Effect of Tungsten on Long-Term Microstructural Evolution and Impression Creep Behavior of 9Cr Reduced Activation Ferrit
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I.
INTRODUCTION
MICROSTRUCTURAL stability on prolonged thermal exposure is an essential criterion for the 9Cr reduced activation ferritic-martensitic (RAFM) steels, the structural material selected for the test blanket module in the International Thermonuclear Experimental Reactor (ITER). The severe operating conditions impose stringent requirements on properties, especially with respect to embrittlement and degradation of high-temperature strength due to microstructural evolution.[1] Prolonged thermal exposure of the tempered martensite structure leads to continuous evolution of secondary phases and substructure changes in the RAFM steels. Although the principal carbide in this steel is M23C6, small amounts of carbonitrides of V and Ta also form during tempering. Further precipitation, coarsening, and spheroidization of carbides occur during aging or under creep conditions. The changes include coarsening of precipitates, reduction in dislocation density, changes in microchemistry, and formation of new phases. 9Cr-1Mo steels and its variants being a workhorse of petrochemical and
V. THOMAS PAUL, Scientific Officer ‘F’, V.D. VIJAYANAND, Scientific Officer ‘D’, C. SUDHA, Scientific Officer ‘F’, and S. SAROJA, Head, MTPD, Professor, are with the Metallurgy and Materials Group, Homi Bhabha National Institute, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, 603 102, India. Contact e-mail: [email protected] Manuscript submitted April 8, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
energy industries, the above aspects have been studied by several research groups all over the world.[2–4] Addition of tungsten to the steel is beneficial as it produces a more stable microstructure that resists recovery of martensite by stabilizing the initial dislocation substructure. It also enhances the high-temperature strength by increasing the binding force among the atoms in solid solution. Alloying elements such as W and Ta exert a profound influence on the kinetics of different processes like formation, dissolution, and coarsening of precipitates. It is reported that in 9Cr steels, tungsten refines the martensitic lath size and resists its coarsening during creep exposure.[5,6] Additionally, the presence of tungsten in carbides reduces its coarsening kinetics by retarding self-diffusion of iron.[7] The role of W and Ta in refining the prior austenite grain size in 9Cr RAFM has also been reported.[8] Furthermore, W is found to be more effective in resisting the recovery of martensite than Ta. Prolonged exposure to high temperatures produces Laves phase (Fe2W) and its volume fraction depends on W content in the steel. But formation of Laves on long-time service temperatures is known to deteriorate the creep properties.[9,10] In the present study, impression creep (IC) is used to compare the creep properties of RAFM steels with two different tungsten contents of 1 wt pct W and 1.4 wt pct W (termed as INRAFM).[11] Further, the change in property is also corroborated with the microstructural evolution in these steels during prolo
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