| T92 steel( NF616) was successfully developed by Nippon Steel Corporation in 1980 s through further alloying on the basis of T/P91 steel, increasing the percent of W element to 1.8%, reducing the percent of Mo element to 0.5%, and adding Nb and B elements. Compared with T/P91 steel, the creep rupture strength of T92 steel was dramatically increased and could be used for the high-temperature steam pipe below 625℃. Currently with the excellent performance, T92 steel has become the new ideal generation steel for(ultra) supercritical power plant unit. However, due to the limited operating time of domestic T92 steel and the difference of operation parameters at domestic and abroad, domestic researchers are lack of the long-term actual operational data, and so the research on the microstructure degradation mechanism during long-term high-temperature service is not clear yet, especially on the creep life assessment method. On the basis of the aforementioned key issues, the research on the relationship between microstructure and mechanical property under various creep conditions(stresses and temperatures) of T92 steel was performed, furthermore the creep life was predicted based on the derived Larson-Miller equation.Besides, in this paper, the microstructure, fracture mechanism and mechanical property of T92 steel crept under different conditions were studied by modern material analysis methods, such as SEM, TEM, Nano Indenter Instrument and so on. The results showed the as-received T92 steel exhibited a typical lath martensitic structure with nano-sized precipitates distributed homogeneously. After long-term creep under high temperatures and stresses, the microstructure of T92 steel cannot keep the old fashion and gradually began to degenerate. Such as the recovery of martensitic laths occurred, the second phases in the microstructure clustered and coarsened along the(sub) boundaries of the prior austenite grains and martensitic laths, Laves phase also began to precipitate. There were two types of M23C6 in the T92 steel, namely, rod-shaped and spheroidal, and the spheroidal type was more stable than the rod-shaped type during creep exposure. Laves phase precipitated adhering to the large M23C6 particles along the(sub) grain boundaries. During the long-term creep exposure, the coarsen rate of Laves phase was obviously faster than that of M23C6. What’s more, when the size of Laves phase reached a critical value, it would induce the nucleation of creep cavities, and so the coalescing of cavities caused the fracture and premature failure of materials. Comparing the mean sizes of precipitates in the area subjected to stresses with that in the area not subjected to stresses, the coarsening kinetics of Laves phase was markedly affected by plastic deformation. Besides, the percentage content of main alloy elements in the matrix, such as Cr, W, Mo, was gradually reduced, because the different types of compounds precipitated from the matrix and grew bigger and bigger, which gradually consumed the alloy element in the matrix. All these softening phenomena in the microstructure of T92 steel brought a decline of micro-hardness of the material together.In this study, the nanoindentation experiments were performed to obtain the micro/nano hardness of martensitic matrix of T92 steel, excluding the effects of second phases and grain boundaries in the microstructure. Combining with the microstructure analysis, the degradation mechanism of martensitic matrix was discussed under different stresses. The results showed that the major strengthening factors of martensitic matrix underwent weakening on several levels during the creep exposure at high temperature, including the increasing in the inter-particle spacing due to the coarsening of Laves phase and M23C6 particles, an increase in the width of martensitic lath for its recovery, as well as the loss of the important solid strengthening elements, including Cr, W and Mo in the matrix. The combined effects of these factors together resulted in the degradation of T92 steel performance after long-term service, and declining in the hardness of martensitic matrix macroscopically.Finally, the accelerated aging data of T92 steel under 600℃, 649℃, 700℃ were fitted and the logarithmic curve was established for the high temperature performance. Furthermore, a creep life prediction equation based on Larson-Miller method was established, and the extrapolating results of persistent strength for T92 steel after 10, 000 h is estimated and comparable to the corresponding value of ECCC. |
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