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김근준(K. J. Kim),나길환(G. H. Na),하태권(T. K. Ha),정재영(J. Y. Jung) 한국소성가공학회 2010 한국소성가공학회 학술대회 논문집 Vol.2010 No.10
Thermodynamic equilibrium of Fe-36Ni-base Invar alloy containing C contents from 0.25 to 0.38 weight percent during solidification has been investigated in this study. From the thermodynamic simulation using FactSage®, it has been revealed that equilibrium phases which can be formed are two kinds of MC-type precipitates, i.e. (Nb, Mo, V)C and MoC, and M₂C carbide. The MoC carbide was expected to be formed at relatively lower temperature below 770℃. Microstructure observation revealed the coarse precipitates in the as-cast ingots, which was Mo-rich non-equilibrium phase and could be resolved by the successive heat treatment. With increasing C contents from 0.25 to 0.38 wt.%, tensile strength of Invar alloy increased from 950 to 1100㎫ after hot rolling and thermal expansion coefficient showed a trivial increase.
하태권(T. K. Ha),김준호(J. H. Kim),나길환(G. H. Na) 한국소성가공학회 2012 한국소성가공학회 학술대회 논문집 Vol.2012 No.5
본 연구에서는 modified Bridgman 법을 이용하여 순 마그네슘, Mg-1Al, Mg-1Zn-0.5Y, Mg-3Li, 그리고 AZ31 합금 단결정을 성공적으로 제조할 수 있었으며, 제조된 결정에 대하여 방위결정, 압축 및 경도시험을 수행하였다. 단결정의 강도와 경도는 방위에 매우 민감한 것을 확인할 수 있었고, 합금원소의 첨가에 의해서도 강도와 가공경화 거동이 영향을 받는 것을 확인하였다. Single crystals of pure Magnesium, Mg-1Al, Mg-1Zn-0.5Y and Mg-3Li alloy were successfully fabricated in this study by employing the modified Bridgman method. To determine the exact orientation of crystals, Laue back-reflection method and pole figure measurement were carried out on each single crystal. Dimensions of single crystals were 10 ㎜ in diameter and 120 ㎜ in length. Hardness and compression tests were conducted and the results revealed that hardness and the strength strongly depended on the orientation. The closer to basal one the orientation was, the higher hardness and compressive strength were. The effect of alloying was not higher than that of orientation. After compressive deformation of single crystals, the orientation of the crystals was found to rotate and to be parallel to the basal orientation.
정재용(J. Y. Jung),김근준(K. J. Kim),나길환(G. H. Na),하태권(T. K. Ha) 한국소성가공학회 2009 한국소성가공학회 학술대회 논문집 Vol.2009 No.5
Superplastic deformation behavior and texture evolution after equal channel angular pressing (ECAP) of Zn-0.3Al alloy were investigated in this study. ECAP was conducted at temperatures form 40℃ to 160℃ on the plate type specimens of 5 mm thickness and 20 mm width. The specimens obtained by ECAP showed typical texture with basal poles tilted away from the ND toward ED. A series of compression and tensile tests was carried out at temperatures from RT TO 200℃ under the strain rate from 0.03 to 10/s. After ECAP of the Zn-Al alloy, elongation was dramatically increased up to 1000% at above 60℃. The effects of ECAP on the texture and anisotropy in the superplastic deformation bebavior were found to be negligible.
하태권(T. K. Ha),김근준(K. J. Kim),나길환(G. H. Na) 한국소성가공학회 2009 한국소성가공학회 학술대회 논문집 Vol.2009 No.10
In the present study, the effect of initial microstructure, cold reduction ratio, and annealing temperature on the spherodization rate of SK85 high carbon steel sheet was investigated. High carbon steel sheet fabricated by POSCO was soaked at 800℃ for 2 hr in a box furnace and then treated at 570℃ for 5 min in a salt bath furnace followed by water quenching to obtain a fine pearlite structure. Cold rolling was conducted on the sheets of fine pearlite by reduction ratios of 20, 30, and 40% and heat treatment for spheroidization was carried out at 600 and 720℃ for the various time intervals from 0.1 to 32 hrs. Area fraction of spheroidized cementite was measured with an image analyzer as a function of cold reduction ratios and duration times.