• Journal of Welding and Joining
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• 제33권6호
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• pp.6-12
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• 2015

Thermal fatigue life of the automobile exhaust manifold is directly affected by the restraint force according to the structure of exhaust system and bead shape of the welded joints. In the present study, the microstructural changes and precipitation behavior during thermal fatigue cycle of the 18wt% Cr ferritic stainless steel weld heat affected zone (HAZ) considering restraint stress were investigated. The simulation of weld HAZ and thermal fatigue test were carried out using a metal thermal cycle simulator under complete constraint force in the static jig. The change of the restraint stress on the weld HAZ was simulated by changing the shape of notch in the specimen considering the stress concentration factor. Thermal fatigue properties of the weld HAZ were deteriorated during cyclic heating and cooling in the temperature range of $200^{\circ}C$ to $900^{\circ}C$ due to the decrease of Nb content in solid solution and coarsening of MX type precipitates, laves phase, $M_6C$ with coarsening of grain and softening of the matrix. As the restraint stress on the specimen increased, the thermal fatigue life was decreased by dynamic precipitation and rapid coarsening of the precipitates.

• 한국자기학회지
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• 제3권1호
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• pp.23-28
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• 1993

복합타게트 방식의 고주파 2극 마그네트론 스퍼터링 장치를 이용하여 Fe-Hf-C계 극미세 결정 연자성 박막을 제조하여 박막조성, 열처리 조건 및 기판과 그 하지층이 자기적 성질에 미치는 영향을 조사하였다. Fe-Hf-C계 초미세 결정 연자성 박막은 Hf 8-10at.%, C 14-18at.%의 조성범위에 서 비정질과 결정질의 경계에 가까운 조성일수록 연자성이 향상되었으며 포화자속밀도 16 kG, 1 MHz 에서의 실효투자율 4000 이상의 연자성을 나타내고 $600^{\circ}C$ 까지도 투자율 3000정도의 열적안정 성을 나타내었다. 기판 및 하지층에 따른 연자성 특성은 미세구조의 변화 보다는 기판/자성막 및 하지층/자성막간의 상호확산에 의해 주로 영향을 받는 것으로 나타났다.

• Journal of Periodontal and Implant Science
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• 제41권5호
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• pp.234-241
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• 2011

Purpose: One of the most frequent complications related to dental implants is peri-implantitis, and the characteristics of implant surfaces are closely related to the progression and resolution of inflammation. Therefore, a technical modality that can effectively detoxify the implant surface without modification to the surface is needed. The purpose of this study was to evaluate the effect of erbium-doped: yttrium, aluminium and garnet (Er:YAG) laser irradiation on the microstructural changes in double acid-etched implant surfaces according to the laser energy and the application duration. Methods: The implant surface was irradiated using an Er:YAG laser with different application energy levels (100 mJ/pulse, 140 mJ/pulse, and 180 mJ/pulse) and time periods (1 minute, 1.5 minutes, and 2 minutes). We then examined the change in surface roughness value and microstructure. Results: In a scanning electron microscopy evaluation, the double acid-etched implant surface was not altered by Er:YAG laser irradiation under the condition of 100 mJ/pulse at 10 Hz for any of the irradiation times. However, we investigated the reduced sharpness of the specific ridge microstructure that resulted under the 140 mJ/pulse and 180 mJ/pulse conditions. The reduction in sharpness became more severe as laser energy and application duration increased. In the roughness measurement, the double acid-etched implants showed a low roughness value on the valley area before the laser irradiation. Under all experimental conditions, Er:YAG laser irradiation led to a minor decrease in surface roughness, which was not statistically significant. Conclusions: The recommended application settings for Er:YAG laser irradiation on double acid-etched implant surface is less than a 100 mJ/pulse at 10 Hz, and for less than two minutes in order to detoxify the implant surface without causing surface modification.

• 한국세라믹학회지
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• 제39권2호
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• pp.199-203
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• 2002

소결 분위기가 금속입자분산 세라믹스기 복합체의 미세 조직 및 물성에 미치는 영향에 대하여 조사하고자 화학적 방법으로 합성한 $Al_2O_3$/Fe-Ni 나노복합분말을 수소 및 아르곤 가스 분위기, 또한 소결온도 등으로 제어하여 열간가압 소결하였다. 수소분위기에서 소결한 복합체는 아르곤분위기의 경우보다 반응상 $FeAl_2O_4$의 형성이 억제되었으며, 증가된 파괴강도 및 인성 값을 나타내었다. 또한, 소결 온도를 낮추었을 경우 기지상 및 금속 분산상의 미세화와 향상된 기계적 성질을 얻을 수 있었다. 소결 조건에 따른 기계적 특성의 변화는 주로 반응상의 형성과 관련된 미세조직 특성에 의존하는 것으로 해석하였다.

