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With the widespread use of ultra-supercritical units, higher requirements have been placed on the high-temperature creep performance of materials. P92 steel, as a mature commercial martensitic steel, has been widely used in main steam pipes, superheaters, and reheaters due to its excellent creep performance. The excellent creep performance of P92 steel benefits from its unique microstructure, including supersaturated solid solution, dislocations, precipitates, and martensite. Martensite has a layered microstructure, containing prior austenite grains, packets, blocks/sub-blocks, and laths, as well as a high density of dislocations. However, during the creep process, the coarsening of precipitates, the formation of new precipitates (Laves phase and Z phase), the coarsening of martensite laths, and martensite recovery can lead to the loss of creep strength. Therefore, it is of great significance to study the microstructure evolution and its effect on creep properties of P92 steel during the creep process.
Methods
Prof. Yong-Hao Lu from the University of Science and Technology Beijing and his team conducted uniaxial creep tests on P92 steel at 650 °C under different stresses (90-125 MPa). The microstructure and creep damage of the creep specimens were characterized using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The evolution of martensite laths, precipitates, and dislocation structures were studied through statistical analysis of EBSD data. In addition, the fracture morphology was also observed to analyze the creep fracture mechanism of P92 steel under different stresses.![]()
Fig. 1. Dimensions of creep specimens and SEM, EBSD, TEM observation positions.Highlights
- Revealed the difference in creep behavior of P92 steel under different stresses and its correlation with microstructure evolution.
- Found that the coarsening of martensite laths is caused by the disappearance of fine martensite packets or blocks, rather than the subgrain boundary migration and annihilation as previously thought.
- Analyzed the creep cavity nucleation and growth of P92 steel under different stresses.
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Fig. 2. (a) Creep strain-time curves under different applied stresses, (b) creep rate-time curves.
This research revealed the microstructure evolution of P92 steel under different stresses at 650 °C, as well as the different creep behaviors of P92 steel under different stress levels. These findings provide insights for in-depth understanding of the creep mechanism of P92 steel.![]()
Fig. 3. TEM and electron channeling contrast images (ECCIs) observed from uniform deformation areas under different applied stresses: (a-b) 125 MPa, (c-d) 100 MPa and (e-f) 90 MPa.
The first author of this work is Shang C.G. from the University of Science and Technology Beijing. Prof. Yong-Hao Lu from the National Center for Materails Service Safety, University of Science and Technology Beijing is the corresponding author of this paper.C.G. Shang, M.L. Wang, Z.C. Zhou, K. Yagi, Y.H. Lu, The microstructure evolution and its effect on creep behaviors in P92 steel under different stresses, Materials Characterization 198 (2023) 112744. https://doi.org/10.1016/j.matchar.2023.112744.
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