• Proceedings of the Korean Society of Precision Engineering Conference
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• 2010.05a
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• pp.215-216
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• 2010

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2018.06a
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• pp.111-111
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• 2018

ASTM A387-Gr.91 강은 우수한 고온 강도, 크리이프 저항성 그리고 내산화성 등으로 인해 화력 및 바이오매스 발전 시설과 같은 고온 설비의 구조재료로 널리 사용되고 있다. 고온 환경에서 높은 강도는 탄화물과 탄질화물에 의한 석출강화가 주 요인으로 작용한다. 열처리 과정에서 Mo, Cr, Mn, 그리고 Fe는 구-오스테나이트 및 마르텐사이트 라스 입계에 $M_{23}C_6$ 탄화물로 석출되며, V, Nb, 및 N은 조직 내부에 미세한 MX 탄질화물로 석출된다. 따라서 합금의 고온 강도는 조직 내 석출물의 개수밀도와 크기에 크게 의존한다. 그러나 적용 환경의 특성 상 고온 노출에 따른 2차상 석출 및 조대화의 조직열화 현상이 발생하며, 이는 재료의 강도를 저하시킨다. 본 연구에서는 ASTM A387-Gr.91 강의 미세조직 열화에 따른 강도저하 및 파괴 양상을 고찰하는데 그 목적을 두었다. 본 연구에서 사용된 ASTM A387-Gr.91 강의 화학성분(wt, %)은 0.1 C, 0.38 Si, 0.46 Mn, 0.25 Ni, 8.38 Cr, 0.93 Mo, 0.18 V, 0.09 Nb, 그리고 나머지는 Fe 이다. 조직열화 및 기계적 강도저하 특성을 평가하기 위한 등온열화는 $650^{\circ}C$의 대기 환경에서 최대 1000시간동안 실시하였다. 열화된 시험편의 미세조직 및 탄화물에 대한 분석은 SEM과 EDS를 이용하여 실시하였다. 그리고 기계적 강도 평가는 인장실험과 비커스 경도시험을 통해 실시하였다. 또한 열화 시간에 따른 파단양상의 변화를 관찰하기 위해 인장시험편의 파단면을 SEM과 EDS를 이용하여 분석하였다. 그 결과, 열화에 따른 마르텐사이트 라스의 소실, 탄화물의 조대화, 그리고 2차상 석출의 조직 열화현상이 나타났다. 또한 기계적 강도는 조직 열화에 따라 저하되는 경향을 나타냈다.

• Analytical Science and Technology
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• v.7 no.4
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• pp.493-504
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• 1994

The morphology and structure of precipitates of formed in Nb steels were investigated using SEM, TEM and XRD. The quantitative analysis of the precipitates was performed by ICP-AES. The potentiostatic etching method was employed as an extraction method using 10% AA-methanol and 15% Na-citrate electrolytes. The two selected potentials relative to SCE(Standard Calomel Electrode), -100mV in 10% AA-methanol solution and -250mV in 15% Na-citrate solution were found to be effective for the extraction. XRD analysis showed that composition of Nb carbonitride in Nb-steel(0.01% C-0.7% Nb-0.004N) was $NbC_{0.65}N_{0.2}$.

• Transactions of Materials Processing
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• v.15 no.8 s.89
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• pp.581-585
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• 2006

Effects of V on both the formation of Ti-Nb-V carbonitrides and mechanical properties of Ti-Nb bearing low carbon HSLA steels were investigated. Hot rolling process was simulated by using Gleeble 3500 system with the steels containing three different levels of V ($0{\sim}0.1wt.%$). Vanadium precipitated as Ti-Nb-V carbonitrides at austenite region but it did not precipitate as VC during austenite to acicular ferrite or bainitic ferrite phase transformation. As V content increased, the amount of Nb precipitates was decreased but the average size of Ti-Nb-V carbonitrides was increased due to larger diffusivity of V than that of Nb. Coarsened Ti-Nb-V carbonitrides could act as heterogeneous nucleation site during ${\gamma}{\rightarrow}{\alpha}$ phase transformation, thus, acicular ferrite transformation was promoted as V content increased, resulting in increase of upper shelf energy.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2002.04b
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• pp.8-8
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• 2002

서멧이란(Cermet) '세라믹 경질상과 금속 결합상의 복합체"를 의미하나 절삭 골구계에서는 좁은 의미의 "TiC 혹은 Ti(CN)을 바탕을 Ni,Co를 결합상으로 하는 초경재료"를 의미한다. 이런한 서멧은 전략 물질적 성격이 강한 Co,W로 구성된 WC-Co 초경 합금을 대체하기 위해 고안되었는ㄴ데 ㄴ놓은 경도와 고온에서의 화학적 안정성, 낮은 비중과 저렴한 원료 가격등이 장점이나 WC-Co에 비해 상대적으로 낮은 인성이 문제점으로 지적 되어왔다.TiC-Ni 서멧의 낮은 인성을 향상시키기 위해 Ti(CN) 고용체를 바탕으로 하고 제2ㅇ, 제3의 탄화물이나 질화물을 첨가하는 개발 연구가 과거 30여 년간 진행되어왔다. 그러나 많은 연구결과가 조성 개발 위주로 진행되어 왔고, 또 실험간의 불일치와 재현성 결여로 인해 보여진 여러 현상에 대한 보편적인 결론을 내리기 힘든 점이 있었다. 특히, 미세구조와 기ㅔ적 특성에 관한 통합적이고 조직적인 연구가 진행되지 못하였다. 이번 발표에서는 발표자 실험실에서 지난 8 년 간 행하여 온 실험 결과를 바탕으로 탄화물서멧에 나타나는 독특한 현상, 고용체 탄질화물의 열역학적 안정성, 서멧 재료의 유심구조, 표면 에너지 및 strain, 에너지가 미세 구조 형성에 미치는 영향, 그리고 표면층 형성 등 서멧 개발에 관한 중요 관심사에 대해 간단히 살펴보려고 한다.해 간단히 살펴보려고 한다.

