흑심가단주철의 2상혼합조직에 관한 연구

김동익,김동규,나형용 ( Dong Ik Kim,Dong Kyu Kim,Hyong Yong Ra ) 한국주조공학회 1984 한국주조공학회지 Vol.4 No.3

The transformation plenomena of γ phase which is formed by heating the ferritic malleable cast iron above Al transformation temperature, and the effects of heating temperature on the hardness were investigated. The results are summarized as follows; 1. By heating the ferritic malleable cast iron above 740℃ α phase surrounded the graphite particle transformed into γ phase. The volume fraction of γ phase is increased linearly with holding time in α + γ region and rapidly with increasing the heating temperature. 2. When the specimens are heated between 740℃-860℃, the grain boundary of ferrite prevent the growth of γ phase. Therefore the interface of γ and α are obserbed along the former ferrite grainboundary. But above that heating temperature the tendency of preventation is weaken, so the interface between γ and α phase are formed irregularly. 3. Having same volume fraction of γ phase, the hardness of specimen are increased with increasing the heating temperature.

급속응고된 Fe-C 2 원계 합금의 응고조직 및 열처리 조직에 관한 연구

나형용,김동익 대한금속재료학회(대한금속학회) 1988 대한금속·재료학회지 Vol.26 No.12

The rapidly solidified structures of Fe-C binary alloy containing 3.8-4.5 wt%C made by using the melt spinner and gas atomizer was analyzed and how the solidified structure of it, of which consolidation temperature is expected to be 700-1000 ˚C, changed was considered. As a result of experiment, the solidified structure consist of the eutectic structure of austenite and cementite, the size of which was within sevaral ㎛, and at the last solidified part, the austenite dendrite was observed. In the solidified structures by using the melt spinner, any meta-stable phase was not observed, but in the powder under the 25㎛ solidified by using the gas atomizer, there much observed meta-stable ε-phase. The microhardness of it showed the initial value of 550-900MVH(㎏/㎟) and even though after annealing. The high value of 700-800MVH(㎏/㎟) was also measured. So, it is possible that these metarials have practical use.

급속응고한 Fe-3.7wt% C-Cr 계 합금분말 압출재의 미세조직과 기계적 성질

나형용,김동익 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.12

The purpose of this work was to develop an engineering materials having a high volume fraction of carbide. For this purpose, rapidly solidified Fe-3.7wt%C-(1∼4)wt%Cr alloy powder was made using high pressure gas atomizer and followed by hot extrusion. In this study, the microstructures and mechanical properties of extruded bars were discussed. The microstructures of extruded bars were found to be the mixture of carbide and fine pearlite. The average carbide particle size was about 2㎛ and the pearlite lamellar spacing was about 0.1㎛. The tensile strength increased with increasing extrusion ratio and reached 85∼90㎏/㎟ when extrusion rate was 13. Toughness was 4∼5㎏/㎟ when extrusion race was 13. Toughness was 4∼5㎏ m/㎠ and hardness was 55∼60 HRc.

급속응고된 Fe-4wt% C-Cr 계 합금분말의 응고조직과 열처리조직에 관한 연구

나형용,김동익 대한금속재료학회(대한금속학회) 1989 대한금속·재료학회지 Vol.27 No.3

The solidified and annealed structures of gas atomized Fe-4wt%C-Cr alloy powders were investigated. The solidified structures were mixtures of austenitic dendrite and (Fe, Cr)₃C/austenite eutectic. Metastable ε-phase was observed in small particle, -25㎛, which was massive solidification products. When the alloy powders were annealed at 600-900 C, growth of carbide which, original1y eutectic type, grew by ostwald ripening was observed. And the volume fraction of carbides had a tendency to increase due to precipitation of carbon which was supersaturated in austenite to carbide. The carbide grew rapid1y above 900℃. Before annealing, the micro hardness of powders marked 950-1300 Hv andwas increased with increasing of Cr contents. After annealed at 1000℃ for 1hr., the microhardness was 850-1500Hv. And in alloy of 4% Cr, the micro-hardness of that was above 1500 Hv, so it is expected that Fe-C-Cr alloy powders can be used as wear-resistance material.

열교환법에 의한 실리콘 단결정 성장

김성균,나형용,김동익,최회진 대한금속재료학회(대한금속학회) 1996 대한금속·재료학회지 Vol.34 No.2

The effects of cooling rate of heater, melt weight, seed direction, base materials of crucible and purity of silicon were investigated in the experiment of the silicon single-crystal(diameter 7㎝) growth by Heat Exchanger Method in this paper. Silicon single-crystal which purity was electronic-grade could be grown at the cooling rate lower than 0.8℃/min when the seed with $lt;100$gt; direction was used and twins in parallel with the radial direction were formed at the cooling rate higher than 0.2℃/min when the seed with $lt;111$gt; direction was used. But, in any case, some polycrystal formation at the low edge part of the crystal could not been avoided. When the alumina with low thermal conductivity instead of graphite with high thermal conductivity was used as the base material of crucible, the trend of the poly-crystal formation at the low edge part of the crystal could be diminished. Especially, Silicon single-crystal which purity was metallurgical-grade could be grown at the cooling rate lower than 0.2℃/min when the seed with $lt;100$gt; direction was used.

