| As the thermal power plant boiler tube serves under high temperature for a long time, sustaining high pressure and corrosion of water vapor at the same time, their property are important for ensuring boiler safe operation.10Cr9MolVNb steel, due to its excellent comprehensive properties, is widely used in high-temperature and high-pressure resistant parts of supercritical and ultra supercritical power generation units, such as main steam pipeline, superheater and reheater, and so on. Therefore, the research on the microstructure and property of T91 martensite steel has guiding significance for reasonable arrangements of the boiler’s running time and life prediction of materials, as well as solving the problems existing in the localization of T91 steel.In this paper, we study the microstructure and property of T91 steel in the process of in-situ tensile. First, the T91 steel is normalized at 1055℃ for 60 min and tempered at 730℃, 765℃ and 800℃ respectively. Then metallurgical microscope is used to analyze the microstructure of T91 steels tempered at different temperatures. And in-situ SEM technique is used to analyze the mechanical properties, the fracture process and the fracture morphology, as well as microstructure changes in the course of tension. TEM is used to analyze the change of microstructure of different tensile stage. According to the principle of alloying of T91 steel, the strengthening mechanism of martensite is studied.The results show that well tempered martensite could be obtained through 765℃ tempering treatment. The higher temperature, the coarser the grain is, and lower temperature leads to tempering partially. The results show that when the tempering temperature increases from 730℃ to 800℃,the tensile strength, the yield strength and the elongation increase first and then decrease, which reach a maximum at 765℃. The sample fractured after necking under the action of tensile stress and the tensile fracture appears to be dimple. The structure of martensite tempered at 730℃ is small and the subgrain structure is not obvious. Lath martensite fragmentation is poor and the dislocation tangle is serious, in the meantime. The lath martensite tempered at 765℃ has fine structure. The subgrain formation is good after high temperature recovery with the fragmentation of lath martensite being well. The dislocation tangle eased and dislocation network gradually formed. After 800℃ tempering, the structure of martensite becomes coarse and the subgrain structure is clearly visible. The dislocation tangle has largely disappeared with obvious dislocation network and carbide arranged as a string of beads in the grain boundary.The influence of temperature and deformation on the microstructure and properties of the specimens was analyzed by comparing the transmission images of the samples before and after tempering. Strengthening and toughening of martensite mainly has the following several reasons:the solid solution strengthening of atoms such as Cr, Mo, interface strengthening and toughening of lath martensite and subgrain, dispersion strengthening of precipitated phases and deposition strengthening of deposit phase, strengthening and toughening of dislocation after tempering, distortion strengthening caused by stretching. Many strengthening mechanism composited together and worked together in actual situation, which reflects the excellent comprehensive property. |
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