• Journal of the Korean Society for Heat Treatment
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• v.21 no.6
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• pp.287-292
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• 2008

Dissolution behavior of complex carbonitrides in a Nb-Ti-V microalloyed steel was quantitatively examined by electrical resistivity, transmission electron microscopy (TEM), and optical microscopy. The electrical resistivity increased with solution treatment temperature up to $1250^{\circ}C$ for a holding time of 15 min. But, an increasing rate of electrical resistivity with temperature was obviously decreased above $1150^{\circ}C$. As the solution treatment temperature increases, irregular shaped Nb-rich carbonitrides disappear and cuboidal Ti-rich carbonitrides are observed. Abnormal grain growth occurs above $1250^{\circ}C$ for a holding time of 15 min. The optimal solution treatment temperature of a Nb-Ti-V microalloyed steel was determined as $1200^{\circ}C$ for a holding time of 15 min.

• Korean Journal of Metals and Materials
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• v.49 no.12
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• pp.917-923
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• 2011

Dissolution and precipitation behaviors of complex carbonitrides in austenite of a V-Nb microalloyed steel were quantitatively examined through electrical resistivity measurement and transmission electron microscopy. The electrical resistivity increased with solution treatment temperature up to $1240^{\circ}C$ and then was saturated at $225n{\Omega}m$ for a holding time of 10 min. The electrical resistivity method was also used to quantitatively measure the isothermal precipitation kinetics of the complex carbonitrides in austenite. Nb-rich precipitates were observed in austenite at the early stages of precipitation, but Nb was replaced by V up to the equilibrium amount within the precipitates with further holding time. The time-temperature-precipitation diagram showed a C-type curve; nose temperature and its incubation time were $900^{\circ}C$ and 100 s, respectively.

• Resources Recycling
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• v.21 no.1
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• pp.75-81
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• 2012

Porous pellets for immobilizing heavy metals were manufactured from coal tailings and iron oxide powder. Coal tailings was pulverized and mixed with iron oxide powder. The mixed powder was granulated into spherical pellets and roasted. Over $1100^{\circ}C$, residual coal in coal tailings reduced iron oxide to ZVI(Zero-Valent Iron). The pellets have 34.63% of porosity, 1.31 g/mL of bulk density, and 9.82.urn median pore diameter. The pellets were reacted with synthetic solutions containing each heavy metals: arsenic(V), copper(II), chrome(VI), and cadmium(II), respectively. On the test of immobilizing heavy metal, the pellets made at $1100^{\circ}C$ were superior to the other pellets made under $1000^{\circ}C$. Immobilizing over 99.9% of 10ppm heavy metal solutions required I hour for arsenic, 2 hours for chrome, and 4 hours for copper. However, immobilizing capacity of cadmium was inferior to that of the other metals and it was decreased in reversely proportion to initial concentration of the solutions.

• Journal of the Korean Society for Heat Treatment
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• v.22 no.5
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• pp.290-297
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• 2009

In case of using Asymmetric Bipolar Pulsed DC (ABPD) power generator, thin film is efficiently deposited as ions are getting higher energy by suppressing target poisoning and electric arc. In this article, the mechanical properties of CNx thin films deposited on the STS 316L were compared with DC and ABPD power generators. The CNx thin films deposited with ABPD clearly improved wear resistance by higher ratio of sp3CN as compared with DC. Nb interlayer affected to increase the value of 10N of adhesion between CNx thin films and substrate. But, CNx thin films deposited with ABPD couldn't endure to wear load and decreased wear resistance as the films were too thinner than substrate. Nevertheless the higher substrate bias energy applied to perform the dense films, it wasn't shown benefits about the wear properties from DC sputtering. But, in case of using ABPD sputtering, the wear resistance was largely improved without changing morphology despite of thin films.

• Journal of the Korean Society for Nondestructive Testing
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• v.27 no.5
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• pp.417-425
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• 2007

The ferritic 9Cr-1Mo-V-Nb heat-resisting steel was experimentally studied in order to characterize its microstructural evolution during creep-fatigue by coercivity measurement. The creep-fatigue test was conducted at $550\;^{\circ}C$ with the tensile holding time of 60s and 600s, respectively. The coercivity decreased until the failure and the hardness monotonously decreased for the whole fatigue life. As the life fraction of creep-fatigue increased, the $M_{23}C_6$ carbide coarsened following the Ostwald ripening mechanism. However, the MX carbonitrides did not grow during creep-fatigue due to so stable at $550\;^{\circ}C$. The width of martensite lath increased because of the dislocation recovery at the lath boundaries. The magnetic coercivity has an influence on the microstructural properties such as dislocation, precipitates and martensite lath boundaries, which interpreted in relation to microstructural changes. Consequently, this study proposes a magnetic coercivity to quantify the level of damage and microstructural change during the creep-fatigue of ferritic 9Cr-1Mo-V-Nb steel.