Papers by Author: Young Sang Na

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Authors: Kuk Tae Youn, Young Mok Rhyim, Jong Hoon Lee, Young Sang Na, Wee Do Yoo, Chan Gyu Lee
Abstract: For hot die steel, failure is mostly caused by heat checking and melt-out on its surfaces which are in contact with molten metals. In the present research, resistances to melt-out and heat checking of surface modified H13 hot die steels, such as gas nitriding(GN) and micro-blasting(MB), were investigated. The evaluation of melt-out behavior was carried out by measuring the mean depth from the original surface after immersion. To examine the thermal fatigue resistance, a cyclic thermal shock system consisting of induction heating and water spray quenching was constructed. The value of Lm is proposed as the index representing the susceptibility to crack initiation and propagation. The melt-out depth was the lowest for the GN treated surface. MB-GN and MB-GN-MB treated specimens also showed good resistance to melt-out. In the case of GN treatment, while the white layer was completely melted out, the diffusion layer still existed even after immersion for 43 hours. This implies that nitriding significantly reduced the rate of melt-out. From the total crack number, it is seen that crack initiation was reduced to the extent of half after surface treatment. This result means that the residual compressive stress and nitrided layer were beneficial to crack initiation resistance.
Authors: Young Sang Na, Young Mok Rhyim, J.Y. Lee, Jae Ho Lee
Abstract: In order to quantitatively analyze the critical strain for the initiation of dynamic recrystallization in Ni-Fe-based Alloy 718, a series of uniaxial compression tests was conducted in the temperature range 927°C - 1066°C and the strain rate range 5 x 10-4 s-1 - 5 s-1 with varying initial grain size. The critical strains were graphically determined based on one parameter approach and microscopically confirmed. The effect of γ'' (matrix-hardening phase) and δ (grain boundary phase)on the critical strain was simply discussed. The constitutive model for the critical strain of Alloy 718 was constructed using the experimental data obtained from the higher strain rate and the temperature range between 940°C and 1040°C.
Authors: Jung Hoon Yoo, Dae Hwang Yoo, Jung Hwa Seo, Ji Ling Dong, Young Sang Na, Kyung Shik Cho, Jong Hoon Lee, Ulugbek Shaislamov, Jun Mo Yang, Chan Gyu Lee, Kee Sam Shin
Abstract: In Zr-Cu-Ni-Al bulk metallic glasses where there are no dislocations, localized plastic deformation in shear bands occurs largely by the formation and migration of defects such as voids, micropores, shear bands and local variations in composition. Thus, the investigation on defects is critical for the understanding and improvement of plastic deformation in metallic glasses. In this study, microstructures and nano defects in the Zr-Cu-Ni-Al BMGs are characterized by variety of techniques, such as X-ray diffractometry, high resolution transmission electron microscopy, scanning transmission electron microscopy and electron holography.
Authors: Nho Kwang Park, Jong Taek Yeom, Young Sang Na, J.S. Lee, In Ok Shim, S.S. Hong
Authors: Young Mok Rhyim, Sang Ho Han, Young Sang Na, Jong Hoon Lee
Abstract: It is well known that the durability of tool steel could be improved by deep cryogenic treatment. It has been assumed that the increase of service life of tool steel caused by decrease of retained austenite and/or by formation of nano-scale fine η-carbide. But the principles of deep cryogenic treatment remain unclear yet. In this research, to manifest the effect of deep cryogenic treatment on wear resistance, the specimen was emerged in liquid nitrogen for 20 hours for deep cryogenic treatment after austenitizing and the following tempering temperature was varied. The microstructure of specimens was observed using TEM and the mechanical properties and wear resistance were examined. As the tempering temperature increased, the carbides became larger and fine carbides were formed above certain temperature. In the case of deep cryogenic treated specimen, the number of carbides increased while the carbides size was decreased, furthermore, the fine carbide forming temperature was lowered also. It was considered that the deep cryogenic treatment increased the driving force for the nucleation of carbides. As tempering temperature increased, hardness decreased while wear resistance and impact energy increased. The deep cryogenic treated specimens showed this tendency more clearly. It was considered that the wear resistance is affected not only to the hardness but also to the precipitation of fine carbides, and this carbide evolution can be optimized through the deep cryogenic treatment.
Authors: Kuk Tae Youn, Young Sang Na, Jong Hoon Lee, Young Mok Rhyim, Wee Do Yoo, Chan Gyu Lee
Abstract: In order to understand the melt-out mechanism of H13 die casting mould material, microstructural investigation was conducted for the immersion-tested surface in Al-alloy melt. The surface condition of H13 tool steel was modified by varying nitriding conditions and by surface blasting. In particular, long-term behavior of melt-out phenomena during immersion testing of up to 43 hours was focused in this article. It was observed that an ion-nitrided H13 surface showed higher resistance towards melt-out than a gas-nitrided or blasted surface. This was related to the surface layers that are normally formed by nitriding.
