• Journal of the Korean Society for Nondestructive Testing
• /
• v.36 no.5
• /
• pp.377-383
• /
• 2016

The effect of long-term aging degradation on magnetic properties of ferritic 11Cr low-carbon steel was investigated. Coercivity and hysteresis loss measured from the hysteresis loops decreased with long-term aging time and showed that the relation was well fitted by a second order exponential function. Vickers hardness also decreased with aging time and resulted in mechanical softening. In addition, the microstructural evolution was observed by the scanning electron microscopy, backscattered electron image and X-ray diffraction. The $Cr_{23}C_6$ precipitates along grain boundary grew fast and Laves ($Fe_2W$) phase on martensitic lath boundaries in interior grains was developed. The solid solution atoms depleted in matrix and lath subgrains recovered owing to precipitate coarsening with long-term aging degradation. There was a close relation with softening of magnetic and mechanical properties.

• Korean Journal of Materials Research
• /
• v.6 no.11
• /
• pp.1146-1152
• /
• 1996

실제 사용시 신뢰성을 보장하기 위하여, 고온에서 장시간 동안의 시효로 인한 ACA COG(Anisotropic Conductive Chip On Glass) 접합 특성의 변화가 연구되었다. 모든 접합 시편들은 16$0^{\circ}C$에서 156시간 동안 유지되었고 시효하는 동안의 접촉저항의 변화는 감소하였다. 특히, 156시간이후, 4000개 /$\textrm{mm}^2$의 입자밀도를 가진 ACA에서는 접촉저항의 벼노하가 나타나지 않았다. 입자크기의 경우 작은 입자를 가진 ACA는 16$0^{\circ}C$에서 시효후에도 접촉저항의 변화를 보이지 않았다. 또한 4000개/$\textrm{mm}^2$ 및 5$\mu\textrm{m}$ 입자를 가진 ACA를 사용한 시편은 접합상태가 안정하였기 때문에 16$0^{\circ}C$에서도 경화수지의 팽창 및 리플로우(reflow)에 의한 영향을 받지 않았다. 따라서 이 ACA에서는 16$0^{\circ}C$에서 156시간 동안 시효한 후에도 오픈(open)이 나타나지 않았다.

• Transactions of the Korean Society of Automotive Engineers
• /
• v.10 no.2
• /
• pp.110-116
• /
• 2002

At elevated temperature, very complex precipitations occur in STS316. To investigate the effect of the precipitation on mechanical properties in SIS316, tensile tests and fatigue crack growth tests were carried out at $650^{\circ}C$ using artificially degraded materials. The material degradation was simulated by aging for up to 20000 hrs. at $750^{\circ}C$, which is equal to 179000hrs (about 20yrs) of service life at $650^{\circ}C$, after conducting solution treatment for 20 min. at $11300^{\circ}C$. The result of the hardness test and the tensile test showed that both properties are closely related to the mean free distance of carbides. Also, from the results of fracture tests at $650^{\circ}C$, ${\triangle}K_{th}$, after values were found to decrease as aging time and microstructure, as the volume fraction of $\sigma$ phase increased.

• Journal of the Korean Society of Manufacturing Process Engineers
• /
• v.21 no.1
• /
• pp.56-65
• /
• 2022

The electrochemical corrosion properties of austenitic AISI 304 steel subjected to a long-term-aging heat treatment were investigated. AISI 304 steel was aged at 700 ℃ for up to 10,000 h. The variation in the microstructure of the aged specimens was observed by optical microscopy and scanning electron microscopy. Electrochemical polarization experiments were performed to obtain the corrosion current density (Icorr) and corrosion potential (Ecorr). Analyses indicated that the metastable intermetallic carbide M₂₃C₆ formed near the γ/γ grain boundary and coarsened with increasing aging time; meanwhile, the δ-ferrite decomposed into the σ phase and into M₂₃C₆ carbide. As the aging time increased, the current density increased, but the corrosion potential of the austenitic specimen remained high (at least 0.04 ㎛/cm²). Because intergranular carbide was absent, the austenitic annealed specimen exhibited the highest pitting resistance. Consequently, the corrosion resistance of austenitic AISI 304 steel decreased as the aging heat treatment time increased.

• Proceedings of the Korean Nuclear Society Conference
• /
• 1996.05c
• /
• pp.73-78
• /
• 1996

Type 316L 스테인레스강에 대한 질소첨가의 효과를 분석하기 위해 질소를 소량(0.024%)첨가한 합금과 적정량(0.15%) 첨가한 합금을 용해하여 입계부식특성 평가를 하였다. Oxalic시험 및 DL-EPR 시험 결과 적정량의 질소를 첨가한 합금이 소량 첨가한 금보다 우수한 예민화 특성을 보였다. TEM에 의한 미세구조 분석 결과 저질소 합금의 경우 비교적 짧은 열처리 시간에 M$_{23}$C$_{6}$ type의 석출물이 입계를 따라 형성되고 시효시간이 경과 할수록 그양이 급격히 증가하는 양상을 보인 반면에 적정 질소 첨가 합금의 경우 탄화물 생성이 비교적 긴 시효시간으로 늦추어져 예민화 실험 결과와 일치된 결과를 보였다. 두 합금 모두에서 탄화물 이외에 Mo의 함량이 매우 높은 석출물이 관찰되었는데 적정질소 첨가강의 경우 시효 시간의 경과에 따라 초기의 작은 cluster들의 형태에서 시작하여 얇은 박막의 형태로 입계면을 따라 성장하는 양상을 보였고, 반면에 저질소합금의 경우 입계를 따른 작은 석출물들이 cluster들로는 성장하였으나 장시간 시효가 진행됨에 따라 탄화물의 성장에 의해 박막 형태로는 성장하지 못하였다. Mo-rich 박막형상 석출물에 대한 분석 결과 20-면체 준결정상의 형태에 매우 가까운 결정구조를 보였으며 때로 η상과 매우 밀접한 회절 패턴도 관찰되었다. 이러한 상은 적정 질소첨가 합금에서의 탄화물 생성 지연과도 관련이 있을 것으로 추정되었다.

