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수퍼 2상 스테인리스강의 피로균열 진전시 발생하는 음향방출신호 특성에 미치는 σ상 석출의 영향
都在潤,李相起,安碩煥,南起祐,姜昌龍,朴仁德 대한금속재료학회 2001 대한금속·재료학회지 Vol.39 No.12
The fatigue crack propagation induced by σ phase precipitation of super duplex stainless steels aged at 600℃ was evaluated and the time-frequency analysis according to acoustic emission was conducted. The fracture mechanism by σ phase precipitation can be found from the time-frequency analysis of acoustic emission signals. As a result of aging at 600℃, the dispersed structure showed faster precipitation rate of σ phase than fibrous structure. The time-frequency analysis of acoustic emission signals obtained during the fatigue test revealed that, the specimens with non-precipitated σ phase induced a dominant frequency of 200∼300 ㎑ without any correlation with crack length. As the fatigue crack propagates (stage Ⅱ_c mode), the frequency range was in a high frequency range above 400 ㎑ in addition to frequency range of 200∼300 ㎑. On the other hand, the frequency range in the specimens with σ phase precipitated was not only 200∼300 ㎑ but also 400 ㎑ because of the fracture mechanism emanating high energy such as dimple fracture and separation of inclusion.
역변태 온도 및 시간이 다른 STS316L의 기계적 및 탄성파 특성
도재윤,탁영준,신기항,남기우 한국산업융합학회 2022 한국산업융합학회 논문집 Vol.25 No.6
In this study, the mechanical properties of 80% cold-rolled austenitic 316L stainless steel were evaluated using specimens subjected to reverse transformation at 500–750℃ for 20 minutes and reverse transformation at 700℃ for 2–60 minutes. Also, for the elastic wave obtained from the tensile test, the dominant frequency according to the reverse transformation condition was investigated by time-frequency analysis. The SEM image of the 80% cold-rolled material was transformed into martensite and showed line and cross shapes. The TEM image showed that line shapes were shown at the grain, and grain boundary of martensite. The higher the heat treatment temperature and the longer time, the larger the grain. Tensile strength decreased as the heat treatment temperature and time increased, but elongation increased. Hardness was proportional to tensile strength. This is because the grain with different directions showed the same direction due to reverse transformation. The dominant frequency was decreased and then increased as the temperature and time increased. This is because the direction of the grain is different at a low temperature and the same direction is shown at a high temperature.
슈퍼이상스테인리스강 용접열영향부의 피로균열전파거동 및 음향방출 특성
도재윤(Jae Yoon Do),김진환(Jin Hwan Kim),안석환(Seok Hwan Ahn),박인덕(In Duck Park),강창용(Chang Yong Kang),남기우(Ki Woo Nam) 대한금속재료학회 ( 구 대한금속학회 ) 2002 대한금속·재료학회지 Vol.40 No.9
Because duplex stainless steel shows the good strength and corrosion resistance properties, the necessity of duplex stainless steel, which has long life in severe environments, has been increased with industrial development. The fatigue crack propagation behavior of heat affected zone(HAZ) has been inrestigated in super duplex stainless steel. The fatigue crack propagation rate of HAZ of super duplex stainless steel was faster than that of base metal of super duplex stainless steel. We also analysed acoustic emission signals during the fatigue test with time-frequency analysis method. According to the results of time-frequency analysis, the frequency ranges of 200-400 kHz were obtained by striation and the frequency range of 500 kHz was obtained due to dimple and separate of inclusion.
오스테나이트 분포에 따른 수퍼 2 상 스테인리스강의 기계적 특성과 피로균열 진전거동
도재윤(J . Y . Do),이상기(S . K . Lee),안석환(S . H . Ahn),남기우(K . W . Nam),강창룡(C . Y . Kang) 한국열처리공학회 2001 熱處理工學會誌 Vol.14 No.4
N/A The characteristics of super duplex stainless steel were investigated on its fibrous structure and dispersed structure. These structures consist of various volume fractions and distributions of the austenite phase that were obtained by changing the heat treatment temperature and cycle. The fibrous structure had higher austenite volume fraction than dispersed structure on the same temperature. As the austenite volume fraction increased in both structures, tensile strength and elongation increased, but hardness decreased. Fatigue life of fibrous structure parallel to rolling direction was shorter than that of perpendicular to rolling direction. Fatigue life of dispersed structure was longer than parallel fibrous structure, and shorter than perpendicular fibrous structure. Fatigue crack propagation race of fibrous structure was faster than that of dispersed structure.
