• Corrosion Science and Technology
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• 제7권2호
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• pp.119-124
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• 2008

Steel is generally not corrosion resistant to water with formation of non protective rusts on its surface. Rusts are composed of iron oxides such as $Fe_3O_4$, $\alpha-$, $\beta-$, $\gamma-$and ${\delta}-FeOOH$. However, steel, particularly weathering steel containing small amounts of Cu, Ni and Cr etc., shows good corrosion resistance against rural, industrial or marine environment. Its corrosion rate is exceedingly small as compared with that of carbon steel. According to the exposure test results undertaken in outdoor environments, the atmospheric corrosion rate for weathering steel is only 1 mm for a century. Atmospheric corrosion for steels proceeds under alternate dry and wet conditions. Dry condition is encountered on steel surface on fine or cloudy days, and wet condition is on rainy or snowy days. The reason why weathering steel shows superior atmospheric corrosion resistance is due to formation of corrosion protective rusts on its surface under very thin water layer. The protective rusts are usually composed of two layer rusts; the upper layer is ${\gamma}-FeOOH$ termed as lepidocrocite, and inner layer is nano-particle ${\alpha}-FeOOH$ termed as goethite. This paper is aimed at elucidating the atmospheric corrosion mechanism for steel in comparison with corrosion in bulky water environment by use of empirical data.The summary is as follows: 1. No corrosion protective rusts are formed on steel in bulky water. 2. Atmospheric corrosion for steel is the corrosion under wetting and drying conditions. Corrosion and passivation occur alternately on steel surface. Steel, particularly weathering steel with small amounts of alloying elements such as Cu, Ni and Cr etc. enhances forming corrosion protective rusts by passivation.

• 오토저널
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• 제4권3호
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• pp.18-23
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• 1982

최근 자동차의 고속경향화 추세는 많은 진전을 보고 있으며 또 앞으로도 많은 연구와 노력이 경주되어야 하리라고 본다. 그 가운데 하나로 스프링의 경량화는 대략 다음과 같은 3방향에서 연구되고 있다. 1) 복합재료, 즉 GFRP(carbon fiber reinforced plastics), 나아가서는 BFRP(born fiber reinforced plastics)의 탄선과 경량성을 이용한 스프링. 2) 형상, 즉 구조적인 면에서 스프링소재를 후판으로 사용함으로서, 다매물에서 단매 또는 소수 매의 거물곡선 스피링(parabolic taper spring)으로의 변진. 3) 고응력 사용, 즉 종래의 스프링량보다 높은 탄성을 가진 재료로써 스프링 응력을 톺게 사용할 수 있는 강종의 개발 등이다. 특히 3)의 탄성한을 \ulcorner이는 문제는 2)의 후재화와 더불어 열처리성, 가공성 등이 문제가 되어 이에 따라 신강종으로서의 변천을 강요하게 된다. 그러므로 국제경쟁력을 강화하기 위하여 각국 들이 활발하게 연구 및 개발을 추진하고 있는 현상이다. 우리나라의 경우 현재 제작기술은 어느 정동 많이 축적되고 있으나, 아직 R&D면에서는 점진적으로 추진되고 있다. 이와 같은 실정에서 자동차산업의 국제화와 더불어 종래 사용된 스프링강재의 재검토가 우리나라에서도 긴급히 필 요하게 되었따. 그 예로서 KS SPS5(JIS SUP9)의 강종이 종래 일본에서 자동차용스프링의 대 종을 이루고 있던것이 최근에는 SUP9A로 전환되고 있다. 이것은 SUP9A는 SUP9보다 기계적 성질이 우수하며, 또한 자동차분야의 세계제일위인 미국의 SAE5160재와 일지되고 있다. 그러므로 국제화 경향과 더불어 SUP9종은 SUP9A로 전환되고 있으므로서 우리 나라의 경우에도 자동차 스프링의 수출용은물론이고, 국내용에도 SUP9A에 해당하는 규격이 제정되고, 또한 빠른 전환이 필요하다고 본다. 이와 같은 국제적인 추세는 SUP6을 SUP7H종으로 더욱 향상된 것이 실용 화되고 있다. 아래에서 이에 대한 기계적 특성을 중심으로 검토키로 한다.

• 한국재료학회지
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• 제22권1호
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• pp.16-23
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• 2012

Research into the development of high strength (1 GPa) and superior formability, such as total elongation (10%), and stretch-flangeability (50%) in hot-rolled steel was conducted with a thermomechanically controlled hot-rolling process. To improve the overall mechanical properties simultaneously, low-carbon steel using precipitation hardening of Ti-Nb-V multimicroalloying elements was employed. And, ideal microstructural characteristics for the realization of balanced mechanical properties were determined using SEM, EBSD, and TEM analyses. The developed steel, 0.06C-2.0Mn-0.5Cr-0.2(Ti + Nb + V), consisted of ferrite as the matrix phase and second phase of granular bainite with fine carbides (20-50 nm) in both phases. The significant factor of the microstructural characteristics that affect stretch-flangeability was found to be the microstructural homogeneity. The microstructural homogeneity, manifest in such characteristics as low localization of plastic strain and internally stored energy, was identified by grain average misorientation method, analyzed by electron backscattered diffraction (EBSD) and hardness deviation between the phases. In summar, a hot-rolled steel having a composition 0.06C-2.0Mn-0.5Cr-0.2(Ti + Nb + V) demonstrated a tensile strength of 998 MPa, a total elongation of 19%, and a hole expansion ratio of 65%. The most important factors to satisfy the mechanical property were the presence of fine carbides and the microstructural homogeneity, which provided low hardness deviation between the phases.