• 한국결정성장학회지
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• 제28권2호
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• pp.91-97
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• 2018

Indium hydroxide 분말이 침전법으로 제조되었다. 반응온도($150{\sim}250^{\circ}C$)와 각 반응온도에서의 유지시간(1~72 h)이 실험 변수로 사용되었다. 각각의 제조된 분말을 X-선 회절 분석기(XRD)와 투과전자현미경(FE-TEM), BET를 통해 입자크기, 미세구조 및 결정상을 관찰하였다. 본 연구에서는 침전법에 의해 제조된 Indium hydroxide가 열처리 시 반응온도와 각 반응온도에서의 유지시간에 따라 어떻게 Indium oxide로 상전이 거동을 하는 지를 고찰하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집A
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• pp.105-110
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• 2001

It has recently been raised main issue how solve the problem of insufficient energy. One of the solution is to increase the thermal efficiency of power generation system. For the purpose of high efficiency, it is necessary to increase the steam temperature and pressure. So, the use of modified $9{\sim}12%Cr$ steel having superior creep rupture strength and oxidation resistance is required to endure such severe environment. The evaluation of creep properties of those heat resistance material is very important to secure the reliability of high temperature and pressure structural components. Since creep properties are determined by microstructural change such as carbide precipitation and coarsening, It is certain that there are some relationship between creep properties and hardness affected by microstructure. In this study, SP-Creep ruptured test for newly developed 9Cr steel being used as boiler valve material was performed, and creep properties of the material were evaluated. Also, hardness test were performed and hardness results were related to the creep properties such as LMP and creep strength to verify the availability of SP-Creep test as creep test method.

• 대한기계학회논문집
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• 제8권6호
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• pp.569-575
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• 1984

본 연구에서는 가장 하혹한 부식 환경 분위기에서 널리 사용되는 오오스테나 이트계 스테인레스 강재 용접부에 대해 비등 염화 마그네슘 용액 중에서의 SCC 발생 및 전파특성을 초기 응력강도계수(initial Stress intensity factors) $K_{Ii}$ 값에 대하여 비교, 검토함으로서 모재, 용접열 영향부, 용착금속 및 비이드 형상에 따른 용 접부의 SCC 감수성을 정량적으로 구명하였다. 또한 이상과 같은 각 부분의 SCC 특성 을 용접부의 조직적 및 기계적 특성 변화와 관련시켜 고찰하였다.

• 열처리공학회지
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• 제28권5호
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• pp.229-238
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• 2015

The effects of Sn addition and solution treatment on corrosion behavior were studied in AZ91 magnesium casting alloy. The addition of 5%Sn contributed to the introduction of $Mg_2Sn$ phase, to the reduction in dendritic cell size and to the increase in the amount of secondary phases. After the solution treatment, trace amount of $Al_8Mn_5$ particles were observed in the ${\alpha}$-(Mg) matrix for the AZ91 alloy, while $Mg_2Sn$ phase with high thermal stability was additionally found in the AZ91-5%Sn alloy. Before the solution treatment, the AZ91-5%Sn alloy had better corrosion resistance than the Sn-free alloy, which is caused by the enhanced barrier effect of the (${\beta}+Mg_2Sn$) phases formed more continuously along the dendritic cell boundaries. It is interesting to note that after the solution treatment, the corrosion rate of both alloys became increased, but the Sn-added alloy showed higher corrosion rate than the Sn-free alloy. The microstructural examination on the corroded surfaces revealed that the remaining $Mg_2Sn$ particles in the solution-treated AZ91-5%Sn alloy play a role in accelerating corrosion by galvanic coupling with the ${\alpha}$-(Mg) matrix.

• 열처리공학회지
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• 제26권1호
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• pp.14-20
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• 2013

Duplex stainless steel, which is a kind of stainless steel with a mixed microstructure of about equal proportions of austenite and ferrite, is generally known as a unique material with excellent corrosion resistance and high strength. However, toughness, strength, and corrosion resistance of the steel could be reduced due to precipitation of topologically closed packed phases such as sigma phase during cooling. In case of large forged products, they have strong possibility that ${\sigma}$-phase precipitates due to difference of cooling rate between surface and inner of the products. Investigation on sigma phase precipitation behavior of duplex stainless steel with change of cooling rate was carried out in this study. Forged SAF 2205 duplex stainless steel was used as specimens to examine the cooling rate effect. Dissolution behavior of sigma phase was also discussed through resolution test of duplex stainless steel containing lots of sigma phase. Experimental results revealed that impact energy was very sensitive to precipitation of small amount sigma phase. However, sigma phase could be removed by short term resolution treatment and impact resistance of the duplex stainless steel was restored.

• 폴리머
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• 제38권2호
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• pp.225-231
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• 2014

고분자 결합제로 vinyl acetate(VA) 함량이 각각 30, 60%인 poly(ethylene-co-vinyl acetate)(EVA)를 사용하여 고농축 복합화약 시뮬란트의 유변학적 특성을 연구하였다. 충전제로는 탄산칼슘 및 research department explosive (RDX)와 물리적 특성이 유사한 Dechlorane을 사용하였다. 회분식 용융 혼련기를 사용하여 농축 현탁계를 혼련하였는데 최대 75 v%까지 충전이 가능하였다. 동적 기계적 물성 변화를 측정한 결과 Dechlorane이 탄산칼슘보다 결합제수지와 더 높은 상호작용을 보였다. 일정 전단속도 방식과 일정 전단응력 방식의 평판-평판 레오미터를 사용하여 현탁계의 미세구조의 변화가 유변물성에 미치는 영향을 조사하였고, Krieger-Dougherty 식을 사용하여 최대 충전 부피분율 및 고유점도를 구하였다. EVA31/Dechlorane 현탁계의 최대 충전 부피분율은 약 70 v%이고, 혼련시 전단응력이 약 2000 Pa 정도 부가되는 것이 적절함을 알 수 있었다.