Effect of Tungsten on Primary Creep Deformation and Minimum Creep Rate of Reduced Activation Ferritic-Martensitic Steel
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CREEP resistant ferritic-martensitic steels have been developed and used successfully for the applications in different sectors like fossil-fired power plants, petrochemical industries, and steam generators of fast nuclear reactors. These ferritic-martensitic steels are now under consideration for clad and wrapper applications in the core of fast fission reactor and for blanket module (TBM) in fusion reactor.[1] For TBM application, the chemical composition of conventional grade 91 steel (9Cr-1Mo0.06Nb-0.2V-0.05N) has been modified with the substitution of elements producing high residual radioactive elements in a fusion reaction environment with the elements having low radioactivity such as Mo by W and Nb by Ta. Strict controls over long half-life residual radioactive and embrittling elements have been exercised to facilitate easy handling and disposal of blanket module at the end of service life.[2–4] These steels containing 9 to 12 wt pct chromium, tungsten, and tantalum are commonly classified as the reduced activation ferritic-martensitic (RAFM) steels.[5–9] These steels have tempered martensitic structure and gain their strength from the presence of phase transformation-induced high dislocation density, dispersion of intragranular fine MX type of J. VANAJA, Scientific Officer (D), and KINKAR LAHA, Head, are with the Creep Studies Section, Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Contact e-mail: [email protected] M.D. MATHEW, Head, is with Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam. Manuscript submitted April 7, 2014. Article published online July 23, 2014 5076—VOLUME 45A, OCTOBER 2014
precipitates, and M23C6 precipitates arranged in the lath and grain boundaries. Such complex microstructure resists recovery during creep deformation and exerts high creep deformation resistance and creep rupture strength to the RAFM steel.[10] Design of nuclear reactor components including TBM of fusion reactor is based on accumulation of creep strain over the stipulated time period. The creep curve is described into three distinct regions namely primary, secondary, and tertiary stages. Recovery, recrystallization, and strain-hardening characteristics affect the creep behavior. Alloying the steel significantly affects these factors which in turn govern the creep curve. The mechanisms of strain-hardening and recovery prevail in all the regions of creep deformation except the dominance of one over the other in each region. In many cases, the resistance to recovery and strain hardening cannot be separated since recovery and strain hardening can occur simultaneously. Once a dislocation distribution is established by strain hardening, recovery tends to change the dislocation distribution through rearrangements into a stable structure. So the role of interaction between solute atom, precipitates, and dislocation is to resist the rearrangement of dislocation structure and, hence, the recovery. Bailey–Orowan equation relating the steady-state cre
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