Microstructural Evolution of a 12Cr-2W-Cu-V-Nd Steel during Three-year Service Exposure

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Microstructural evolution of 12Cr–2W–Cu–V–Nb steel tubes (ASME SA213-T122) after one-year and three-year service exposure tests in a Japanese practical boiler has been investigated from a standpoint of the phase stability of precipitates. The test tubes consist of tempered martensite and δ-ferrite, and the main precipitates are MX-type carbonitride, M<sub>23</sub>C<sub>6</sub> carbide and Laves phase. TEM observations on thin films show that the MX has precipitated in a plate-shaped with a coherent or semi-coherent relationship with the matrix inside grains. An estimation of the lattice misfit between MX and the matrix suggests that the coherent strain has been high enough to enhance the shear stress and then strongly interact with dislocations. Another important point is that morphology and compositions of MX have been stable under the present service conditions, thereby the creep strength as well as tensile strength has kept high after long-term service exposure.<br>The long-term exposure to the present service temperature has enhanced the precipitation of Fe<sub>2</sub>(W, Mo) Laves phase inside grains, resulting in a marked reduction in the dissolved W and Mo in matrix. It is found that the kinetics of W-partitioning between matrix and Laves phase can be successfully expressed by the Johnson–Mehl–Avrami type equation and applied to estimate the actual temperatures of the exposed tubes.<br>It is concluded that the kinetics of Laves phase precipitation and morphology of MX have mainly controlled a microstructural stability in the 12Cr–2W–Cu–V–Nb steel, and also give helpful suggestion to increase the creep resistance during the long-term service exposure.

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  • ISIJ international  

    ISIJ international 40(11), 1156-1163, 2000-11-15 

    The Iron and Steel Institute of Japan

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