Seed를 이용한 일방향응고 Ti-46A1-1.5Mo-0.2C 합금의 크리프 특성

李鎬年,丁仁洙,吳明勳,山口正治,韋當文 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.1

Creep properties of directionally solidified Ti-46A1-1.5Mo-0.2C alloys with well aligned lamellar microstructure have been dramatically improved. The ingots were manufactured by seeding technique directional solidification. Directionally solidified Ti-46A1-1.5Mo-0.2C alloys showed excellent creep properties. However, the creep resistance decreased as the angle between lamellar orientation and loading axis increased. It is found that the control of the lamellar orientation is most important to improve the creep resistance of TiAI alloys. In the results of TEM analysis of creep specimen before and after creep test, it was found that no carbides were formed before creep test. But, after creep test, carbides were formed in γ lath, γ / γ interfaces, γ / α_2 interfaces and dislocations. In the case of creep test at 750℃, the quantity of carbides found in dislocations decreased as the load level increased. It was thought that superior thermal stability of Ti-46A1-1.5Mo-0.2C alloy suppressed the dynamic recrystallization and the coarsening of the lamellar microstructure, and the excellent creep resistance was maintained for long time due to this thermal stability.

천이크리프를 고려한 구형압입 크리프 물성평가법

임동규(Dongkyu Lim),이진행(Jin Haeng Lee),김민수(Minsoo Kim),이형일(Hyungyil Lee) 대한기계학회 2013 大韓機械學會論文集A Vol.37 No.11

기존 단축 크리프시험에서는 천이 크리프의 영향을 무시하고 정상상태 크리프만을 고려하기 때문에 실제 크리프 특성을 나타내지 못한다. 이에 본 연구에서는 천이크리프를 고려한 압입 크리프 물성평가법을 제시한다. 다양한 재료에 대해 구형압입시험 전산모사를 이용해 크리프 물성변화에 따른 거동을 살펴보고, 크리프 특성을 무차원 변수들의 회귀식으로 표현한다. 이를 토대로 천이크리프를 고려한 압입 크리프 물성평가 프로그램을 생성했다. 제시한 물성평가 프로그램을 통해 압입 하중-변위 곡선으로부터 크리프지수, 계수값들을 각각 1.1%, 2.3% 오차범위에서 예측할 수 있다. Creep through nanoindentations has attracted increasing research attention in recent years. Many studies related to indentation creep tests, however, have simply focused on the characteristics of steady-state creep, and there exist wide discrepancies between the uniaxial test and the indentation test. In this study, we performed a computational simulation of spherical indentations, and we proposed a method for evaluating the creep properties considering transient creep. We investigated the material behavior with variation of creep properties and expressed it using regression equations for normalized variables. We finally developed a program to evaluate the creep properties considering transient creep. By using the proposed method, we successfully obtained creep exponents with an average error less than 1.1 and creep coefficients with an average error less than 2.3 from the load-depth curve.

W치환 2205 이상 스테인리스강의 크리프 특성 평가에 관한 연구

김기엽,최병호,남기우,안용식 한국비파괴검사학회 2004 한국비파괴검사학회지 Vol.24 No.1

22Cr-5Ni 2상 스테인리스강의 합금원소 중 몰리브덴(Mo) 원소를 텅스텐(W)으로 치환하여 크리프거동에 미치는 영향을 조사하였다. 크리프시험은 600℃와 650℃의 온도에서 실시하였다. 650℃ 크리프 시에는 취약한 금속간화합물인 σ상이 석출하였으며 이것은 크리프수명을 급격히 감소시키는 역할을 하였다. Mo에 대한 W원소의 치환은 일반적으로 σ상의 석출을 지연시킨다고 알려져 있으나, 2wt.%까지 W 치환을 실시한 본 연구범위에서는 크리프 수명 및 최소 크리프속도에 뚜렷한 영향을 미치지 못하였다. 크리프손상의 정도를 평가하기 위하여 크리프시편에 대한 초음파검사를 실시하였다. 크리프온도 600℃에서는 크리프시편의 수명이 증가함에 따라 음속이 비례적으로 증가하였으나 650℃에서는 반대로 감소하였다. The effect of the substitution of Tungsten(W) for Molybdenum(Mo) on the creep behaviour of 22Cr-5Ni duplex stainless steel(DSS) has been investigated. Creep tests were carried out at 600℃ and 650℃. Intermetallic σ phase is precipitated during creep at 650℃, at which creep rupture time was much lower compared with at 600℃. The substitution of W for MO in the duplex stainless steel was known to retard the formation of σ phase. Minimum creep rate and creep rupture time, however, were hardly influenced by the substitution of 2wt.% W. An ultrasonic measurement for the creep specimens has been carried out for the evaluation of creep damage. The sound velocity increases propotionally with the increase of creep rupture time at 600℃ of creep temperature. On the contrary, the sound velocity decreases with the increase of rupture time at 650℃, which can be correlated with the microstructural evolution during creep.

