| G110alloy is an aging strengthening Fe-Ni base superalloy developed by China Iron and Steel Research Institute for the turbine blades of650℃ultrasupercritical power plants. It is developed on the basis of Nimonic80A with less content of Ni but higher strength at high temperatures. The combination of excellent strength, oxidation resistance as well as corrosion resistance at650℃are highly demanded. Therefore, it is critical to investigate the microstructural evolution and creep rupture life of G110at650℃.The high contents of Cr (22%) and Ni (32-40%) elements provide good oxidation and corrosion resistance for G110alloy. Ni is an austenite-forming element and could stabilize the austenite. The solution strengthening effect of Ni is larger than Fe, due to its good strength and ductility. The content of Ni can significantly affect the tensile strength and creep rupture life of G110alloy by affecting the amount, size, content and strength of y’. Ni is one the forming elements of y’phase, which is the main strengthening phase in G110alloy. Therefore, five heats of G110alloy with varied Ni contents were developed and investigated in detail in this study.The heat treatment parameters are of great importance for a newly developed alloy. For G110alloy with solution and aging heat treatment, the microstructures and mechanical properties with different solution and aging temperatures are studied. The optimal heat treatment parameters were1020℃/lh(WQ)+760℃/10h(AC).With the increase of Ni content, the tensile strength at room temperature and high temperature increase, while the ductility decease. The impact toughness is relatively stable. On the condition of optimal heat treatment, with the increase of Ni content, the types of the precipitates are unchanged, among which the content of MC carbide is relatively stable while the y’precipitates content gradually increases. The tensile strength at high temperature increases with the increase of Ni content. On the contrary, the creep rupture life of G110alloy decreases as the content of the undesired a-Cr phase increases with the content of Ni.Long time aging experiments at650℃were carried out to investigate the microstructural stability and mechanical property change during long time service. The following conclusions have been drawn.(1) a-Cr phase precipitated during long time aging.(2) The content of y’ phase and a-Cr increased with aging time, while the MC carbide was relatively stable.(3) The size of y’precipitates kept small (≤30nm) in the first5000hours aging.(4) The size of the MC carbide is stable during aging.(5) The size of a-Cr phase gradually increased during aging. In the first1000hours, the a-Cr phase was in the form of blocky shape. Later lamellar and rod shaped a-Cr phase could be observed both at grain boundaries and grain interior.As the main strengthening phase in G110alloy, the content of y’precipitates is larger than the combination of all the other precipitates. During long time aging at650℃, the tensile strength increased with the increase of y’precipitates amount in the first1000hours. Afterwards, the increase in the size of γ’precipitate as well as a-Cr phase led to the slow increase of tensile strength. However, as the size of y’precipitate kept relatively small and the amount of y’ precipitate is much larger than that of a-Cr phase, no obvious drop in tensile strength was observed during the whole aging period.The105h creep rupture strength is one of the key properties for the material selection of turbine blades. The105h creep ruptures strength extrapolation of G110alloy is calculated to be190MPa in this study, which is larger than required (l00MPa). But failure analysis, the creep cracks mainly initiated at the grain boundaries of G110alloy. The rapid increase of a-Cr phase led to the coarsening of the grain boundaries. In contrast, the precipitation of needle-shaped a-Cr phase inside the grains did not contribute to the creep crack initiation. |
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