Papers by Author: Jang Hyun Sung

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Authors: Young Chul Park, Jung Ho Kang, Jin Kyung Lee, Jang Hyun Sung, Seok Heum Baek, Young Jik Jo
Abstract: Shape memory alloy (SMA) has been used to improve the tensile strength of composite materials. Because it produces compressive residual stress in the matrix by using its shape memory effect. In order to fabricate a shape memory alloy composite, TiNi alloy and Al2024, Al6061 were used as reinforcing material and matrix, respectively. In this study, TiNi/Al2024 and TiNi/Al6061 shape memory alloy composites were made by the hot press method. The fatigue limit of a shape memory alloy composite determined the volume ratio and reduction ratio. The probabilistic stresslife curve (P-S-N curve) about the shape memory alloy composite makes up using statistical method.
Authors: Dae Kyoung Yoo, Hea Joeng Lee, Chang Yong Kang, Kwang Ho Kim, Yung Hee Kim, Jang Hyun Sung
Abstract: Generally, solution nitriding (nitrogen permeation) is not applied to ferritic stainless steel, which has low nitrogen solubility in the ferrite phase. This study has investigated phase changes, nitride precipitations and hardness variations of Fe-11Cr-0.1Ti (409L) ferritic stainless steel following nitrogen permeation and tempering heat treatments. The strong affinity between nitrogen and Ti enabled the permeation of nitrogen to 409L ferritic stainless steel. The nitrogen-permeated surface changed to a martensitic phase with a hardness range of between 520 and 585Hv, depending on the nitrogen permeation temperature and time, while the surface nitrogen content was about 0.04%~0.05%. When tempering the NPSA (solution annealing after nitrogen permeation) treated specimen at 450 °C, a maximum hardness of 550Hv was obtained, probably due to the precipitation of very fine rod and square type titanium nitrides, while the minimum hardness of 365Hv was obtained at a tempering temperature of 650°C, owing to the precipitation of coarse TiN.
Authors: Jung Wook Kim, Sudeep Abraham, Kwang Ho Kim, Jang Hyun Sung, John J. Moore
Abstract: High-temperature oxidation behaviors of Ti-Al-Si-N and Ti-Al-N films were comparatively investigated in this work. Two kinds of Ti0.75Al0.25N and Ti0.69Al0.23Si0.08N films were deposited on WC-Co substrates by a DC magnetron sputtering method using separate Ti3Al(99.9%) and Si(99.99%) targets in a gaseous mixture of Ar and N2. Si addition of 8 at.% into Ti-Al-N film modified its microstructure to a fine composite comprising, Ti-Al-N crystallites and amorphous Si3N4, and to a smoother surface morphology. While the solid solution Ti0.75Al0.25N film had superior oxidation resistance up to around 700, the composite Ti-Al-Si-N film showed further enhanced oxidation resistance. Both Al2O3 and SiO2 layers played roles as a barrier against oxygen diffusion for the quaternary Ti-Al-Si-N film, whereas only the Al2O3 oxide layer formed at surface did a role for the Ti-Al-N film. Oxidation behavior and mechanical stability of the films after oxidation were compared between two films using instrumental analyses such as XRD, GDOES, XPS, and scratch test.
Authors: Chang Yong Kang, Don Wook Son, Jang Hyun Sung, Ki Woo Nam
Abstract: The damping capacity and strength of Fe-6Al-25/34Mn alloys have been studied for the development of new materials with high strength and damping capacity. Particularly, the effect of α’(including α) and ε martensite phases, which constitute the microstructure of cold rolled Fe-Al-Mn alloys, has been investigated in terms of the strength and damping capacity of the alloys. The damping capacity rises with increasing the degree of cold rolling and reveals the maximum value at 32% reduction. The damping capacity is strongly affected by the volume fraction of ε martensite. The phases such as α’ and austenite(γ)   on damping capacity. Considering that tensile strength increases and elongation decreases with increasing the volume fraction of α’-martensite, it is proved that tensile strength is mainly affected by the amount of α’martensite.
Authors: Jee Hyun Sung, Yun Chul Jung, Yung Hee Kim, Jang Hyun Sung, Chang Yong Kang
Abstract: The microstructural changes of Fe-22%Mn-12%Cr-4%Co-0.3%Ti-x%Si (mass %) damping steels have been studied during solution nitriding at 1100°C. In the solution nitriding free specimens, austenite, ferrite and ε-martensite were formed all over the specimens. However, the original microstructures to form in the solution nitriding free specimens were changed into single austenite phase by the solution nitriding and fine nitride with coherent relation with matrix precipitated in the austenite. With increasing Si contents, the amounts of coherent precipitates and stacking faults decreased and increased, respectively. It has been confirmed that Si hinders the permeation of nitrogen from the surface to the interior of specimen, suppresses the precipitation of nitrides such as TiN and AlN, and reduces the energy for the formation of stacking fault and that solution nitriding is a useful method to improve the surface properties of damping steels.
Authors: Jung Hyun Kong, Jang Hyun Sung, Sang Gweon Kim, Sung Wan Kim
Abstract: Microstructural changes and hardness variations in SKD11 steel have been investigated during the processes of carbide dispersion (CD) carburizing, austenitization, subzero treatment and tempering. The carbon content of the surface region increased up to 3.0% after CD carburizing, and the surface phases consisted of two predominant types of M7C3 carbides (large primary eutectic M7C3 carbide and secondary M7C3 carbide), retained austenite and martensite. After austenitization, the carbon content of the surface region decreased to 2.4%. At the same time, surface hardness was reduced from 900Hv for the CD carburizing condition to 830Hv after austenitization. On the other hand, the hardness at the interior region of the austenitized steel displayed a 100Hv higher value than that of the CD carburizing steel. In spite of removal of the retained austenite, subzero treatment (at -100) of the austenitized steel resulted in a decrease in hardness, probably due to the softening of the martensite matrix. However, tempering (at 200 for 4 hours) of the subzero treated steel raised its hardness up about 70Hv compared to steel tempered without the subzero condition, due to precipitation of fine nano size carbides below 50nm.
Authors: Hea Joeng Lee, Jung Hyun Kong, Dae Kyoung Yoo, Young Chul Park, Jang Hyun Sung
Abstract: This study examined the phase changes, nitride precipitation and variations in hardness of Fe-18Cr-1Mo-0.2Ti (436L) ferritic stainless steels after a nitrogen permeation heat treatment at temperatures ranging from 1050°C to 1150°C. The strong affinity between nitrogen and Ti/Cr enabled the permeation of nitrogen into the 436L ferritic stainless steels. The nitrogen content of the surface layers ranged from 0.40% to 0.87%, depending on the nitrogen permeation temperatures. The nitrogen-permeated surface layers changed into martensite plus retained austenite(RA) with rod type M2N and square type TiN precipitates. Ups and downs of hardness with increasing depth below the surface was observed, depending on the volume fraction of the RA, nitrogen content and quantity of precipitates. The maximum hardness of the nitrogen permeated surface layer was 700Hv.
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