873~1273 K에서 열화된 강화흑연강의 기계적 특성 연구

남기우(Ki Woo Nam),이수철(Soo Cheol Lee) 한국비파괴검사학회 2013 한국비파괴검사학회지 Vol.33 No.2

벌레 모양 흑연철로 알려진 강화흑연강(CGI)은 주철보다 더 강하고 더 가벼운 것이 요구되는 응용제품에서 인기를 얻고 있는 금속이다. 최근 강화흑연강 디젤엔진 블록에 사용되고 있다. 본 연구에서는 디젤엔진의 배기 매니폴드에 사용되는 것을 고려하여, 873~1273 K에서 1~96 시간 열처리를 실시하여 CGI340의 기계적 특성을 평가하였다. 열처리를 실시한 시험편의 인장강도는 모재에 비하여 전체적으로 낮은 인장강도를 나타내었다. 열처리 시간이 증가할수록 인장강도가 감소하였으며, 열처리 온도가 높고 시간이 길어질수록 더 많이 감소하였다. 초음파 피로시험에 의한 피로한도는 모재는 약 130 ㎫, 1173 K(96 hrs) 시험편은 약 100 ㎫을 나타내었다. 경도는 열처리 시간이 증가할수록 감소하였으며, 열처리 온도가 높을수록 경도의 분포가 낮아졌다. 그리고 CGI340에 분포하는 구상흑연의 평균경도는 120 Hv, 벌레 모양 흑연의 평균경도는 114 Hv로 기지조직인 페라이트보다 낮은 경도를 나타내었다. 열처리 온도 및 시간에 따라서 구상흑연 및 벌레 모양 흑연에 대한 변화는 일정하지 않지만, 열처리 시간이 길어짐에 따라 기지조직의 결정립이 커지는 것을 확인하였다. Compacted graphite iron(CGI), also known as vermicular graphite iron, is a metal which is gaining popularity in applications that require either greater strength, or lower weight than cast iron. Recently compacted graphite iron has been used for diesel engine blocks. Considering that using in exhaust manifold of the diesel engine, CGI340 was conducted the heat treatment during 1 hour to 96 hours from 873 to 1273 K. Mechanical characteristics were evaluated. The obtained results are as follows; The tensile strength of the heat treated specimens showed overall lower tensile strength than that of the base metal. Tensile strength decreases with increasing of heat treatment time, and the higher heat treatment temperature and the longer time, were more reduced. The fatigue limit by the ultrasonic fatigue test was approximately 130 ㎫ of base metal, 100 ㎫ of 1173 K (96 hrs) specimen, respectively. The hardness decreases with increasing heat treatment time, and the higher the heat treatment temperature was lowered hardness distribution. In CGI340, average hardness of nodular graphite was 120 Hv, average hardness of vermicular graphite was 114 Hv. This showed lower hardness than the base structure ferrite. The nodular graphite and vermicular graphite according to the heat treatment temperature and time didn’t have a consistent change. However, the grain size of base structure grew with increasing of heat treatment time.

집중하중하의 균열을 갖는 부정정보의 소성붕괴거동

남기우(Ki-Woo Nam) 한국해양공학회 1996 韓國海洋工學會誌 Vol.10 No.4

The paper focuse on the effect of a crack subjects to collaspe behaviors of statically indeterminate beams under concentrated load. Through the experiment and calculation, it was revealed that the collaspe load of statically indeterminate beams is much higher than that of statically determinate beams. The cumulative AE event counts of statically determinate beams was less than that of statically indeterminate beams, and the center notch beams was revealed less than that of the side notch beams.