Authors: Young Mok Rhyim, Kuk Tae Youn, Young Sang Na, Jong Hoon Lee
Abstract: The Effect of die surface modification on the physiochemical melt-out phenomenon was investigated. To measure the melt-out resistance more accurately, the mean depth measurement method after immersion in molten Al-alloy was proposed instead of the conventional weight change method. The validity of the mean depth method was verified by the comparison with the field service test of core-pin. The several kind of nitriding, such as gas nitriding, ion nitriding, salt-bath nitriding and TNHT process of PHILOS TECHNOLOGIES, INC were employed to change surface condition and their effects on melt out resistance were examined by the immersion test and the field service test of core pins. The melt-out depth was decreased with the nitriding treatments and in case of ion-nitrided specimen showed 2.8 times lower dissolution depth than as heat treated specimen. The white compound layer play a important role in the prevention of physiochemical reaction, therefore, the TNHT process, which does not form the white layer, was less effective in the view point of melt-out phenomenon only. The field test result using core-pins for producing automobile engine parts showed similar tendency with the mean depth method and this implies that the mean depth method reflects the melt-out resistance of the modified surfaces effectively.
Authors: Young Sang Na, S.G. Kang, K.Y. Park, Jong Hoon Lee
Abstract: Micro-forming is considered to be a suited technology to manufacture very small metallic parts (several μm~mm). Zr-based bulk metallic glass, Zr62Cu17Ni13Al8, has been expected to be a promising metallic material for micro-forming process due to their isotropy, low flow stress in a wide supercooled liquid region and good stability of amorphous matrix. Therefore, one can expect that micro-forming of Zr62Cu17Ni13Al8 might be feasible at a relatively low stress in the supercooled liquid state without any crystallization during hot deformation. In this study, micro-formability of Zr62Cu17Ni13Al8 bulk metallic glass was investigated for micro-forging of U-shape pattern. Microformability was estimated by comparing Rf values (=Af/Ag), where Ag is corss-sectional area of U groove, and Af the filled area by material. Micro-forging process was also simulated and analyzed by applying the finite element method. The micro-formability of Zr62Cu17Ni13Al8 was increased with increasing load and time in the temperature range of the supercooled liquid state. In spite of the similar trend in the variations of Rf values, FEM simulation results showed much higher Rf values than the experimental Rf values. This disagreement was analyzed based on the stress overshoot phenomena of bulk metallic glasses in the supercooled liquid region. FEM simulation of the microstamping process was applicable for the optimization of micro-forming process by carefully interpreting the simulation results.
Authors: Hong Yan Liu, Kee Sam Shin, Jung Hoon Yoo, Ji Ling Dong, Quoc Bao Huynh, Hui Yu, Chan Gyu Lee, Young Sang Na, Kyung Shik Cho, Jong Hoon Lee
Abstract: Zr62Cu17Ni13Al8 in the supercooled liquid state is expected to be micro-formable at a relatively low stress. We used X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and quantitative high-resolution TEM (HRTEM) to investigate the microstructures of Zr62Cu17Ni13Al8 amorphous alloy after compression test. The alloy exhibited the homogeneous amorphous microstructure with some crystalline phases dispersed in the matrix. According to the XRD results, under the certain strain rate in the supercooled liquid state, the alloy showed higher crystallization at the higher heat treatment temperature. However, at the same heat treatment temperature, the alloy deformed under low strain rate showed higher crystallization. The β crystalline phase particles with spherical shape were detected by SEM and TEM. The sample with higher strain rate and temperature showed longer shear bands. Nano-voids formed by the coalescence of excess free volume in shear bands were investigated by quantitative HRTEM. Compared with the undeformed area, in the shear band, nanovoids were identified in the deformed area through quantitative HRTEM simulation.
Authors: Young Mok Rhyim, Kuk Tae Youn, Wee Do Yoo, Young Sang Na, J.H. Lee
Abstract: The influence of thermal fatigue test temperature on crack propagation behavior of the surface treated tool steel for die-casting was investigated. For this purpose, thermal fatigue system consisted of induction heating and water spray quenching unit was constructed to simulate the service condition and Lm is proposed as the index representing the susceptibility to crack initiation and propagation. The thermal fatigue tests were conducted at the maximum temperature of 600°C, 700°C and 720°C using as-heat treated or nitrided specimens. The ion nitrided specimen showed lower Lm value than as-heat treated at all test temperature. But in the case of maximum and average crack length, the ion nitrided specimen exhibited higher value than those of as-heat treated specimen.
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