Improving the creep properties of 9Cr-3W-3Co-NbV steels and their weld joints by the addition of boron
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I. INTRODUCTION
NEW ferritic steels that can be used in ultrasupercritical fossil power plants operating at temperatures above 923 K are being developed worldwide.[1,2,3] One of the techniques employed for the development of the new steels is the controlled addition of boron, and new steels containing boron are being tested in different countries.[4–8] Results from these studies have shown that the creep properties of the newly developed boron-containing ferritic steels are significantly superior to those of the more conventional steels like the P91, P92, and P122 steels. The distribution of boron in these steels has been examined using -autoradiography, which has shown that boron preferentially segregates along the prior-austenite and lath boundaries.[9,10] The boron content in the precipitates has been examined using auger electron spectroscopy (AES),[9] field-emission–AES (FE-AES),[6] and atom-probe–field-ion microscopy,[5,11] which showed the partial replacement of the carbon by boron in M23C6. Transmission electron microscopy studies on the boron-containing steel have shown that the presence of boron in the carbides retards coarsenSHAJU K. ALBERT, Scientific Officer “G,” is with the Materials Joining Section, Materials Technology Division, IGCAR, Kalpakkam 603 102 Tamil Nadu, India. Contact e-mail: [email protected] MASAYUKI KONDO, STX Researcher, Welding Metallurgy Group, MASAAKI TABUCHI, Group Leader, High Temperature Materials Group, FUXING YIN, Senior Researcher, Metallurgical Processing Group, KOTA SAWADA, Researcher, Creep Group, and FUJIO ABE, Director, Heat Resistant Design Group, are with the National Institute for Materials Science, Tsukuba 305-0047, Japan. Manuscript submitted April 29, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
ing of these carbides and the recovery of the dislocation substructure.[8] Thus, based on all these results, it is concluded that boron is effective in delaying the coarsening of the particles, which, in turn, delays the dislocation substructure coarsening. This is also supported by the fact that the beneficial effect of boron is noticed more in the creep tests conducted at low stress levels with a long rupture time than in those conducted at high stress levels with a short rupture time.[6] One of the major problems that limit the application of the ferritic steels is type IV cracking in their weld joints.[12,13] This is characterized by fracture in the fine-grained heataffected zone (FGHAZ) of the weld joints with a rupture life much shorter than that obtained for the parent metal. Hence, it is necessary to investigate the resistance of the newly developed boron-containing steels to type IV cracking, if they have to be used in product forms such as plates and pipes for which welding is unavoidable in actual fabrication. However, no studies on the type IV cracking of the boron-containing steels have been reported so far. In the present study, the creep properties of the parent metals and weld joints of newly developed 9Cr-3W-3Co-NbV steels containing different le
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