Improved Creep Behavior of Ferritic-Martensitic Alloy T91 by Subgrain Boundary Density Enhancement
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INTRODUCTION
THERE is much interest in increasing the application temperature and design stresses of ferritic-martensitic (F-M) 9 to 12 pct Cr steels, such as T91, to increase the efficiency of thermal power plants. These steels are favored for high-temperature applications in boiler and turbine components in power plants, due to their excellent mechanical properties, such as a low coefficient of thermal expansion, high thermal conductivity, and high thermal shock resistance. In spite of its attractive properties, the F-M alloy T91 suffers from oxidation/ corrosion and grain boundary/matrix creep at higher temperatures and unacceptably low toughness at lower temperatures. Application at higher temperatures has been restricted by the limited creep strength. The creep strength of the F-M alloy T91 is controlled by solid solution strengthening (W and Mo in solution); precipitation hardening (carbides and carbonitrides on subgrain boundaries, lath boundaries, packet boundaries, and prior austenite grain boundaries (PAGBs) and in the matrix); and dislocation hardening (subgrain G. GUPTA, formerly Graduate Student, with the Materials Science and Nuclear Engineering Department, the University of Michigan, Ann Arbor, MI, USA, is Process Engineer, Intel. Contact e-mail: [email protected] GARY S. WAS, Professor, is with the Materials Science and Nuclear Engineering Department, the University of Michigan, Ann Arbor, MI, USA. Manuscript submitted July 2, 2007. Article published online December 4, 2007 150—VOLUME 39A, JANUARY 2008
boundaries and mobile dislocations).[1] Creep deformation is controlled at elevated temperatures by the recovery and softening of the tempered martensite structure. In the initial service condition, the microstructure is predominantly characterized by a relatively coarse dispersion of M23C6 and M2X precipitates, together with a much more finely scaled dispersion of M23C6, M2X, MX, M6X, V4C3, etc., precipitates. However, during creep, the large precipitates coarsen, complex variations in the fine dispersion of precipitates occur,[2–5] and the subgrain width is also substantially increased by creep deformation, due to the absorption of excess dislocations.[6] The extent of coarsening increases with temperature and stress.[7,8] The transformed microstructure in these steels consists of elongated subgrains that evolve during creep into coarse, equiaxed subgrains, while the dislocation density decreases.[9] The substructure, consisting of subgrain boundaries and carbides, contributes significantly to the creep strength of T91. Subgrain boundaries are sites at which both precipitates and dislocations are concentrated; they act as strong obstacles to moving dislocation segments during plastic deformation. The subgrain boundaries contribute to creep strength by inducing long-range internal stresses that are inversely proportional to the distance between the subgrain boundaries. An enhanced density of such boundaries should reduce the effective obstacle distance and increase the internal stress, thus reducing th
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