Bainitic chromium-tungsten steels with 3 Pct chromium
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INTRODUCTION
SINCE the late 1970s, ferritic/martensitic steels have been candidate structural materials for the first wall and blanket structures of magnetic fusion power plants. The first steels considered in the U.S. Fusion Reactor Materials Program were Sandvik HT9 (nominally Fe-12Cr-1Mo-0.25V0.5W-0.5Ni-0.2C, here designated 12Cr-1MoVW);[1] modified 9Cr-1Mo (nominally Fe-9Cr-1Mo-0.2V-0.06Nb-0.1C, designated 9Cr-1MoVNb);[2] and 2.25Cr-1Mo (Fe-2.25Cr1Mo-0.1C).[3] All compositions are in weight percent. During the mid 1980s, national fusion programs began to develop ‘‘reduced-activation’’ steels.[4,5,6] Irradiation of a ferritic steel first wall by neutrons from the fusion reaction will activate (transmute to radioactive isotopes) elements of the steel. The induced radioactivity in reduced-activation steels will decay much more rapidly than in conventional steels, thus simplifying the disposal of the radioactive structure after its service lifetime. Such steels also have been referred to as fast induced–radioactivity decay (FIRD) steels.[4] Alloying elements in FIRD or reduced-activation steels that produce long-lived radioactive isotopes during irradiation, such as Mo, Ni, Nb, Cu, and N, must be eliminated or minimized. In FIRD ferritic/martensitic steels developed in the European Union, Japan, and the United States, molybdenum was replaced by tungsten in the conventional Cr-Mo steels to produce Cr-W steels; niobium was replaced by tantalum.[4,5,6] Just as the work on conventional steels emphasized the high-chromium steels 9Cr-1MoVNb and 12Cr-1MoVW,[1,2,3] work on reduced-activation steels has concentrated on 7 to 10 pct Cr steels.[4,5,6] Initial studies at Oak Ridge National Laboratory (ORNL) were on steels with 2.25 to 12 pct R.L. KLUEH and P.J. MAZIASZ, Senior Research Staff Members, and D.J. ALEXANDER, Research Staff Member, are with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376. Manuscript submitted June 20, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
Cr.[7,8,9] Of the original eight ORNL steels examined, an Fe2.25Cr-2W-0.25V-0.1C (2.25Cr-2WV) steel was the strongest,[7] but its Charpy impact properties were inferior to those of an Fe-9Cr-2W-0.25V-0.07Ta-0.1C (9Cr2WVTa) steel.[9] The new martensitic 9Cr-2WVTa steel had comparable tensile properties[8] and superior Charpy impact properties[9] to those of 9Cr-1MoVNb and 12Cr-1MoVW steels. Despite the excellent behavior of the 9Cr-2WVTa, there are advantages for low-chromium bainitic steels.[10] Because bainitic steels often have excellent strength and toughness in the as-quenched or as-normalized condition, it might be possible to use such steels without a postweld heat treatment (PWHT), which is not possible for any of the martensitic 9Cr steels. This would be a very important consideration for the construction of a complicated structure such as a fusion power plant. The ability to use such a steel in the as-normalized condition (without tempering) would also provide economic advantages (cost, time, energy
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