• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.776-780
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• 2002

This paper describes the concept and the characteristics of hyper interfacial bonding developed as a new concept joining process for UFG (ultra-fine grained) steel. Hyper interfacial bonding process is characterized by instantaneous surface melting bonding which involves a series of steps, namely, surface heating by high frequency induction, the rapid removing of heating coil and joining by pressing specimens. UFG steels used in this study have the average grain size of 1.25 ${\mu}{\textrm}{m}$. The surface of specimen can be rapidly heated up and melted within 0.2s. Temperature gradient near heated surface is relatively steep, and peak temperature drastically fell down to about 1100K at the depth of 2~3mm away from the heated surface of specimen. Bainite is observed near bond interface, and also M-A (martensite-austenite) islands are observed in HAZ. Grain size increases with increasing heating power, however, the grain size in bonded zone can be restrained under 11 ${\mu}{\textrm}{m}$. Hardened zone is limited to near bond interface, and the maximum hardness is Hv350~Hv390.

• 열처리공학회지
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• 제4권3호
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• pp.39-46
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• 1991

The effects of austenitizing temperatures and times and cooling rate on the characteristics of continuous air cooling have been investigated for 0.3%C-Mn steels microalloyed with vanadium. Transformation start temperatures have been found to be measured from temperature-time curve directly obtained with continuous air cooling and to decrease with increasing austenitizing temperature, cooling rate and Mn contents. The coarsening behavior of austenite grain size has been measured to abnormally grow at $1050^{\circ}C$ and rapidly grow at $1200^{\circ}C$. It has been found that the volume fraction of pearlite was linealy proportional to the reciprocal square root of austenite grain size. The hardness has been measured to increase with increasing cooling rate up to $250^{\circ}C/min.$ and to remain relatively unchanged in the range of $250{\sim}400^{\circ}C/min.$ showing that hardness valves for steel with a higher Mn content increase more than those for steel with a lower Mn content. The impact property has been found to decrease with increasing of austenite grain size but does not linealy change with the reciprocal square root of austenite grain size.

• 대한기계학회논문집A
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• 제33권7호
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• pp.673-680
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• 2009

In this study, die cooling system using the nitrogen gas has been applied to hot aluminum extrusion process for refining grains and reducing of grain growth. Computational fluid dynamics(CFD) has been carried out to evaluate die cooling effect by nitrogen gas, and the results of CFD have been used to FE-simulation for the prediction of the extrudate temperature in hot extrusion process. Experimental hot extrusion has been performed to observe microstructure and to measure temperature of extrudate. The results of FE-Simulation have been good agreement with those of experiment. Finally, process condition of hot extrusion can be established to reduce grain growth of Al6061 through the experiment.

• 한국전기전자재료학회논문지
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• 제15권1호
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• pp.24-29
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• 2002

In this paper, the microstructure and the dielectric properties of Sr$\_$1-x/CaxTiO$_3$(0$\leq$x$\leq$0.2)-based grain boundary layer ceramics were investigated. The sintering temperature and time were 1420∼152 0$\^{C}$ and 4 hours in N$_2$ gas, respectively. The average grain size and the lattice constant were decreased with increasing content of Ca, but the average grain size was increased with increase of sintering temperature. The second phase foamed by the thermal diffusion of CuO from the surface leads to verb high apparent dielectric constant, $\xi$$\_$r/>50000 and low dielectric loss, tan$\delta$<0.05. X-ray diffraction patterns of Sr$\_$1-x/CaxTiO$_3$ exhibited cubic structure, and the peaks shifted upward and the peak intensity were decreased with x. This is due to the lattice contraction as Sr is replaced by Ca with a smaller ionic radius. The specimens treated thermal diffusion for 2hrs in 1150$\^{C}$ exhibited nonlinear current-voltage characteristic, and its nonlinear coefficient(a) was overt 7.

• 한국전기전자재료학회논문지
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• 제11권4호
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• pp.274-280
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• 1998

Polycrystalline $Sr_2Nb_2O_7$ ceramics with very high Curie temperature were sintered using the powder derived by the chemical coprecipitation method (CCP). The phase evolution and grain-orientation of sintered samples were examined by XRD, while sintering behavior, dielectric properties and polarization were studied by SEM and ferroelectric tester. Extremely high degree of grain-orientation was observed along the (0k0) direction, which resulted in anisotropic dielectric properties of the sintered samples, with the dielectric constant values approaching those for single crystal. Thin film fabrication of $Sr_2Nb_2O_7$ in the pyroniobate family was also attempted on $SiO_2$/Si(100), Pt/$SiO_2$/Si(100), Pt/Ti/$SiO_2$/Si(100) and Pt/$ZrO_2/SiO_2/Si_2(100)$ substrates, using metalorganic decomposition (MOD) process. Neodecanoate precursor solution was prepared by mixing strontium neodecanoate with niobium neodecanoate synthesized from niobium ethoxide. It was found that $Sr_2Nb_2O_7$ single phase appeared in XRD patterns the samples annealed above $950^{\circ}C$. The effect of substrate type on film microstructure and dielectric properties was observed.

