Papers by Author: Jeong Tae Kim

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Authors: Young Seok Song, M.R. Lee, Jeong Tae Kim
Abstract: To check long term degradation behavior of hydraulic forged superalloy 718 during exposure at high temperature, an Alloy 718 ingot with a diameter of 400mm was manufactured by the vacuum melting process, VIM followed by VAR. The ingot was broken down for uniform microstructure and mechanical properties by a controlled cogging process using a hydraulic press. To investigate long term degradation behavior of impact properties and hardness, the specimens were exposed to 600oC, 650oC and 700oC for holding times up to 12,112 hours. Impact energy absorption tests were performed at room temperature. The fractured area and the microstructure of the impact specimens were observed by OEM and SEM and Brinell hardness tests were also performed. The changes of impact energy and hardness are remarkably different for each temperature condition. The results suggest that the impact properties and hardness of Ni based superalloy 718 is strongly related to temperature and time during high temperature exposure.
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Authors: Young Seok Song, M.R. Lee, Jeong Tae Kim
Abstract: Alloy 718 ingot with a diameter of 400mm was made by the vacuum melting process ; VIM followed by VAR. Compression tests were conducted on samples collected from columnar zone of the VIM/VAR-processed Alloy 718 ingot in wide temperature and strain rate ranges, i.e. 750~1,200OC and 10~0.001s-1 in order to understand the deformation behavior and evolution of microstructure. Tensile tests at high temperature were also conducted on samples in temperature ranges, i.e. 750~1,100OC. Effects of process parameters on the flow behavior as well as on the microstructure evolution during compression tests at high temperatures are considered. As a result of the deformation simulation, The VIM/VAR ingot was heat-treated for homogenization, and casting structure of the ingot was broken down for uniform microstructures and mechanical properties by controlled cogging process using a hydraulic press. The observation of the microstructure and grain size distribution was carried out to evaluate the effects of optimum process parameters during cogging and mechanical property tests were performed in this study.
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Authors: Kuk Cheol Kim, Byung Hoon Kim, Jin Ik Suk, Dong Soo Kim, Jeong Tae Kim
Abstract: The demand for ultra supercritical (USC) power plants has increased due to the need for high thermal efficiency and reduced CO2 emissions. For turbine materials, high-cycle and lowcycle fatigue life at USC service temperatures are needed to verify material integrity due to the heat-up and cool-down process of power plant operation and due to turbine variations during operation. In this paper, fatigue characteristics for 9~12 Cr steels as candidate USC bucket materials were investigated. First, the fatigue life between DS2B2 and COST B2 steel were compared. COST B2 is the commercial steel with improved high temperature properties by adding boron, and DS2B2 is the new steel developed by Doosan by adding Co and adjusting Mo and W based on the same Mo equivalent value (%wt. Mo + 1/2 %wt. W). DS2B2 steel was found to have longer low cycle fatigue life than COST B2. Second, the effect of boron on fatigue life for bucket materials based on COST B2 steel was investigated. At room temperature, as boron content increased, low cycle fatigue life became superior, whereas, at 593oC the fatigue life was similar. For high cycle fatigue, as boron content increased, fatigue life increased due to the strengthening effect by the addition of boron.
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