전기화학적 분극법에 의한 1.0Cr-0.5Mo강의 경년열화 평가에 관한 연구

나의균,김훈,이종기,Na, Eui-Gyun,Kim, Hoon,Lee, Jong-Gi 대한기계학회 1998 大韓機械學會論文集A Vol.22 No.1

The contents of this paper include a non-destructive technique for evaluating the degradation of the boiler superheater tube in a fossil power plant through an electrochemical polarization test. Correlation between Ip of polarization parameter and SP-DBTT for the superheater tubes in long-term use was obtained. 1.0Cr-0.5Mo steel was degraded by softening, and the degree of degradation was dependent upon carbides with Cr and Mo elements. Since brittle fracture at low temperature and ductile fracture mode at high temperature were shown, similarity between standard Charpy and small punch tests could be found. In addition, SP-DBTT showing the degree of degradation was higher, as the time-in use of the materials got longer. Electrolyte including picric acid of 1.3 g in distilled water of 100ml at 25.deg. C temperature and sodium tridecylbenzene sulfonate with 1g could be applied to evaluate the degradation of 1.0Cr-0.5Mo steel by means of the electrochemical polarization test. Ip and Ipa values measured through the electrochemical test are the appropriate parameters for representing the degradation of the superheater tube(1.0Cr-0.5Mo steel) for the fossil power plant. It is poassible to evaluate the degradation of materials with different time histories electrochemically, by Ip value only, at field test.

암석폐재의 고화체 합성기술의 개발과 파괴인성평가에 관한 연구

나의균,유효선,김진용,이종기,정세희,Na, Eui-Gyun,Yu, Hyosun,Kim, Jin-Yong,Lee, Jeong-Gee,Chung, Se-Hi 대한기계학회 1997 大韓機械學會論文集A Vol.21 No.9

The hot press apparatus to obtain the solidified rocks with 60mm of diameter against rock waste was developed, and the optimum conditions for solidification were founded out, of which were 300.deg. C of temperature and 1hr of holding time. The solidified rocks reinforced with the fibers (carbon, steel) were made by means of a hydrothermal hot press method. Fracture toughness of those was obtained using the round compact tension(RCT) specimens. Load and displacement behaviours of the solidified rocks reinforced with the fibers were dependent upon the fiber volume fraction and kind of the fibers. Strength and fracture energy of the solidified rocks with steel were much larger than those of the solidified ones with carbon because of the Bridge's effect, multiple cracking and crack branching phenomena.

수열 Hot Press법에 의한 화강암 폐재의 고화체형성 및 평가기술의 개발에 관한 연구

나의균,정세희,Na, Eui-Gyun,Chung, Se-He,Takahashi, Hideaki 한국재료학회 1995 한국재료학회지 Vol.5 No.3

본 연구에서는 석재공장 주변의 오염원으로 되고 있는 화강암 폐재의 고화체의 형성과 평가 기술에 관한 내용을 다루었다. 이를 위해, 최근 고화체의 함성기술로서 높이 평가받고 있는 수열 Hot press법을 이용하여 분말형태의 화강암 폐기물을 고화시키는데 필요한 조건을 찾아내었다. 아울러 고화체의 기계적 성질과 파면의 양상 및 수열실험동안 발생된 생성물 사이의 상호 관계를 고찰하였다. 고화체의 기계적 성질은 수열실험조건에 의존성이 있었으며, 적절한 고화조건은 반응온도 30$0^{\circ}C$, 유지시간 1시간이었다. 또한 고화체의 파면은 반응온도 및 유지시간에 따라 현저히 다른 양상을 보였으며, 수열실험동안 다양한 화합물이 생성되었다. 그 중에서 Xonotlite와 Talc는 고화체의 강도를 저하시키는 주된 화합물이었다.

