the Japan sodium-cooled fast reactor (JSFR). This use of PNC-FMS for core structural applications in the fast reactor does, however, necessitate dissimilar metallurgical joining of this metal to components manufactured from 316-grade stainless steel. Although conventional fusion welding processes, such as tungsten inert gas arc-welding and electron beam welding, have been previously applied to dissimilar welding,[1] in this instance they inevitably result in a problem associated with a reduction of the weld toughness due to the reduction in toughness resulting from the retention of large d-ferrite grains within the martensitic structure of the fusion zone. To mitigate this problem, a post-weld heat-treatment is usually required before use. Because
YUTAKA S. SATO, Associate Professor, HIROYUKI KOKAWA, Professor, and HIROMICHI T. FUJII, Assistant Professor, are with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, 6-6-02 Aramaki-aza-Aoba, Aoba-ku, Sendai 980-8579, Japan. Contact e-mail: ytksato@material. tohoku.ac.jp YASUHIDE YANO, Senior Research Engineer, and YOSHIHIRO SEKIO, Research Engineer, are with Japan Atomic Energy Agency, 4002 Narita-cho, Oarai-machi, Ibaraki 311-1393, Japan. Manuscript submitted January 9, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
this problem primarily arises from melting and solidification, it could conceivably be alleviated by using a solid-state joining process, such as friction-stir welding (FSW).[2] In recent years, FSW has shown considerable promise for joining aluminum alloys, as well as magnesium, copper, titanium, and steel.[3–5] For nuclear applications, some studies[6–11] have examined the FSW of oxide dispersion-strengthened (ODS) ferritic steel, which is one of the most promising cladding tube materials for fast reactors. Typically, fusion welding processes cause an aggregation of oxide particles in the fusion zone, whereas FSW can produce a homogeneous distribution of oxide particles in the stir zone, although some coarsening or slight aggregation of the particles is often observed.[6,7,11] Several papers[12–16] regarding the FSW of high-Cr ferritic stainless steels have also been published, in which, depending on the chemical composition and welding parameters, either a fine ferrite or martensite + ferrite structure was observed in the stir zone. In either instance, an improvement in the mechanical properties over the base material and fusion weld was noted. The authors[17,18] have previously examined the mechanical properties and microstructure of friction-stir-welded PNC-FMS. This work described a stir zone consisting of a fine microstructure of fer
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