Fine carbide-strengthened 3Cr-2WVTa bainitic steel
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3/12/04
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Fine Carbide–Strengthened 3Cr-3WVTa Bainitic Steel ZHENG CHEN, ZHI-WEI SHAN, N.Q. WU, V.K. SIKKA, MING-JIAN HUA, and SCOTT X. MAO Both the 3Cr-3WV and the 3Cr-3WVTa steels exhibit an acicular bainite microstructure under the normalized and the normalized-and-tempered condition. The addition of Ta to the 3Cr-3WV steel substantially decreases the prior austenite grain size, but it has little effect on the bainite packet size. Fine TaC precipitates are formed in the normalized 3Cr-3WVTa specimen. After further tempering of 3Cr-3WVTa steel, fine TaC particles are further precipitated and dispersed within grains. The carbides at the prior austenite grain boundaries in the Ta-containing steel are much smaller than those in the steel without Ta. Tensile tests and fracture toughness (KIC) tests have been performed on both the 3Cr-3WV and 3Cr-3WVTa steels at room temperature. The 0.2 pct yield strength of the Ta-containing steel is higher than that of the steel without Ta, especially under the normalized-and-tempered condition. The 3Cr-3WVTa steel is primarily strengthened by a secondary-phase precipitation mechanism represented by the formation of fine carbides after tempering. The 3Cr-3WVTa steel exhibits higher fracture toughness than the 3Cr-3WV steel. The toughening mechanism is also discussed based on the dependence of the calculated fracture stress upon the carbide size and the bainite packet size.
I. INTRODUCTION
REDUCED-ACTIVATION steels have been developed for fusion application since the mid 1980s. Such steels are also referred to as fast induced-radioactivity decay materials.[1,2,3] Alloying elements such as Mo, Ni, Nb, Cu, and N, which produce long-lived radioactive isotopes during irradiation, must be eliminated or minimized in the reduced-activation steels. Recently, tungsten, which exhibits no such degradation under irradiation, was proposed as a replacement for molybdenum, because it was found to behave like molybdenum as a precipitation-strengthening element in low alloy steels.[4] Similarly, it was suggested that the strengthening function of niobium could be replaced by vanadium, titanium, and especially tantalum, which has characteristics in common with niobium.[4] With the replacement of molybdenum by tungsten, and niobium by tantalum, a martensitic 9Cr-2WV(Ta) steel has been developed, which has tensile properties and superior Charpy impact performance as good as those of the 9Cr-1MoVNb steel.[5] Low-chromium bainitic steels have advantage over martensitic 9Cr-2WV(Ta) steels.[6] Since bainitic steels have excellent strength and toughness in the as-quenched or as-normalized condition, it is possible to use such steels without a postweld heat treatment, which is not possible for any martensitic 9Cr steels. This would be a very important consideration for construction of a complicated structure such as a fusion power ZHENG CHEN, Research Associate, Department of Mechanical Engineering, University of Pittsburgh, is Associate Professor, Department of Materials Engineering, Ea
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