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Study on the Microstructure and Stress Relaxation Behavior of INCONEL Alloy 783

Author: ShenZhi
Tutor: DanAiDang；ShenHongWei
School: Shanghai Jiaotong University
Course: Materials Science
Keywords: Superalloy low-CTE stress relaxation activation volu
CLC: TG132.32
Type: Master's thesis
Year: 2011
Downloads: 89
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Abstract


Low coefficient of thermal expansion (Low CTE) superalloy INCONEL 783 as aerial materials have been made for bolts applied in ultra supercritical steam turbine power set, and even probably in higher service temperature just like above 650oC. However, the effect of these turbine power set on Inconel 783 has few reported about the security. In this thesis, two alloys from homemade and import were selected for evaluating the heating treatment processes through the combination of JMatPro software and experiment data, and also analyzing the high-temperature stress relaxation behavior and microstructure evolution. The results were following:Solution temperature of standard heat treatment was reasonable for nominal composition of alloy 783, but lower for real alloys with high content Nb, Al and C addition.According to thermodynamic calculation, alloy 783 was sensitive to precipitation ofηphase. Lower 650oC was suggested in order to prevent theηprecipitation. In addition, Cr, Fe additions benefit for stabilizingγ′phase, and theηphase is suppressed by the reduction of Co, Nb.The aging ofβphases is not only favorable to the increase of oxidation resistance but also useful to slow upγ’phase precipitation and control theηphase formation.Compared to the homemade alloy, the number ofγ′precipitation in imported alloy matrix increase and lattice misfit becomes larger.γ′precipitation started in the step ofβaging, and generated positiveγ/γ′lattice misfit. The lattice misfit increases with ageing, but negative lattice misfit was found inγ’second aging. Stress relaxation curves of alloy 783 at 575oC~700oC exhibits two stages including the quick drop and near steady-state. Short time relaxation indicates this alloy can be used at 600oC and below, while longer term exposure is needed to assess the reliability at 625oC.Aging process can influence relaxation performance. The reverse and long term exposure aging lead to the weaker relaxation resistance. The solution temperature and solution or not have not caused the change in the relaxation characteristics at 600oC, but significantly increase at higher temperature.The homemade alloy shows better performance than the imported alloy for 1000h relaxation at 600oC. And at 650oC, it is slight inferior than the imported alloy undergoing 1160oC solid-solution, but better than that with standard heat-treatmentTwo alloys at first stage relaxation reveal the extremely high stress indicies, reducing significantly with the increase of time and temperature. Activation volume ranges from 50 to 80 b3, which increase and then reduce with temperature, reduce straightly with time.The aging process has a large influence on activation volume, but no obvious change was found from second solid-solution and temperature.The Heusler precipitate fromβphases under heat treatment, long-term aging and stress relaxation. These phases are strongly depending on temperature and time but not show effect with relaxation performance.The stress relaxation of alloy 783 is assisted by thermal-activation, which related toγ′particle coarsening and dislocation recovery, and controlled by dislocation cross-slipping and climbing. At the initial stage, cross-slipping may occupy the predominant position; climbing will take control gradually as creep continues.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > The alloy learn with a variety of properties of alloys > Special physical properties of alloys > Special thermal properties of alloys > Melting point alloys , refractory alloy , high temperature alloys
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