• 한국자기학회지
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• 제5권5호
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• pp.716-719
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• 1995

Pd/(Pt/Co/Pt) modulated multilayer films have been deposited on various substrates with Pd/Pt buffer layers. Films grown at different temperatures have very distinct magnetic properties and surface microstructures. Atomic force(AFM) and scanning tunneling (STM) microscopies studies of these films reveal that films deposited at room temperature have small grain structures with an average grain size of about $140\;{\AA}$. However, much larger grains of about $1200\;{\AA}$ in size are observed in the films grown on buffer layers which were deposited at $500^{\circ}C$. These large grains are found to actually consist of smaller grains of about $170{\AA}$ in diameter. SQUID magnetic and Kerr hysteresis loop measurements indicate that multilayer films with large grains exhibit high magnetic coercivities of around 5 kOe. A subgrain growth model is proposed to understand the observed grain structures in the multilayers.

• Journal of Welding and Joining
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• 제13권1호
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• pp.89-102
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• 1995

The evaluation of reheat cracking susceptibility in CrMoV turbine rotor steel was performed using thermally simulated heat affected zones. The examinations were carried out in terms of microstructural characterization, microhardness measurement and a Charpy type notch opening three point bend test. It was found that reheat cracking susceptibility increased as the peak temperature increased. This effect was due to the combined effects of the carbide dissolution and unrestricted grain growth at 1350.deg. C peak temperature. Reheat cracking susceptibility was estimated based on microhardness measurement and prior austenite grain size. It was established that for this particular material, reheat cracking in coarse grained heat affected zone can be eliminated if the microhardness is below about 360DPH and the grain size is below about 30.mu.m. It is evident that reheat cracking susceptibility can be eliminated or reduced by carefully controlling the welding parameters such that a refined structure is produced in the coarse grained heat affected zone.

• 한국재료학회지
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• 제5권2호
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• pp.197-202
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• 1995

저압 화학 기상증착법으로 제작한 비정질 실리콘의 표면 형태 및 결정 성장 과정을 증착조건과 열처리 조건의 변화에 따라 조사하였다. 비정질에서 결정으로 변화하는 전이온도인 570~$590^{\circ}C$에서 증착한 시편은 (311)조직의 거친 표면으로 성장하였다. 같은 증착 온도에서 두께가 두꺼울수록 다결정에서 비정질로 변화하였다. 증착하는 과정에서의 결정화는 기판에서부터 시작되지만, 진공상태를 그대로 유지하고 비정질 실리콘을 전이온도에서 열처리하면 표면 실리콘 원자가 이동하여 결정화하였다.

• 한국재료학회지
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• 제14권11호
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• pp.795-801
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• 2004

The injection molded Fe sintered bodies were fabricated using two kinds of Fe powders haying 50 nm and $3\sim5{\mu}m$ in diameter. In the using of Fe powder having 50 nm in diameter, the comparatively dense bodies ($94\sim97\%$) were obtained even at low sintering temperature ($600\sim700^{\circ}C$), while in the sintered bodies ($1000^{\circ}C$) using $3\sim5{\mu}m$ Fe powder, their relative densities showed low values about $93\%$, although they were strongly depend on the sintering temperature and volume ratio of Fe powder and binder. In the sintered bodies using of 50 nm Fe powders, the volume shrinkage and grain size increased as the sintering temperature increased, but the values of hardness decreased. In the sample sintered at $650^{\circ}C$, the values of relative density, volume shrinkage and grain size were $96\%,\;37\%\;and\;0.97{\mu}n$, respectively and the minimum value of wear depth was obtained due to combination of fine grain and comparatively high density.

• 한국세라믹학회지
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• 제28권4호
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• pp.261-268
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• 1991

The dielectric properties and microstructure of SrTiO3-based grain boundary layer (GBL) capacitor were investigated, and SrTiO3 GBL capacitor was made by penetrating the Frit (PbO-Bi2O3-B2O3 system). The Nb2O5-doped SrTiO3 ceramics were fired for 4-hours, at 145$0^{\circ}C$ in H2-N2 atomsphere to get semiconductive ceramics. The grain size of SrTiO3 sintered at reduction atmosphere had increased as the amount of Nb2O5 increases and then decreased as the amount of Nb2O5 exceeded 0.2 mole%. Insulating reagents which contained PbO-Bi2O3-B2O3 system frit and oxide mixture were printed on the each semiconductive ceramics and fired at varying temperature and for different holding time. The optimum dielectric properties could be obtained by second heat treatment at 110$0^{\circ}C$ for 1 hour, when frit paste was printed. A SrTiO3-based GBLC had the apparent permitivity of about 3.2$\times$104, the dielectric loss of 0.01~0.02 and the stable temperature coefficient of capacitance. The influence of frit paste on dielectric properties was similiar to that of oxide paste but the stability of temperature property of capacitance was improved.