Atom probe field ion microscopy investigation of boron containing martensitic 9 Pct chromium steel
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I. INTRODUCTION
THERE is a growing demand to increase the creep strength and resistance to stress-corrosion cracking of steels used in power plants. In order to satisfy this demand, a number of new, advanced, ferritic-martensitic steels have been developed in the last decade which show promising behavior, mainly improved creep strength compared with the materials used previously, i.e., Fe-2.25 mass pct Cr-1 pct Mo and X20 CrMoV 12-1.[1] Moreover, the tungstencontaining NF616 and HCM 12A steels, which have been recently approved by ASME, have surpassed the creep resistance of X10 CrMoVNb 9-1 (or P/T 91) steel.[2] In the European research program (COST 501), a steel with 1 mass pct Mo and 1 mass pct W was developed and was designated as E911. In addition, a boron-containing variant B2 without tungsten was also developed. Interestingly, this boron-containing steel exhibited improved creep strength, which is at least at the level of the NF616.[3] The boron-containing steel grade is a modification of X10 CrMoVNb 9-1 (or P/T 91) containing 1.5 mass pct Mo by addition of 100 ppm boron. It was developed by Bohler Edelstahl (Kapfenberg, Austria) and produced via the electroslag melting (ESR) method.[4] This steel, which does not contain d-ferrite phase, showed good strength and ductility. The hardenability was found to be good even for large components.[5] Moreover, the creep behavior in the temperature range from 600 8C to 650 8C shows advantages compared with other materials from the group of 9 to 12 pct Cr steels. This difference in creep strength between conventional steels and boron-containing steels has been related to microstructural evolution.[3] The microstructure of the boron-containing steel has been extensively studied by transmission electron microscopy[3] and is summarized as follows. The boron-containing steel is usually received from steel producers after solution annealing and tempering treatment. In the as-received condition, the microstructure is comprised of small ferrite laths, which P. HOFER, formerly Graduate Student, Institute of Materials Science, Welding and Forming, Technical University Graz, is with the Andriz Co., Graz, A-8010 Austria. M.K. MILLER, Senior Research Staff Member, S.S. BABU, Development Staff Member, and S.A. DAVID, Corporate Fellow, are with the Oak Ridge National Laboratory, Oak Ridge, TN 37831. H. CERJAK, Professor, is with the Institute of Materials Science, Welding and Forming, Technical University Graz, A-8010 Graz, Austria. Manuscript submitted August 2, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
were originally martensitic laths. Even after tempering treatment, the ferrite laths have a high dislocation density. In addition, a fine distribution of M23C6 carbides along the ferrite-lath boundaries and within the laths was observed. The M23C6 phase is a stable phase based on Cr23C6 and can take other elements in solution. The size distributions of these carbides are finer in boron-containing steels compared to that of other steels. During thermal and/or creep exposure a
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