Papers by Author: Young Seok Song

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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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