SUS 316스테인리스 강 TIG용접부의 부식특성평가에 관한 연구

나의균(Eui Gyun Na),고승기(Seung-Kee Koh),김훈(Hoon Kim),이상근(Sang-Keun Lee) 대한기계학회 2012 대한기계학회 춘추학술대회 Vol.2012 No.5-3

본 연구에서는 고온고압용으로 많이 사용되고 있는 스테인리스 강을 대상으로 부식특성을 평가하였다. 시험재는 모재, 용접후 열처리재 및 용접재를 대상으로 하였다. 용접후열처리는 730℃까지 온도를 올린 후 4시간동안 유지한 다음 노냉을 시켰다. 부식특성 평가는 양극분극법을 이용하였으며, 분극실험에서 H₂SO₄+KSCN용액을 이용하였다. 또한 저변형률실험법을 이용하여 인공해수 중에서 용접재와 모재 등의 부식특성을 평가하였다. 아울러 시험편의 미시균열 손상의 정도를 알아보기 위해 음향방출실험도 동시에 실시하였다. 후열처리재의 부식의 정도가 가장 심했으며, 모재가 부식환경에 가장 안정적이었다. 용접재의 음향방출 결과는 전 변형구간에 걸쳐 음향방출신호가 나탔으며, 용접후 열처리재의 경우에는 항복강도 이후에는 거의 음향방출신호가 탐지되지 않았다. The purpose of this study is to evaluate the corrosive characteristics for the TIG weld region for SUS-316 stainless steel. Basemetal, weldment and heat treated specimens were prepared and heat treatment was conducted under the 730℃ of temperature, 4hours of holding and cooling in furnace. Anodic polarization test was applied to evaluate the corrosive characteristics for the specimens in the environment of H₂SO₄+KSCN solution. Besides, corrosive characteristics for the specimens was evaluated using slow strain rate test(SSRT) and acoustic emission(AE) test was carried out to examine the sources of micro-crack initiation simultaneously. In case of post weld heat treated specimen, degree of corrosion was the greatest and basemetal was shown to be most stable among 4 specimens. Lots of acoustic emission signals were emitted for the weldment over the all plastic deformationg range. However, In case of the PWHT specimen, acoustic emission signals were produced within the yield strength almost only unlike the weldment. Besides, number of AE counts for the weldment was the largest and those for the PWHT specimen was decreased considerably.

SA-516강 다층용접부 용접후 열처리재의 음향방출신호 특성 평가

나의균(Eui-Gyun Na) 한국비파괴검사학회 2011 한국비파괴검사학회지 Vol.31 No.5

본 연구에서는 SA-516 압력용강의 다층용접재와 용접후 열처리재를 대상으로 음향방출신호 특성을 평가하였다. 또한 예균열 선단에서 형성되는 소성영역의 크기와 음향방출신호와의 관계를 고찰하였으며, 실험 후 파단면을 관찰하여 음향방출원을 규명하여 용접후 열처리의 유효성을 평가하였다. 용접재 및 후 열처리재 모두 용접된 판 두께방향의 중앙부에서 표준 샤르피 시험편을 채취하여 날카로운 균열(예균열)을 내고난 다음, 4점굽힘과 음향방출실험을 동시에 실시하였다. 후 열처리재와 용접재 공히 탄성영역에서 음향방출신호는 발생하지 않았으며, 항복하중과 최대하중 사이에서 발생하였고, 최대하중 이후의 소성 심화영역에서는 신호가 발생하지 않았다. 후 열처리재의 음향방출신호 강도는 시험편의 채취 위치에 관계없이 용접재에 비해 작았으며, 균열선단에서 소성영역의 진전형태는 용접재에 비해 훨씬 단순한 양상을 보였다. 후 열처리재의 파단면에는 용접재와는 달리 산화물의 분포가 훨씬 적었으며, 이는 열처리로 인해 용접부의 음향방출원이 감소하였다는 점에서 볼 때 열처리 효과는 있었다. In this study, evaluation of acoustic emission signals characteristics for the post weld heat treated (PWHT) multi-pass weldment and weldment was dealt. Charpy standard specimens were taken from the lowest, middle and highest regions of the weld block. Pre-crack was made using the repeated load. Four point bend and AE tests were conducted simultaneously. Regardless of the specimens, AE signals were absent within elastic region and produced in the process of plastic deformation. AE signals for all specimens were not emitted after the maximum load. Value of signal strength for the all PWHT specimens was lower than that of the weldment. Besides, relations of plastic deformation zone size and accumulated AE counts for the PWHT specimens were more simple compared with the weldment. In case of the PWHT specimen, particles on the fractured surface decreased prominently compared with the weldment due.to PWHT. From these results, it can be concluded that PWHT was effective in reducing the AE sources for the weldment.