Mechanical, Tensile Creep and Viscoelastic Properties of Thermoplastic Polyurethane/Polycarbonate Blends

Alper Kasgoz 한국섬유공학회 2021 Fibers and polymers Vol.22 No.2

This study investigated the morphological, mechanical and solid-state creep properties of thermoplasticpolyurethane/polycarbonate blends. Blend film samples were prepared via the solution-mixing method. The morphologicaland mechanical properties of the samples were investigated by scanning electron microscope (SEM) and universal testmachine. Solid-state creep tests were also performed by dynamic mechanic analyzer (DMA) under a stress value of 3 MPa atdifferent temperatures (30, 40, and 50 °C). Morphological observations indicated that the blend samples had a compatiblestructure due to the polar nature of PC and TPU. In the mechanical tests, it was found that the tensile modulus value improvedsignificantly by incorporation of PC, whereas the strain at break and toughness values reduced. Accordingly, the blendsample that contained PC at the rate of 10 % (wt.) showed a higher tensile modulus and lower toughness than neat TPU by2.85 and 0.56 times, respectively. In the creep strain analyses, the viscoelastic structure and long-term creep performance ofthe samples were analyzed by the Burger model, time-temperature superposition (TTS) approach and Findley model. Theexperimental values and the model predictions indicated that incorporation of PC into the TPU phase improved the creepresistance of TPU significantly. For example, the creep strain value of neat TPU could be reduced by 68 % and 98 % in therespective cases of the PC concentrations of 10 % (wt.) and 50 % (wt.). Finally, the experimental creep-recovery behavior ofthe samples was investigated, and the permanent strain values were determined by the Weibull Distribution Function (WDF).

크리프 물성평가를 위한 구형압입 수치접근법

임동규(Dongkyu Lim),이진행(Jin Haeng Lee),최영식(Youngsick Choi),이형일(Hyungyil Lee) 대한기계학회 2013 大韓機械學會論文集A Vol.37 No.10

본 연구에서는 증분소성이론에 기초한 구형압입이론을 크리프 물성을 평가하기 위한 압입이론으로 확장했다. 먼저 크리프변형률 기울기가 일정한 지점을 유효 응력-변형률속도 최적 관측지점으로 선정했다. 구형압입시험 전산모사를 이용해 크리프 지수와 계수를 변화시켜 가면서 이에 따른 재료의 거동을 무차원 변수들 (ξ, ψ)의 회귀식으로 표현해 크리프 물성평가를 위한 새로운 수치 접근법을 구축했다. 이를 토대로 구형압입시험으로부터 재료의 크리프지수 및 계수를 예측하는 물성평가 프로그램을 개발했다. 압입 하중-변위 곡선으로부터 크리프지수는 평균 1.5%, 크리프계수는 평균 1.0% 이내의 오차범위에서 물성치들을 얻을 수 있다. In this study, the theory of spherical indentation based on incremental plasticity is extended to an indentation method for evaluating creep properties. Through finite element analysis (FEA), the point where the elastic strain effect is negligible and the creep strain gradient constant is taken as the optimum point for obtaining the equivalent strain rate and stress. Based on FE results for spherical indentation with various values of creep exponent and creep coefficient, we derive by regression an equation to calculate creep properties using two normalized variables. Finally a program is generated to calculate creep exponent and creep coefficient. With this method, we obtain from the load-depth curve creep exponents with an average error of less than 1.5 % and creep coefficients with an average error of less than 1.0 %.

