Study on Underwater Welding ( Report 1 ) - Its Weldability -

남기우 한국어업기술학회 1983 수산해양기술연구 Vol.19 No.1

최근 각 방면의 해양개발에 대한 관심이 높아짐에 따라 해양구조물의 가공 및 보수수단으로서 수중용접기술의 개발연구가 선진국 등에서 시작되고 있으나 국내에서는 아직 이 방면의 연구를 찾아 볼 수 없다. 본 연구에서는 두께 10 mm의 국산구조용강판(SM41A)에 대하여 자작한 중력식용접장치에 의하여 6종의 국산피복아아크용접봉을 써서 수중용접을 실시하여 그 용접성을 조사하였고, 각 계통의 용접봉의 실용성을 검토하였다. 중요연구 결과는 다음과 같다. 1. 운봉비가 1일 때 용접봉의 각도 40˚, 60˚, 80˚인 경우 용접봉각도가 비이드(bead) 외관에 큰 영향을 미치지 않는다. 2. 용접봉직경이 일정할 때 용접전류의 증가에 따라 입열량이 증가하므로 비이드 외관에 큰 영향을 미치나 용접봉직경에 대한 각 적정전류에서의 비이드외관은 양호하게 나타난다. 3. 수중용접에서 고산화티탄계 용접봉을 사용하였을 경우, 적정용접전류와 직경 사이에는 상관성이 존재하며, 그 적정용접전류의 범위는 비교적 넓다. 4. 저수소계 및 고셀롤로우스계 용접봉을 제외한 4종(일미나이트계, 라임티타니아계, 고산화티탄계, 철분산화철계)의 경우, 용접봉의 안정한 수중아아크가 유지되고, 양호한 수중용접이 얻어졌다. 특히 라임티타니아계, 고산화티탄계 및 일미나이트계는 용접결함이 없는 양호한 수중용접이 가능하다. 5. 수중용접부에 대한 매크로(macro)조직과 미시현미경조직은 마르텐사이트와 베이나이트조직을 나타내고, 그 외는 미세한 페라이트, 퍼얼라이트조직으로서 용접결함이 없는 양호한 용접부를 갖는다. 6. 시험표면층인 3층, 4층 및 6층 용접에서의 본드부근의 열영향부에 각각 경도의 피이크가 있으며, 그 외의 부분은 대체로 경도가 균일함이 확인되었다. Recently, many studies for developing the underwater welding techniques have been carried out in the advanced countries as a manufacturing process and a repairing method according as a great deal of interest in development for various marine industrial fields has been gradually increased. But no study on such underwater welding is available at present in our country. In this study, underwater welding was carried out for welding of domestic structural steel plates (SM41A) of 10 mm thickness, using six types of domestic coated arc welding electrodes on a self-made gravity type underwater welding device, resulting in investigation for the underwater weldability of the domestic structural steel plates as well as for the underwater welding properties and practicability of the domestic welding electrodes.

Si₃N₄ 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.

SiO₂ 콜로이달에 의한 Si₃N₄ 복합 세라믹스의 상온굽힘강도 및 균열치유 현상

남기우,김종순,이희방,Nam, K.W.,Kim, J.S.,Lee, H.B. 대한기계학회 2009 大韓機械學會論文集A Vol.33 No.7

Strength characteristics of $Si_3N_4$ composite ceramics has been studied as functions of heat-treatment temperature and additive $SiO_2$. $SiO_2$ colloidal could significantly increase the bending strength. Crack healing temperature decreased 300 K by additive $TiO_2$. Bending strength of specimen added $SiO_2$ is higher than that of non-added $SiO_2$. Moreover, bending strength of specimen with $SiO_2$ colloidal coating is much higher that of non-coated specimen. In in-situ observation, crack-healed specimen at 1,573 K shows phenomenon like a fog on the surface. By SPM, both crack-healed specimen, non-coating and coating of $SiO_2$ colloidal, at 1,273 K were healed completely but both of 1,573 K exist crack. This was made by evaporation of $SiO_2$ at high temperature. Crack-healing materials of $Si_3N_4$ composite ceramics is crystallized $Y_2Si_2O_7$, $Y_2Ti_2O_7$ and $SiO_2$. A large amount of Si and O, and little C were detected by EPMA. Si and O increase but C decreases according to heat treatment temperature. Specimens with additive $SiO_2$ were more detected Si and O than that of non-additive $SiO_2$. Specimen with $SiO_2$ colloidal coatings were much more detected O.