Si<sub>3</sub>N<sub>4</sub> 세라믹스의 균열 치유와 굽힘 강도 특성
남기우,박승원,도재윤,안석환,Nam, Ki-Woo,Park, Seung-Won,Do, Jae-Youn,Ahn, Seok-Hwan 대한기계학회 2008 大韓機械學會論文集A Vol.32 No.11
Crack-healing behavior of $Si_3N_4$ composite ceramics has been studied as functions of heat-treatment temperature and amount of additive $SiO_2$ colloidal. Results showed that optimum amount of additive $SiO_2$ colloidal and coating of $SiO_2$ colloidal on crack could significantly increase the bending strength. The heat-treatment temperature has a profound influence on the extent of crack healing and the degree of strength recovery. The optimum heat-treatment temperature depends on the amount of additive $SiO_2$ colloidal. Crack healing strength was far the better cracked specimen with $SiO_2$ colloidal coating on crack surface. After heat treatment at the temperature 1,273 K in air, the crack morphology almost entirely disappeared by scanning prob microscope. At optimum healing temperature 1,273 K, the bending strength with additive $SiO_2$ colloidal 0.0 wt.% without $SiO_2$ colloidal coating recovered to the value of the smooth specimens at room temperature for the investigated crack sizes $100\;{\mu}m$. But that with $SiO_2$ colloidal coating increase up to 140 %. The amount of optimum additive $SiO_2$ colloidal was 1.3 wt.% and crack healed bending strength with $SiO_2$ colloidal coating increase up to 160 % to smooth specimen of additive $SiO_2$ colloidal 0.0 wt.%. Crack closure and rebonding of the crack due to oxidation of cracked surfaces were suggested as a dominant healing mechanism operating in $Si_3N_4$ composite ceramics.
STS316L 용접부의 피로거동 및 초음파시험에 의한 열화특성 평가
남기우,박소순,안석환,도재윤,박인덕 한국비파괴검사학회 2003 한국비파괴검사학회지 Vol.23 No.2
STS316L had been used as the structural material for energy environmental facilities, because austenite stainless steels like 316L have superior mechanical properties of which toughness, ductility, corrosion resistant and etc. However, those welded structures are receiving severe damage due to increasing of the aged degradation. Most studies until now have been carried out against fatigue behaviors of weldments, and were not well studied on nondestructive evaluation methods. In this study, the fatigue crack propagation behavior of STS316L weldment usually used for vessels of the nuclear power plant was investigated. Also, the degradation characteristics of 316L stainless steel weldments were evaluated by the ultrasonic parameter such as ultrasonic velocity, attenuation factor and time-frequency analysis. The results of this study can be used as a basic data for the prediction of the fatigue crack life of weldments structures without disjointing or stopping service of structures in service.주요용어: STS316L 용접부, 음속, 감쇠계수, 피로거동, 시간-주파수 분석, 단시간 퓨리에 변환, 열화 STS316L은 인성, 연성, 부식저항성 등 우수한 기계적 성질 때문에 에너지환경설비 등 구조물에 널리 이용되고 있다. 그러나, 용접구조물은 가혹한 환경하에서 경년열화가 심각하다. 지금까지의 연구들은 주로 용접부의 피로균열진전거동에 관하여 수행되었고, 피로균열진전속도식의 재료상수와 비파괴적 평가의 상관성에 관한 연구는 거의 없었다. 본 연구는 STS316L 용접부의 피로균열진전 거동을 조사하고, 초음파 파라미터로 열화특성을 평가하고 시간-주파수 분석으로 해석을 하였다. 본 연구 결과는 용접구조물의 사용기간 중 피로수명 등을 예측하기 위한 기초 데이터로서 사용될 수 있다.
316L 스테인리스 용접부의 피로균열진전 거동 및 열화특성
박소순,남기우,안석환,도재윤,박인덕 대한기계학회 2002 대한기계학회 춘추학술대회 Vol.2002 No.3
Recently, by the passing of a year, there have been many accidents in an movable atomic power plant. Therefore a measure to counter to prevent emergency accidents is in need. In this study, we investigated the fatigue crack propagation of 316L stainless steel weldments usually used for vessels of an atomic power plant. As the results of this investigation, it can be use as a basic investigation data to predict the fatigue crack life. And we tried to investigate deterioration property of 316L stainless steel weldments by the Ultrasonic Parameter, such as Ultrasonic Velocity, attenuation factor and Time-Frequency analysis.