Influence of Tungsten and Tantalum Content on Evolution of Secondary Phases in 9Cr RAFM Steels: An Experimental and Comp
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I.
INTRODUCTION
REDUCED activation ferritic martensitic steels are obtained by the replacement of Mo and Nb by W and Ta, respectively in modified 9Cr-1Mo steel. These steels were developed for nuclear applications in order to reduce the environmental effects during the disposal of irradiated material waste.[1–4] These steels are candidate structural materials for the Test Blanket Module of ITER due to their excellent resistance to radiation-induced swelling, He embrittlement, and improved high temperature mechanical properties[5–9] in the temperature range of 823 K to 873 K (550 C to 600 C).[1–9,11] The physical properties of RAFM steels are similar to that of conventional 9Cr-1Mo ferritic steels. However, due to differences in the chemical composition, the mechanical properties of RAFM steels are superior,[10] which provides an advantage for use at higher operating temperatures. Hence, it is necessary to understand the role of alloying elements, especially W and Ta on microstructural evolution during long-term exposure at temperatures exceeding 873 K (600 C). The present RAVIKIRANA is with the Homi Bhabha National Institute, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India, and also with the Department of Physics and Nanotechnology, SRM University, Kattankolattur 603203, India. R. MYTHILI and S. SAROJA are with the Metallurgy & Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India. Contact e-mail: [email protected] Manuscript submitted January 16, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
study aims to understand the phase evolution in steels with different W and Ta contents during thermal exposure using computation methods like JMatPro and is compared with the experimental data. An elaborate study of microstructural and microchemical changes in the steel with 1 wt pct W and 0.06 wt pct Ta with aging in the temperature range of 773 K to 923 K (500 C to 650 C) has been made. The influence of varying W and Ta content on the secondary phases like M23C6, MX, and Laves phase has also been studied during accelerated aging at 923 K (650 C). Correlation of the results obtained from computations as a function of W and Ta content with experimental data showed a reasonable agreement between the estimated phase fraction and composition for different aging conditions.
II.
EXPERIMENTAL PROCEDURE
The steels used in the present study were supplied by M/s MIDHANI Hyderabad, India in the form of 12 mm thick plates. Chemical composition of the steels is listed in Table I. The steels were produced through vacuum induction melting followed by arc refining with a strict control over the purity of raw materials. Specimens of dimensions 10 9 10 9 12 mm3 were normalized at 1253 K (980 C) for 30 minutes followed by tempering at 1033 K (760 C) for 1 hour, (N&T) which are used as the starting materials for further experiments. A systematic study on the kinetics of phase evolution as a function of temperature and time was studied in normalized and tempered (N&T) 1W-0.06Ta
Table I.
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