• Analytical Science and Technology
• /
• v.6 no.1
• /
• pp.141-147
• /
• 1993

The behavior of reverted austenite in Fe-Ni-Mn(Ti) maraging steels has been investigated in the temperature range from $400^{\circ}C$ to $550^{\circ}C$ using TEM equipped with EDX. Four kinds of reverted austenite appeared depending on the aging temperatures and time : Widmanstatten, granular, lath-like and recrystallized austenite. The reverted austenites are enriched in Ni and Mn due to the dissolution of precipitates and redistribution of alloying elements. Widmanstatten austenite appears unformly in the lath martensite having the K-S orientation relationship with the martensite lath, while lath-like martensites showed K-S and N relations depending on the chemistry and heat treating condition. The recrystallized austenite forms at $550^{\circ}C$ after long aging times : some becomes unstable and transforms to lath martensite on cooling.

• Korean Journal of Materials Research
• /
• v.9 no.9
• /
• pp.905-912
• /
• 1999

Tensile properties of powder metallurgy (PM) processed Cu-7.5Ni-5Sn alloys, either as-received or additionally solution heat treated, were examined as a function of aging time. It was found that the as-received Cu-7.5Ni-5Sn alloys showed an abrupt increase in tensile strength after aging at $350^{\circ}C$ for 20 minutes, due to the metastable ${\gamma}$\\` precipitation and a marginal Spinodal decomposition. The resolutionized PM Cu-7.5Ni-5Sn alloys, on the other hand, showed a gradual increase in tensile strength from the very early stage of aging. The overall tensile strength of resolutionized PM Cu-7.5Ni-5Sn alloys, however, was lower than that of the as-received and aged counterparts, due to the grain growth during resolutionization. Afterprolonged aging for the as-received PM Cu-7.5Ni-5Sn alloys, a considerable amount of discontinuous precipitates formed along the grain boundaries. The formation and growth kinetics of such discontinuous precipitates appeared to be dependent on the heat treatment conditions, and affect the mechanical properties greatly.

• Journal of the Korean Society for Nondestructive Testing
• /
• v.19 no.6
• /
• pp.411-419
• /
• 1999

It has been well recognized that a long term service at elevated temperature of $350^{\circ}C{\sim}550^{\circ}C$ induces embrittlement damage due to carbide precipitation and/or P, Sb and Sn segregation at grain boundaries and thereby deteriorates the grain boundary strength of heat resisting components in the energy-related plants. Therefore, it is very important to assess quantitatively the extent of embrittlement damage of heat resisting components to secure the reliable and efficient service condition and to prevent brittle failure in service. However, because fracture tests are limited in size and number of specimen obtained from the structural components, nondestructive test method is required. In this study, the optimum electrochemical parameters are investigated and discussed to evaluate nondestructive embrittlement damage for aged 2.25Cr-1Mo steels by means of electrochemical polarization test method (ECPTM) in proper corrosive environment. In addition, the electrochemical test results are compared with embrittlement degree evaluated by semi-nondestructive SP test.

• Proceedings of the KAIS Fall Conference
• /
• 2009.12a
• /
• pp.267-270
• /
• 2009

오늘날 전자 장비는 매우 다양한 기능의 채용으로 많은 양의 부품을 필요로 하고 그에 따른 전체 시스템의 무게와 크기를 줄이기 위해 소형화와 슬림화가 필수적이다. 따라서 다수의 부품 사용과 고성능화로 인한 열 발생 밀도도 크게 증가하여 열로 인한 온도 상승이 부품의 성능에 매우 중요한 요소로 작용하고 있다. 기기에서의 지속적인 열 발생은 기기를 탑재하고 있는 전체 시스템의 성능에도 큰 영향을 미치며, 특히 고온의 외기에 장시간 노출된 디지털전광판의 시효문제는 시급히 해결해야 할 당면 과제이기도 하다. 본 연구에서는 이와 같은 LCD 패널의 발열로 인한 기기의 성능 저하와 다운 현상을 예방할 수 있도록 열전소자를 이용한 알루미늄 압출형 히트싱크에 대한 열 저항기술을 개발하고, 냉각모듈에 대한 설계기술을 확보하여 LCD 패널용 고효율의 냉각시스템의 개발 과제를 수행했다.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.35 no.7
• /
• pp.717-722
• /
• 2011

The purpose of this study is to predict the residual stress in Al6061 during T6 heat treatment. In this study, the variable residual stress in case of the solid solution($530^{\circ}C$, 2h) and artificial ageing($175^{\circ}C$, 9h) of Al6061 subjected to T6 heat treatment is determined at different ageing times. A heat treatment experiment is conducted to determine the heat transfer coefficient, on the basis of which the residual stress during the T6 heat treatment is predicted. In order to take into account the relaxation of residual stress during artificial ageing, a Zener-Wert-Avrami function is used and elasto-plastic nonlinear analysis is conducted through FE-simulation. Further, the residual stress is measured by using the X-ray diffraction(XRD) method, and the result is compared with the result from the FE-simulation. It is found that the residual stress predicted form the FE-simulation is in good agreement with the residual stress measured by using the XRD method.