Effect of Carbon Reduction on the Toughness of 9CrWVTaN Steels
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INTRODUCTION
COMPARED with austenitic steels, the decreased activation of ferritic/martensitic steels possesses not only good mechanical property and thermal conductivity but also excellent resistance to void swelling. Therefore, they are structural material candidates for future fission reactors. Currently, JLF-1 (among the Japanese Low activation Ferritic steel series), Eurofer 97 (the European reference material), 9Cr-2WVTa (Fe-9Cr-2W-0.25V0.12Ta-0.1C) and CLAM (Chinese Low Activation Martensite) steels are the reduced activation ferritic/ martensitic (RAFM) steels developed in Japan, Europe, United States, and China, respectively.[1] In these steels, both Mo and Nb are removed and replaced by W and Ta to obtain the low activation property. The typical microstructure of these steels is composed of tempered lathy martensite with precipitates dispersed in the matrix. The precipitates are M23C6, where M is mainly Cr with substitution of Fe, and MX, the carbonitride of Ta or V. These precipitates are of great importance to the microstructure stability. However, some recent work[2,3] has shown that with increasing service time, WEI YAN, Associate Professor, WEI WANG, Research Associate, and YIYIN SHAN and KE YANG, Professors, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning 110016, P.R. China. Contact e-mail: [email protected] PING HU, Ph.D. Student, and LIFEN DENG, M.S. Student, are with the Institute of Metal Research, Chinese Academy of Sciences, and with the Graduate School of Chinese Academy of Sciences, Beijing 100049, P.R. China. WEI SHA, Professor, is with the School of Planning, Architecture & Civil Engineering, Queen’s University Belfast, Belfast BT9 5AG, United Kingdom. Manuscript submitted July 30, 2011. Article published online January 5, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
the M23C6 carbide grew too fast to pin the dislocation movement and could not prevent grain boundaries or lath boundaries from migrating, resulting in premature fracture. Seeking thermally stable particles to achieve a highly stable microstructure has always been a goal of heat-resistant steels. Oxides such as fine Y2O3 and YTiO3 have impressive thermal stability and could prevent the microstructure from degradation. Thus, oxide-dispersion strengthened (ODS) steels such as Eurofer 97-ODS and CLAM-ODS steels have been developed.[4–6] However, the microstructures of ODS steels are usually anisotropic because of the manufacture process and their ductile-brittle transition temperatures (DBTT) are very high,[7,8] even though the steels showed better thermal stability.[9,10] Meanwhile, the fabrication involves the complicated and expensive process of powder alloying. It will be difficult to put the ODS steels into industrial-scale practice. Besides oxides, nitrides are also thermally stable. For a metal element, its nitride has a slower growth rate than its carbides under the same condition.[11] Taneike et al.[12] found that M23C6 carbide was suppressed in the 9 pct Cr martensitic steel when
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