• 대한기계학회논문집
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• 제12권3호
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• pp.505-512
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• 1988

본 연구에서는 상술한 $\boxDr$한계정유길이$\boxUl$를 찾아내는 것을 주요수단으로 하여, 그 전파 하한계응력으로서의 피로한도와의 관계를 정량적으로 검토하고, 미소크랙전파 거동이 미시조직 또는 피로부하이력등에 민감함을 감안하여, 이들 인자가 미소크랙 전 파의 하한계조건에 미치는 영향에 관하여 검토하기로 하였다.

• 대한기계학회논문집A
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• 제30권8호
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• pp.965-975
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• 2006

High pressure turbine blades are one of the key components in fossil power plants operated at high temperature. The blade is usually made of 12Cr steel and its operating temperature is above $500^{\circ}C$. Long term service at this temperature causes material degradation accompanied by changes in microstructures and mechanical properties such as strength and toughness. Quantitative assessment of reduction of strength and toughness due to high temperature material degradation is required for residual life assessment of the blade components. Nondestructive technique is preferred. So far most of the research of this kind was conducted with low alloy steels such as carbon steel, 1.25Cr0.5Mo steel or 2.25Cr1Mo steel. High alloy steel was not investigated. In this study one of the high Cr steel, 12Cr steel, was selected for high temperature material degradation. Electrochemical polarization method was employed to measure degradation. Strength reduction of the 12Cr steel was represented by hardness and toughness reduction was represented by change of transition temperature, FATT. Empirical relationships between the electrochemical polarization parameter and significance of material degradation were established. These relationship can be used for assessing the strength and toughness on the aged high pressure blade components indirectly by using the electrochemical method.

• 대한기계학회논문집
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• 제17권6호
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• pp.1529-1540
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• 1993

본 연구에서는 금속 열처리를 위한 고온면의 막비등 급냉각에 관한 1차적 연구로서 과냉과도 비등곡선의 정확한 형상과 냉각제의 냉각조건이 강재의 과냉과도 비등열전달에 미치는 영향, $A_1$변태점 부근의 $A_1$냉각속도와 상변태열량광의 관계, 그리고 상변태열이 냉각곡선에 미치는 영향 등을 규명하고자 한다.

• 열처리공학회지
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• 제16권5호
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• pp.253-259
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• 2003

In this study, to find out the reason of deterioration in surface roughness of steel and cast iron after Tufftride, it has been investigated on the relation between the surface roughness and various factors such as the evolution of compound layer, surface morphology, and surface hardness, and change of pore ratio in the compound layer during Tufftride at $580^{\circ}C$. It is found that the surface roughness was increased with the evolution of compound layer during Tufftride of steel and cast iron. The change of surface roughness after Tufftride was reduced with decreasing tho content of carbon and cementite ($Fe_3C$) in the materials. in the cast irons, the various shaped graphites that was exposed to the surface should induce the discontinuous growth of the compound layer, and this resulted in the incoherent interfaces between matrix and compound layer and the deterioration of surface roughness. In the steels, the existence of cementites in the matrix resulted in the incoherent interfaces between matrix and compound layer. It is considered that during Tufftride the surface roughness must be mainly influenced by the formation of the incoherent interface between compound layer and matrix that is affected by some factors such as the microstructure, the composition, and the hardness of the matrix.

• 한국결정성장학회지
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• 제16권6호
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• pp.278-285
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• 2006

발전소의 탈황설비에 적용하기 위한 B-H 코팅조성물을 탄소강에 코팅하여 $65^{\circ}C{\sim}80^{\circ}C$에서 경화시켰다. 이들 시편을 $100^{\circ}C$와 $120^{\circ}C$의 70 wt% $H_2SO_4$ 용액에 접촉시킨 후 코팅조성물의 조성, 비이커스 경도, SEM에 의한 미세구조 등을 분석하였다. 황산에 대한 내산성은 내산성 시험후 코팅 조성물의 $SO_3$양과 관련이 있었다. 경화온도가 높을수록, 경화시간이 짧을수록 내산성이 좋았으나 고온일 경우에는 균열이 더 많이 발생하여 내산성을 떨어뜨렸다 B-H 코팅 조성물의 고온 내산성을 향상시키기 위한 조건은 경화온도 $65^{\circ}C$이상, 경화시간 12시간 이상이었다.

• 한국기계가공학회지
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• 제16권5호
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• pp.63-68
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• 2017

This study determines the selection of an appropriate jig material for the blade edge of the medical sapphire knife. The physical properties of the jig material affects the edge shape and chipping phenomenon in machining of the medical sapphire knife. If a grinding wheel is used, brittle workpieces such as sapphire are easily damaged by the propagation of cracks because the grinding force significantly increases. It is important to constantly maintain the grinding force in the grinding process of the brittle materials. The grinding force can be kept constantly by inducing the elastic deformation of the Jig material because the elastic deformation of brittle work-piece is negligibly low. The chipping phenomenon may be reduced by selecting the proper Jig material. Aluminum, copper, stainless steels and carbon steel were used as Jig materials. The experiment was conducted using a cast iron grinding wheel, which was installed on a conventional grinding machine with the ELID grinding system. The thickness and width of the chipping area were measured using an optical microscope and FE-SEM to analyze the shape of the blade edge. According to the experiment result, the chipping phenomenon decreased, and the sharp edge was formed when the jig materials with low elastic modulus were used.

• 대한기계학회논문집
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• 제12권6호
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• pp.1327-1334
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• 1988

본 연구에서는 회전굽힘응력하에서 비관통노치를 갖는 재료의 피로거동을 분 석하기 위하여 우선 시험편의 반지름에 해당한 심공노치를 갖는 시험편을 준비하고 이 시험편이 고, 중간, 저응력을 받는경우 작용응력의 대소에 의한 표면 및 내부크랙전파 특성과 함께 크랙면형상변화 관계를 검토하였다.