열교환법에 의한 실리콘 단결정 성장과정의 수치해석

김성균,나형용,김동익,최회진 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.11

Numerical simulations including the fluid flow, conduction and radiation heat transfer have been performed using the geometry of real furnace for the single-crystal growth by heat exchanger method. The finite difference method based on the control volume approach and SIMPLE algorithm were used to solve the momentum and energy equations. Almost all parts of the furnace including the heater, insulating materials and crucible were considered in the calculation domain and the latent heat was accounted by an iterative heat evolution method. Silicon with low thermal conductivity was selected as a model material in order to compare the results with the previous report on the copper single-crystal growth. The effects of cooling rate of the heater, crucible material, crucible shape and melt weight on single-crystal growth were investigated together with the role of natural convection in melt. The optimum process conditions such as the critical cooling rate and the critical ratio of the height to the radius of crystal for the silicon single-crystal growth by heat exchanger method were determined. According to the simulations, among many parameters crucible shape was a dominant processing parameter to control single crystal growth when a thermal conductivity of the specimen was low. The lower the thermal conductivity of base material of crucible was then that of the specimen, the less the region of poly crystal formed at the edge of the crystal was.

급속응고에서의 밴드조직

김성균,나형용,김동익,최회진 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.2

The microstructural evolutions during the transition between the supersaturated solid solution and the solutal dendrite in rapid solidification were qualitatively examined using the model of banded structure formation proposed by Carrard et al., It was shown that the positive average temperature gradient at solid-liquid interface is a necessary condition for the banded structure formation, and the banded structure can not be formed during rapid undercooled solidification. Also, it was shown that the banded structure is hardly anticipated in melt-spun aluminum alloy and, on the other hand, it can be formed at the free surface side of the melt-spun Ag-Cu alloy ribbons. These predictions are in good agreement with the reported experimental results on banded structure formation.

열교환법에 의한 구리단결정 성장

김성균,나형용,김동익,최회진 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.5

The effects of cooling velocity, melt weight and purity of copper were reported on copper single-crystal growth by Heat Exchanger Method in this paper. Copper single-crystals could be grown at the cooling velocity lower than 1℃/min. Higher purity(99.995wt%) of copper single crystal less than 7㎝ height(radius 3.5㎝) was grown when the cooling velocity was 0.34℃/min. and the height less than 3.5㎝(radius 3.5㎝) was grown with the plane front of solid-liquid interface and of semi-ellipse, and the free surface of single crystal was solidified in the end. However in the lower purity(99.93wt%) of copper, it should be solidified with plane front at the initial stage, and cell, dendrite formed finally at the cooling velocity lower than 0.5℃/min. The significant segregation were caused when the solid-liquid interface was changed from plane front to cell structure.

두 벽사이에서의 방향성응고시 고-액계면의 형태

김성균,나형용,김동익,최회진 대한금속재료학회(대한금속학회) 1994 대한금속·재료학회지 Vol.32 No.10

The steady state shapes of solid-liquid interface under various conditions, during directional solidification of pure materials between two straight walls, were calculated by numerical analysis. The transition from the interface shapes governed by the interface energy to that governed by the mobility can occur as the pulling rate changes. At a high pulling rate where the interface shapes are governed by the mobility anisotropy, there is a critical mobility anisotropy above which the faceted interface appears. At a high pulling velocity and with a high mobility anisotropy, the faceted interface shapes depend on the wetting angle, ψ between the interface and the wall only at the range of π/4$lt;ψ$lt;5π/4. At the other range of wetting angle the macroscopic interface shape is independent of the wetting angle. The faceted interface can be composed of the planes with high crystallographic indices if the mobility anisotropy, at the high pulling velocity, is slightly larger than the critical value or if the wetting angle, at the high pulling velocity and with the high mobility anisotropy, is at a range of π/4$lt;ψ$lt;π2.

열교환법에 의한 구리단결정 성장과정의 수치해석

김성균,나형용,김동익,최회진 대한금속재료학회(대한금속학회) 1994 대한금속·재료학회지 Vol.32 No.12

Numerical simulation for copper single crystal growth by a heat exchanger method has been performed including the effects of fluid flow, conduction and radiation heat transfer using the actual geometry of growing furnace. The finite difference method based on a control volume approach and SIMPLE algorithm were used to solve the momentum and energy equations. Almost all parts of the furnace including heater, insulating materials and crucible were considered in the calculation domain and the latent heat was also accounted by an iterative heat evolution method. The effects of cooling rate of heater and crucible position and melt weight were investigated together with the role of natural convection in melt. The optimum process conditions for single crystal growth were determined. The calculated results of critical cooling rate, solidification time and the critical ratio of the height to the radius of crystal were found to be in a good agreement with the experimental results.