AE기법에 의한 압력용기 강 용접부의 변형거동 평가에 관한 연구

나의균(Eui-Gyun Na),유용재(Yong-Jae Yu) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6

The purpose of this study is to evaluate the mechanical behaviors of the weldment of pressure vessel steel and to verify the sources of fracture initiation using an AE technique. Four point bend and AE tests were conducted against the modified Charpy specimens which were extracted from the submerged arc welded block in air. AE signals were absent within an elastic region, regardless of the specimens. Maximum signal strength of the weldment was the highest, followed by PWHT specimen and basemetal. In case of the weldment, there are lots of oxides on the fractured surfaces, in which are formed in the process of welding in air. AE signals were produced due to the debonding between matrix and oxides, inclusions. Plastic deformation does not attribute to production of AE signals.

구조용 강 용접후열처리가 음향방출신호에 미치는 영향

나의균(Eui Gyun Na),오석형(Seok-Hyung Oh),김장권(Jang-Kweon Ki) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.5

The purpose of this study is to evaluate the effects of post weld heat treatment on acoustic emission signals for the multi-pass weldment of the structural steel. 26 multi-pass welds were conducted to the longitudinal direction and then PWHT was done. Heat treatment conditions are 650℃ of heat temperature and 1.5 hour of holding time. Specimens were extracted from the welded block. Standard Charpy specimens which consist of 10mm x 10mm x 55mm were given to the test. Number of AE signals for the PWHT specimen decreased compared with the weldment and that is independent upon the extraction region of the specimens. Plastic deformation behaviors at notch tip for the PWHT specimen were not complex unlike the weldment due to PWHT. Obtained results can be useful in monitoring the safety of the multi-passed weld structures non-destructively.

비파괴법에 의한 압력용기 강 다층용접부의 균열선단에서 소성변형 역성장거동 평가에 관한 연구

나의균(Eui-Gyun Na),이상근(Sang-Guen Lee) 한국비파괴검사학회 2009 한국비파괴검사학회지 Vol.29 No.5

본 연구에서는 비파괴법을 이용하여 압력용기 강 다층용접부에서 채취한 표준 샤르피 시험편의 균열선단에서 형성되는 소성변형 거동을 평가하였다. 모재, 후열처리재 및 용접재를 대상으로 시험편에 기계적인 노치를 가공한 후 예균열(pre-crack)을 낸 다음 4점 굽힘실험과 음향방출(acoustic emission: AE)실험을 동시에 실시하였다. 균열선단은 용접재외 후열처리재의 경우 용융선 근처에 위치하도록 하였다. 하중이 가해짐에 따라 균열선단에서 형성되는 소성영역의 크기는 응력확대계수를 이용하여 구했으며, 각 시험편에 대한 소성역의 크기와 음향방출 특성과의 관계는 축적된 AE 에너지 관점에서 고찰하였다. 시험편에 관계없이 탄성역역에서는 거의 AE신호가 감지되지 않았으며, 대부분의 AE신호는 소성변형 과정에서 발생하였다. 또한, 용접재가 모재와 후열처리재에 비해 AE신호가 훨씬 많이 발생하였다. 모재와 후열처리재 및 용접재의 균열선단에서 소성영역 크기와 축적된 AE에너지와의 관계는 현저히 다르게 나타났으며, 용접재의 AE counts는 모재와 후열처리재에 비해 많이 발생하였다. The purpose of this study is to evaluate the behaviour of the plastic deformed zone at crack tip on the standard Charpy specimens which were taken from the multi-passed weld block of the pressure vessel steel. Notch was machined on the standard Charpy test specimens and pre-crack which was located around the fusion line was made under the repeat load. Four point bend and acoustic emission tests were carried out simultaneously. The size of plastic region at crack tip was calculated using stress intensity factor. Relationships between characteristics of acoustic emission and plastic zone size at crack tip were discussed through the cumulative AE energy. Regardless of the specimens, AE signals were absent within the elastic region almost and most of AE signals were produced at the plastic deformation region from yield point to the mid-point between yield and maximum load. More AE signals for the weldment were produced compared with the base-metal and PWHT specimen. Relations between plastic deformed zones at crack tip and cumulative AE energy for the weldment and PWHT specimen were different quietly from the base-metal. Besides, number of AE counts for the weldment was the larger than those of the base-metal and PWHT specimen.