Effects of microstructure on material properties of modified 9Cr-1Mo steel subject to creep-fatigue

Mayu Muramatsu,Takayuki Suzuki,Yuji Nakasone 대한기계학회 2015 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.29 No.1

The creep-fatigue properties of modified 9Cr-1Mo steel were studied to elucidate the effects of the microstructure on specimens deformedby fatigue and creep-fatigue via stress holding for 30 min at 550°C. First, creep-fatigue behaviors up to 7.8% nominal strainswere obtained with respect to the deformation conditions. Next, the micro Vickers hardness values were determined to measure thecreep-fatigue properties of the deformed materials. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM)were used to determine the grain diameters and grain average misorientations (GAMs) and distributions of dislocations and precipitates,respectively. These observations explained the creep-fatigue properties of the deformed specimens. The differences in the properties ofmaterials subjected to fatigue and creep-fatigue are discussed both macroscopically and microscopically.

Development of Creep and Mechanical Properties for SMAW Cr-Mo-X Steel Welds

Sanghyun Bae,Yongchul Kim,Stephen Liu 대한용접·접합학회 2024 대한용접·접합학회지 Vol.42 No.1

This research describes the development of new welding materials with high strength and high heat resistance for Cr-Mo-X type steels. Through thermodynamic simulation, the newly designed welding electrode had excellent mechanical properties, especially in high temperature tensile testing at 600℃ with 393.2 ㎫ tensile strength, 385.3 ㎫ yield strength and 23.5% elongation. Refinement of second phase particles were key to the success of the newly designed electrodes. Presence of M23C6 is noticeable in the new electrode welds which contributed to the high temperature strength and creep properties. Compared to conventional products, it was confirmed that the amount of M23C6 precipitates, measured in area fraction, increased substantially, from 5.9 to 11.9%. Z-phase, known to degrade weld mechanical properties, was reduced through adjusting chemical composition of the experimental welds. The reduction in Z-phase precipitates improved the brittle fracture resistance.

지오그리드의 크리프 한계 변형률에 관한 해석학적 고찰

전한용,정연인,Jeon Han-Yong,Chung Youn-In 한국섬유공학회 2006 한국섬유공학회지 Vol.43 No.3

New procedure for the evaluation of creep reduction factor using performance limit strain concept was introduced and confirmed through the creep test results. To determine the performance limit strain of the textile geogrid used in this study, the Sherby-Dorn Plots were applied and the results were compared with the results that applied existed limit strain criteria (GRI test method GG-4). The creep-limited strain of the geogrid samples that was deter-mined by using the Sherby-Dorm Plots was all 11%. This value is higher than the existing creep-limited stran value of 10%. From this 11% limit strain, the creep reduction factors were calculated at 100,000 hours design. It resulted in 1.45 for all of the geogrid samples (8 t/m, 10 t/m). Finally, when it was compared with the creep reduction factors using 10% criteria, there was slight decrease in reduction factor values of about $0.06{\sim}0.14$.

Annealing effect on microstructure, hardness and creep properties of the foils of alloy 617

Sanjeev Kumar Sharma,Feng Xuin Li,Ki-Ju Kang 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5

The rolling and annealing effect on the microstructure and high temperature creep properties of Ni-base alloy 617 foils were investigated. Two types of foil specimens with different thickness reduction were prepared by thermo-mechanical processing, which is a combination of cold rolling and static recrystallization. The thermo mechanical processing (TMP) is a way to considerably influence the grain size. The recrystallization and grain growth were observed readily at both lower and higher annealing temperatures. The grain sizes were strongly dependent on the thickness reduction and the annealing temperature. The grain sizes were observed bigger at higher annealing temperature than lower annealing temperature. Due to the microstructure analysis, the carburization was observed at lower annealing temperature and lower thickness reduction. The uniform coarse grain sizes increase the resistance for creep deformation and crack growth owing to the precipitation hardening. The lower thickness reduction foils were found better creep properties.

고온용 압력용기의 크리프 균열성장 잔여수명평가 사례 연구

백운봉,이해무,박종서,윤기봉 한국안전학회 2001 한국안전학회지 Vol.16 No.3

Creep crack growth lift of high temperature pressure equipments was assessed for various crack locations and for various material properties. Surface cracks at the inner and outer surface of the vessel in the axial and circumferential directions were considered. The crack was located in the weld metal, in the parent metal or at the weld interface. Results shored that the crack at the weld interface was the most dangerous one. The crack located outside is weaker than that located inside. Safety factors of the case in which improper material properties were used the to unavailability of the correct material properties were discussed.