Microstructural stability during creep of Mo- or W-bearing 12Cr steels
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I. INTRODUCTION
THE active development and evaluation of ferritic-martensitic steels with improved creep strength has been underway for some time, in an effort to obtain higher operating temperatures and pressures in fossil power plants.[1,2,3] These steels have a basic composition of 9 to 12 pct chromium, and some advanced variants have additions of molybdenum and tungsten for solid-solution strengthening and of vanadium and niobium to create a fine dispersion of carbides/ nitrides. The as-received microstructure in 12Cr steels is that of tempered martensite, with a high dislocation density, fine subgrain size, and precipitated particles of relatively coarse M23C6 and fine MX. During service, these steels are exposed to stresses at temperatures close to the steam temperature (above 593 ⬚C) for times in excess of 10,000 hours, and changes in the as-received dislocation and particulate microstructure occur progressively throughout this time. In addition, the Laves phase, Fe2(Mo, W), is known to form during service in some variants of 12Cr steel, and there is a concomitant depletion of W and Mo from the solid solution, although difficulties in quantitatively analyzing the precipitation of Laves phase have resulted in a few systematic studies.[4,5] The effects of Laves-phase formation on creep performance have not been clearly established, but one view is that it is the principal factor limiting creep performance in 12Cr alloys.[5] The specific objective of the present work has been to provide experimental data on the evolution with time of YOSHIKUNI KADOYA, Research Engineering Manager, is with Takasago Research and Development Centre, Mitsubishi Heavy Industries, Takasago, 676-8686 Japan. Contact e-mail: [email protected] BRIAN F. DYSON, Visiting Professor, and MALCOLM McLEAN, Professor, are with the Materials Science Department, Imperial College of Science, Technology and Medicine, London SW7 2BP, United Kingdom. Manuscript submitted March 12, 2001 METALLURGICAL AND MATERIALS TRANSACTIONS A
each component of the particulate microstructure in two types of 12Cr steel: a Mo-bearing variant and a W-bearing variant. These data have enabled a thermodynamic and kinetic understanding of the appearance of the various phases in these systems. The consequent particle-evolution equations have been incorporated into a predictive model of creep lifetime which is reported in another article.[6]
II. EXPERIMENTAL PROCEDURES AND CALCULATION METHOD A. Test Materials The materials used were taken from rotors of a Mo-bearing 12Cr steel[2] and also from rotors of a W-bearing 12Cr steel.[3] Tables I and II show the chemical compositions and heat treatments given to the materials studied. The major differences in the chemical compositions of the Mo-bearing 12Cr steel and the W-bearing 12Cr steel are as follows: 1.5 wt pct Mo and 0 wt pct W for the former and 0.3 wt pct Mo and 1.8 wt pct W for the latter. In addition, Table II shows that there are differences in the heat-treatment conditions, quenching temperatures, and temperature
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