Papers by Author: Young Mok Rhyim

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Authors: Kuk Tae Youn, Young Mok Rhyim, Jong Hoon Lee, Chan Gyu Lee, Yun Chul Jung
Abstract: It is well known that the main failure mechanisms in die-casting mould are heat checking due to thermal fatigue and melt-out caused by chemical reaction between die and molten alloys. Thermal fatigue tests were carried out using the thermal cycle simulator to establish the proper method to estimate the thermal fatigue resistance of hot die steel. In this study, the thermal shock tester consisted of induction heating and water spray cooling unit was constructed to evaluate thermal crack propagation resistance and the sum of crack length per unit specimen length, Lm is proposed as the index representing the susceptibility to crack initiation and propagation. Also, new concept of measurement for the melt-out behavior was suggested. AISI H13 hot work tool steel was solution treated and tempered at various temperatures, to control the hardness and toughness that have effect on the behavior of thermal crack propagation. The result of thermal fatigue test showed that there is optimum value of hardness and impact energy to maximize the thermal crack propagation resistance. The influence of nitriding on melt-out resistance was also investigated. The dissolution rate due to melt-out phenomenon tended to be smaller for thicker compound layer. Furthermore, the resistance to melt-out was affected by the compound layer thickness rather than that of diffusion layer. The results of the both tests properly reflect the effect of materials properties on failure modes of die-casting mould and it means those test methods are suitable to evaluate the durability of hot work tool steel for die-casting.
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: 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: Kuk Tae Youn, Young Mok Rhyim, Won Jon Yang, Jong Hoon Lee, Chan Gyu Lee
Abstract: The influence of surface treatment such as nitriding, TNHT(Ti Nano Heat-treatment, PHILOS TECHNOLOGIES, INC.) and PVD coating on the thermal crack propagation behavior of hot work die steel was investigated. To examine the thermal fatigue resistance, the cyclic thermal shock system consisted of induction heating and water spray quenching unit was constructed and Lm is proposed as the index representing the susceptibility to crack initiation and propagation. Thermal stress depending on test temperature was also simulated by FEM. The TNHT specimen showed lower Lm value than as-heat treated specimen but, in the case of maximum and average crack length, the TNHT specimen exhibited higher value than those of as-heat treated specimen. This means that the small number of large cracks were initiated and propagated selectively in the TNHT specimen. This result can be caused by two contrary effects of diffusion layer, those are introduction of the residual compressive stress good for mitigation of thermal tensile stress and very high surface hardness harmful for crack initiation. However, Lm value of salt-bath nitriding specimen was very high due to the white layer in spite of the existence of diffusion layer.
Authors: Myoung Seok Sung, Yoon Bok Lee, Young Seok Kim, Young Mok Rhyim, Jin Chun Kim, In Bae Kim, Yang Do Kim
Abstract: CdSe nanoparticles with average particles size of ~ 3.4 nm were successfully prepared by the sonochemical reduction of a mixed solution of CdCl2, Na2SeSO3 and CH2CH2OH)2NH at room temperature. CdSe nanoparticles characterized by TEM, XPS and XRD analysis revealed the formation of high purity cubic phase CdSe nanoparticles with lattice constant of a=6.415Å. CdSe nanoparticles showed relatively large range of particle size distribution from 20 to 50 nm as the sonication time increased. UV-Vis absorption spectra showed a red-shift in the wavelength region of 545-590 nm as increasing sonication time due to the gradual growth of CdSe nanoparticles in the solution. This paper will mainly present the effects of ultrasonic irradiation on the formation and properties of CdSe nanoparticles.
Authors: Shin Ho Kim, Yoon Bok Lee, Seung Chan Cho, Young Mok Rhyim, Jin Chun Kim, Young Seok Kim, In Bae Kim, Yang Do Kim
Abstract: Cadmium selenide (CdSe) have been prepared by the reduction of mixed solution of cadmium chloride (CdCl2), sodium selenosulfate (Na2SeSO3) and diethanolamine ((CH2CH2OH)2NH) at temperatures of 80-200°C under Ar atmosphere. Characteristics of CdSe nanoparticles were analyzed using transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) and UV-Vis absorption spectra. CdSe samples prepared at relatively low temperature of below 100°C showed cubic structure while samples synthesized at temperature above 100°C showed hexagonal phase. The increased reaction temperature caused the particle size increase as well as the agglomeration of particles to form a hollow sphere. UV-Vis absorption spectra also showed a red-shift as increasing reaction temperatures due to the gradual growth of CdSe nanoparticles in the solution. The relatively sharp absorption band from samples synthesized at 80 and 100°C can be attributed to homogeneous particle size distribution compared with those of prepared at elevated temperatures. This paper will mainly discuss the phase stability of CdSe nanoparticles as well as the hollow sphere formation.
Authors: Wee Do Yoo, Jong Hoon Lee, Kuk Tae Youn, Young Mok Rhyim
Abstract: The microstructures and mechanical properties of 17-4 PH stainless steel at each steps of heat treatment, such as homogenizing, solid solution treatment followed by aging treatment, longterm aging at 400 °C, and recovery treatment, in order to obtain a better understanding of the embrittlement phenomena on aging, was investigated. As the homogenizing treatment time increased, the length of δ-ferrite decreased and elongated shape of δ-ferrite turned to sphere shape with the decrease of volume fraction. The solution treated specimen mainly consists of lath martensite with a small fraction of elongated δ-ferrite. The spherical particles existed a little in the martensite matrix, while no precipitates were found in the δ-ferrite at the solution treated specimen. As the aging treatment temperature increased, the strength decreased while the toughness increased. The fcc Cu-rich particles precipitated in the δ-ferrite during the long-term aging at 400 °C after the solution heat treatment. This precipitation causes the aged hardening after long-term aging accompanied by decreases in elongation and charpy V-notch energy absorption. The strength and elongation was restored after recovery treatment and the fcc-Cu precipitated were almost dissolved into the δ-ferrite matrix.
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