날카로운 균열을 갖는 다층용접부의 음향방출거동 평가에 관한 연구

나의균(Eui Gyun Na),오석형(Seok-Hyung Oh),고승기(Seung-Kee Koh),이상근(Sang-Keun Lee) 대한기계학회 2011 대한기계학회 춘추학술대회 Vol.2011 No.5

본 연구에서는 압력용기 날카로운 균열을 갖는 강 다층용접부를 대상으로 음향방출거동을 평가하였다. 용집된 판으로부터 표준 샤르피 시험편을 가공하여 날카로운 균열을 내고 난 다음 굽힘과 음향방출실험을 동시에 실시하였다. 이어 음향방출인자와 굽힘하중과 굽힘량 곡선과의 관계에서 음향방출거동을 평가하였으며, 균열선단에서 소성역역의 진전거동을 음향방출법에 의해 평가하였다. 시험편에 관계없이 탄성영역애서는 음향방출신호가 거의 발생하지 않았으며, 소성영역에서 발생하였다. 음향방출신호강도는 용접재의 경우가 모재와 후 열처리재에 비해 가장 큰 값을 보였다. 주사전자현미경 관찰결과 용접재의 경우 용접시 발생하는 산화물 등이 모재와 후 열처리재에 비해 훨씬 많이 분포되었음을 확인할 수 있었다. The purpose of this study is to evaluate the behaviors of acoustic emissions(AE) at multi-pass weld region with sharp crack for the pressure vessle steel. Charpy standard specimens were extracted from the welded plate. Machined notch was made and then sharp crack was made using repetitive load. Four point bend and AE tests were conducted simultaneously. After tests, relationships between AE parameters such as AE strength etc. and load-deflection curves were obtained. Besides, relations of plastic deformed zone size at sharp crack tip and AE parameter were discussed. AE signals were absent within an elastic region, regardless of the specimens. Most of AE signals are produced in the process of plastic deformation. The deformation modes of basemetal and weldment are quiet different. Value of AE strength for the weldment was larger than those for PWHT specimen and basemetal. SEM observations were made. In case of the weldment. Much more particles such as oxides which are formed in the process of weld in air were distributed on the fractured surfaces compared with the PWHT specimen and basemetal.

강재 변형거동 평가과정에서 음향방출법의 적용법에 관한 연구

나의균(Eui-Gyun Na),김훈(Hoon Kim),이상근(Sang-Keun Lee) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5

The purpose of this study is to obtain the data related with the relationship between AE signal characteristics and mechanical behaviors for the pressure vessel steel. Various kinds of noises are introduced into the AE data in the course of experiments. Accordingly, real AE data have to be obtained after tests. AE test was carried out under four point bending load. Among AE signals, counts and signal strength are used to find out meaning of destructive mechanics. After tests, this paper shows the procedures of filtering the noises against parent of the pressure vessel steel. Relationships between plastic zone size and cumulative AE energy are shown. Besides, AE signals were absent within an elastic region, regardless of the specimens. Most of AE signals are produced in the process of plastic deformation. The deformation modes of basemetal and weldment are different respectively.