Influence of Alloy Content and Prior Microstructure on Evolution of Secondary Phases in Weldments of 9Cr-Reduced Activat
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HIGH chromium (9 to 12 pct) ferritic/martensitic steels with good thermal conductivity and superior void swelling resistance find application as core structural materials in nuclear fission reactors. The Cr-Mo steels are not suitable for service in high energy-high flux neutron environment, as encountered in nuclear fusion reactors due to the induced radioactivity of constituent elements like Mo and Nb.[1] Hence, a new variant of this class of steel the ‘reduced activation’ ferritic/martensitic steel (RAFM) was developed by (1) substituting W for Mo and Ta for Nb and (2) minimizing the other radio active elements, keeping its mechanical/thermal properties unaltered.[2–4] 9Cr-RAFM steel with good mechanical property, weldability, adequate creep strength [up to 873 K (600 C)], and limited radiological activation in addition to better void swelling resistance was chosen for fabrication of Test Blanket Modules (TBM) in the International Thermo-nuclear Experimental Reactor.[5,6]
V. THOMAS PAUL, Scientific Officer E, is with the Material Synthesis and Structural Characterisation Division, Physical Metallurgy Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603102, Tamil Nadu India. C. SUDHA, Scientific Officer F, and S. SAROJA, Head, are with the Microscopy and Thermo-physical Properties Division, Physical Metallurgy Group, Indira Gandhi Centre for Atomic Research. Contact e-mail: [email protected] Manuscript submitted November 10, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Development of RAFM steels have focused on tailoring the alloy chemistry, especially W and Ta contents to achieve satisfactory mechanical properties and weldability.[7,8] These and similar efforts led to enhanced understanding on the effect of varying W and Ta concentrations on creep rupture strength,[9,10] prior austenite grain (PAG) size refinement,[11] evolution and coarsening of precipitates,[12–14] and weldability.[15] Since welding is an inevitable fabrication process, study on structural and mechanical properties of the welded joints of 9Cr-RAFM steels is considered as a thrust area of research.[16–19] For fabrication of TBM components, fusion welding techniques like electron beam welding, tungsten inert gas (TIG) welding, and laser beam welding are chosen where the extent of heat-affected zone (HAZ) is minimum compared to other conventional methods.[20] However, in fusion welding, the weldment would possess a highly heterogeneous microstructure depending on the distance of a region from the heat source and number of passes employed.[21,22] Since it is the microstructure that decides the response of a weldment to stresses subsequent to post-weld heat treatment or during long-term service exposures[23,24] many experimental studies focused on the change in structure and property of weld and HAZ upon varying the heat input and cooling rates during welding.[25,26] Recently, genetic algorithm-based computation was attempted for optimizing the chemistry of RAFM steels in order to achieve desired volume fraction and coarsening rate of
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