응력집중 표면균열의 성장 및 관통거동

남기우 대한기계학회 1992 대한기계학회 춘추학술대회 Vol.- No.-

Crack-healing behavior and bending strength of Al2O3/SiC composite ceramics according to the amount of added Y2O3

남기우 한양대학교 세라믹연구소 2010 Journal of Ceramic Processing Research Vol.11 No.4

Three Al2O3/SiC composite ceramics were prepared, which included 1, 3 or 5 wt.% Y2O3, and their high-temperature bending strengths and in-situ crack-healing behaviors examined. A surface elliptical-crack of about 100 ߦ in diameter was introduced in the specimens using a Vickers hardness indenter. From in-situ observations, the Al2O3/SiC composite ceramic with 3 wt.%Y2O3 showed superior crack-healing ability than the 1 and 5 wt.% Y2O3 ceramics. The as-cracked specimen with 3 wt.% Y2O3showed strength recovery on heating for one hour at 1473 K in air, which may have been due to the lower crack-healing temperature on the addition of 3 wt.% Y2O3. The heat-resistance limit temperatures of the crack-healed Al2O3/SiC composite ceramics were 1073, 1373 and 873 K for 1, 3 and 5 wt.% Y2O3, respectively.

알루미나 탄화규소 복합세라믹스 균열치유재의 강도와 탄성파 특성

남기우,김해숙,김미경,김진욱,안석환 대한기계학회 2007 大韓機械學會論文集A Vol.31 No.4

Nondestructive evaluation of SiC/SiC composites by wavelet analysis of an elastic wave signal

남기우,문창권,김진욱 한양대학교 세라믹연구소 2013 Journal of Ceramic Processing Research Vol.14 No.4

We studied the nondestructive evaluation of the elastic wave signal of SiC ceramics and SiC/SiC composite ceramics under monotonic tensile loading. The elastic wave signal of cross and unidirectional SiC/SiC composite ceramics were obtained by the pencil lead method and a bending test. This was applied for the time-frequency method which used the discrete wavelet analysis algorithm. The time-frequency analysis provides the time variation of each frequency component involved in a waveform, which makes it possible to evaluate the contribution of the SiC fiber frequency. The results were compared with the characteristic of the frequency group from SiC slurry and fibers. Based on the results, if it is possible to shift up and design a higher frequency group, we can then make a superior material better than those of exiting SiC/SiC composites.

Cracked-healing and the bending strength of Si3N4 composite ceramics with SiO2 additions

남기우,S. H. Park,J. S. Kim 한양대학교 세라믹연구소 2009 Journal of Ceramic Processing Research Vol.10 No.4

This study analyzed the crack-healing behavior of Si3N4 composite ceramics based on variations in the heat-treatment temperature and the amount of colloidal SiO2 added. Semi-elliptical cracks about 100 μm length were obtained from a Vickers indenter using a load of 24.5 N. The results showed that the use of an optimum amount of added colloidal SiO2 and the coating of colloidal SiO2 on the cracks could significantly increase the bending strength. Meanwhile, 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 is dependent on the amount of added colloidal SiO2 employed. The crack healing strength was far greater in the case of the cracked specimen when colloidal SiO2 had been coated on the crack surfaces. When scanned with a microscope probe, the cracks were found to have almost entirely disappeared when heat treated at a temperature of 1,273 K. The bending strength of the SSTS-1 cracked specimen not coated with the colloidal SiO2 recovered to the same extent as the smooth specimen at the optimum healing temperature of 1,273 K. However, the bending strength of the cracked specimen with the colloidal SiO2 coating increased by up to 140%. The optimum amount of the added colloidal SiO2 was found to be 1.3wt.%. Moreover, the bending strength of the crack-healed specimen with the colloidal SiO2 coating was up to 160% greater than that of the smooth specimen with 0.0 wt.% of the added colloidal SiO2 coating. Crack closure and the rebonding of the cracks caused by the oxidation of cracked surfaces were thus identified as the dominant healing mechanism of Si3N4 composite ceramics.

평활시험편에 있어서 균열형상비 변화에 따른 표면균열의 피로수명과 균열관통거동

남기우 대한기계학회 1991 대한기계학회 춘추학술대회 Vol.- No.-

피로하중을 받는 HT80강의 LBB거동

남기우,안등주,小倉信和,오세규 대한기계학회 1989 대한기계학회 춘추학술대회 Vol.- No.-