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안규백,배홍열,노병두,안영호,최종교,우완측,박정웅,An, Gyubaek,Bae, Hong-Yeol,Noh, Byung-Doo,An, Young-Ho,Choi, Jong-Kyo,Woo, Wanchuck,Park, Jeong-Ung 대한용접접합학회 2015 대한용접·접합학회지 Vol.33 No.1
Recently, there have been the increase of ship size and the development of oil and gas in arctic region. These trends have led to the requirements such as high strength, good toughness at low temperature and good weldability for prevent of brittle fracture at service temperature. There has been the key issue of crack arrestability in large size structure such as container ship. In this report for the first time, crack arrest toughness of thick steel plate welds was evaluated by large scale ESSO test for estimate of brittle crack arrestability in thick steel plate. For large structures using thick steel plates, fracture toughness of welded joint is an important factor to obtain structural integrity. In general, there are two kinds of design concepts based on fracture toughness: crack initiation and crack arrest. So far, when steel structures such as buildings, bridges and ships were manufactured using thick steel plates (max. 80~100mm in thickness), they had to be designed in order to avoid crack initiation, especially in welded joint. However, crack arrest design has been considered as a second line of defense and applied to limited industries like pipelines and nuclear power plants. Although welded joint is the weakest part to brittle fracture, there are few results to investigate crack arrest toughness of welded joint. In this study, brittle crack arrest designs were developed for hatch side coaming of large container ships using arrest weld, hole, and insert technology.
Cr 첨가가 고망간강의 중성 수용액 환경 내 유동가속부식 거동에 미치는 영향
정영재 ( Yeong Jae Jeong ),박진성 ( Jin Sung Park ),방혜린 ( Hye Rin Bang ),이순기 ( Soon Gi Lee ),최종교 ( Jong Kyo Choi ),김성진 ( Sung Jin Kim ) 한국부식방식학회 2021 Corrosion Science and Technology Vol.20 No.6
The effect of Cr addition to high Mn steel on flow-accelerated corrosion (FAC) behavior in a neutral aqueous environment was evaluated. For comparison, two types of conventional ferritic steels (API X70 steel and 9% Ni steel) were used. A range of experiments (electrochemical polarization and impedance tests, weight loss measurement, and metallographic observation of corrosion scale) were conducted. This study showed that high Mn steel with 3% Cr exhibited the highest resistance to FAC presumably due to the formation of a bi-layer scale structure composed of an inner Cr enriched Fe oxide and an outer Mn substituted partially with Fe oxide on the surface. Although the high Mn steels had the lowest corrosion resistance at the initial corrosion stage due to rapid dissolution kinetics of Mn elements on their surface, the kinetics of inner scale (i.e. Cr enriched Fe oxide) formation on Cr-bearing high Mn steel was faster in dynamic flowing condition compared to stagnant condition. On the other hand, the corrosion scales formed on API X70 and 9% Ni steels did not provide sufficient anti-corrosion function during the prolonged exposure to dynamic flowing conditions.
압흔 주위 변형 양상 분석을 통한 구조용 강의 Luders strain 평가
최열,최종교,주웅용,권동일,안정훈 대한금속재료학회(대한금속학회) 2001 대한금속·재료학회지 Vol.39 No.1
The continuous indentation test was used to evaluate Lu¨ders strain of various steels such as ultra-fine grained steels and AISI 1020 steels. Inhomogeneous pile-up deformation in the indentation test could be related to Lu¨ders strain in tensile test. The pile-up deformation could be characterized as a localized flow phenomena. We analyzed the pile-up area as the parameter of Lu¨ders strain, and described the pile-up amount more quantitatively by considering the relation between plastic-zone size and pile-up radius. Finally, we derived the master curve for some steels, which included the ratio of elastic modulus to hardness as plastic-zone size. For various steels in this study, the values of Lu¨ders strain obtained from the master curve are consistent with those measured from the tensile test.
변수일,최종교,김홍표 대한금속재료학회(대한금속학회) 1986 대한금속·재료학회지 Vol.24 No.1
The effects of nonmetallic infusions and microstructures on the sulfide stress-corrosion cracking (SSCC) of welded line-pipe steels have been investigated by using electrical potential method. SSCC tests were conducted in H₂S-saturated aqueous solution containing 5 wt.% NaCl and 0.5 wt.% CH₃ COOH at temperature of 35±2℃ by using smooth U-bend specimens. Sulfide stress-corrosion (SSC) crack initiation and propagation took place in the weld fusion zone (WFZ) of two different kinds of American Petroleum Institute (API) grade X-65 due to the presence of elongated nonmetallic inclusions such as manganese sulfides. However, SSC crack of API grade J-55 initiated and also propagated along the pearlite band nearly perpendicular to the specimen surface of heat affected zone (HAZ) of both upper and lower half regions of the full thickness due to the globular morphology of nonmetallic inclusions such as calcium sulfides in WFZ produced by Ca treatment. And the unstable crack propagation was arrested in the midthickness region due to the orientation change of pearlite band nearly by 90˚, which gives rise to the occurrence of the stripes in the SSCC fracture surface. The SSCC kinetics of line-pipe steel weldments have been divided into the three distinct stages, namely crack incubation, slow crack propagation and unstable crack propagation stages, and were discussed in terms of nonmetallic inclusions and microstructures. It is suggested that the susceptibility to SSCC was rather affected by nonmetallic inclusions than microstructures.
전위차법 (電位差法) 에 의한 35wt% MgCl2 수용액중에서의 304 계 스테인레스강의 응력부식균열의 생성 및 전파거동에 관한 연구
변수일,최종교,이우봉 대한금속재료학회(대한금속학회) 1988 대한금속·재료학회지 Vol.26 No.11
The behaviour of stress corrosion(SC) crack initiation and propagation of Type 304 stainless steel in 35 wt% MgCl₂ has been studied as a function of temperature and applied stress at the applied electrochemical potential of-106mV_(SHE) by using electrical potential method. In the present study. the electrical potential method allows to differentiate the incubation period from the propagation period for SC crack. The optical microscopy revealed that the pit is a primary source of SC crack initiation under the applied stress lower than yield strength. SC crack initiation stage is composed of three steps, 1) pit initiation, 2) pit growth to critical size, and 3) crack propagation in the lateral and the through-thickness directions. The portion if anodic dissolution trenches to pits as sites of SC crack initiation increased with applied stress higher than yield strength. The activation energy of the electrochemical reaction contribution to the SC crack initiation and the energy of the stress contribution to the SC crack initiation under 212MPa are evaluated to be 30.5 kJ/㏖ and 14.0 kJ/㏖ respectively. The fractography of SC crack propagation showed the transition of fracture mode from fully transgranular mode to the mixed zone with intergranular mode in the interior of the specimen. The intergranular portion is nearly independent of temperature, however it increases with the applied stress.