Microstructural Evolution of Boron Nitride Particles in Advanced 9Cr Power Plant Steels
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INTRODUCTION
VARIOUS types of heat-resistant steels, such as Grade 91 and 92, have been developed in recent years driven by the need to reduce fossil fuel consumption and hence CO2 emission during power generation, through an increase in the thermal efficiency of steam power plant by elevating the allowable steam pressure and temperature.[1,2] The recently developed ferritic 9Cr-3W-3Co-VNb steels with controlled additions of B and N have shown superior creep resistance at elevated temperatures compared with other conventional 9 wt pct Cr steels and are considered to be potential candidates in future for 923 K (650 C) applications.[3–6] The microstructure of the 9Cr-3W-3Co-V-Nb steels after the preservice heat treatment is generally similar to that of conventional 9 to 12 Cr steels, which typically exhibit a tempered martensite matrix with a fine dispersion of secondary phase particles.[3,7–9] Owing to the high normalizing temperature used with the 9Cr-3W-3Co-V-Nb steels, the prior austenite grain size of these steels after heat treatment is much larger compared with conventional 9 to 12 Cr steel. The large grain size is believed to be beneficial to the creep resistance of weld joints.[4] In addition to the large grain size, two of the important alloy design concepts behind LETIAN LI and RYAN MacLACHLAN, Ph.D. Students, MARK A.E. JEPSON, Lecturer, and RACHEL THOMSON, Professor, are with the Department of Materials, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK. Contact e-mail: [email protected] Manuscript submitted January 13, 2012. Article published online February 8, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
the ferritic 9Cr-3W-3Co-V-Nb steels are (1) addition of B to stabilize both Cr-rich carbides (M23C6) against coarsening and grain boundaries during creep[10,11]; and (2) carefully controlling the addition of N in such a way to produce a fine dispersion of MX (where M is V or Nb, and X is N or C) particles thus strengthening the martensitic microstructure.[10–12] The levels of B and N need to be carefully optimized to avoid the formation of boron nitrides (BN), which, if formed, can offset the beneficial effects induced by B and N additions simultaneously. In this study, BN particles were observed to be present in two experimental 9Cr-3W-3Co-V-Nb-type steels after a standard normalizing heat treatment. Previous studies carried out by Abe and co-workers[10,13] have shown that BN can be avoided by carefully controlling the B and N levels in the steel-making process. However, this approach normally requires the suppression of N to a level of ~100 ppm or lower, which can be difficult for large-scale air castings. Therefore, in the current study, attempts have been made to both prevent the precipitation of BN and to also dissolve the BN present after normalizing in two steels which contain higher N than the suggested maximum[13] by altering the heat-treatment conditions. Thermodynamic calculations were first employed to assist in designing the new heattreatment conditions. Then, a series of
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