SiC 첨가한 ZrO₂의 기계적 특성에 대한 와이블 통계 해석

남기우(Ki Woo Nam),김선진(Seon Jin Kim),김대식(Dae Sik Kim) 대한기계학회 2015 大韓機械學會論文集A Vol.39 No.9

비커스 경도 실험은 세라믹스 재료의 경도를 특성화하는데 사용되는 일반적인 실험법이다. 그러나 경도도 하나의 확률변수로 취급하는 것이 일반적이다. 본 연구의 목적은 단상 ZrO₂ 와 SiC 첨가한 ZrO₂/SiC 복합 세라믹스의 굽힘강도와 비커스 경도의 통계적 성질을 조사하는 것이다. 본 연구에서는 와이블 통계 해석에 기초하여 그들의 결과를 특성치와 변동을 비교 고찰하였다. 굽힘강도 및 비커스 경도는 모두 와이블 분포에 비교적 잘 적합할 수 있음을 알았다. 또한 단상 ZrO₂ 와 SiC 첨가한 ZrO₂/SiC 복합 세라믹스와 그들의 열처리재에 대한 비커스 경도의 확률분포에 대한 척도 및 형상 파라메터 값을 평가하였다. The Vickers hardness test is a common method used to characterize the hardness of ceramic materials. However, the hardness is not a deterministic value, but is a random variable. The objective of this paper is to investigate the statistical properties of the bending strength and a set of Vickers hardness values in single ZrO₂ and composite ZrO₂/SiC with a SiC additive. In this work, we compare the characteristic value and variation with the results based on Weibull statistical analysis. The probability distributions of the bending strength and Vickers hardness agreed relatively well with the Weibull distribution. We evaluate the scale parameter and shape parameter in asreceived ZrO₂ and ZrO₂/SiC composite ceramics,as well as in their heat treated ceramics.

SiC 첨가한 ZrO₂의 기계적 특성에 대한 와이블 통계 해석

남기우(Ki Woo Nam),김선진(Seon Jin Kim),김대식(Dae Sik Kim) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.신뢰성 No.-

흡수된 CFRP의 AE에 의한 파괴거동과 신뢰성 평가

남기우(Ki-Woo Nam),김선진(Seon-Jin Kim) 한국해양공학회 1996 韓國海洋工學會誌 Vol.10 No.4

Recently carbon fiber reinforced plastic (CFRP) has been used structural materials in corrosive environment such as for water, chemical tank and pipes. However, mechanical properties of such materials may be change when CFRP are exposed to corrosive environment for long periods of time.<br/> The degradation behavior of carbon fiber/epoxy resin composite material in distilled water is investigated using acoustic emission (AE) technique. Fracture toughness tests are performed on the compact tension specimens that are pilled by two types of [0₂/90₂]₃s and [0/90]6s. During the testes, AE test was carried out to monitor the damage of CFRP by moisture absorption. The data was treated by 2-parameter Weibull distribution and the fracture surface was observed by scanning electron microscope.

레이저 TWB된 이종접합강의 기계적 특성

남기우(Ki Woo Nam),박상현(Sang Hyun Park),이규현(Kyu Hyun Lee) 대한기계학회 2011 대한기계학회 춘추학술대회 Vol.2011 No.10

This study had the laser tailor welded blanks(TWB) for light weight with high strength steel of top part, SABC1470, and cold roller steel of bottom part, SPFH590 and SPFC980 for the security of passengers . TWB different materials specimen qualified the high strength in SABC1470 material by heat treatment. The tensile properties according to waiting time up to water cooling and the deformation behavior of high speed bending by statically indeterminate condition like center pillar were evaluated. In same temperature, the heat treated TWB specimen shows a decreasing trend the tensile and yield strength according to waiting time up to water cooling. SABC1470 + SPFH590 TWB specimen of 850 ℃ was moderate conditions that can protect passenger by accident. Considering its safety of passenger by accident, center pillar is safe to use with SABC1470 of top part and SPFH590 of bottom part.

Si₃N₄ 세라믹스의 균열 치유와 굽힘 강도 특성

남기우(Ki Woo Nam),박승원(Seung Won Park),도재윤(Jae Youn Do),안석환(Seok Hwan Ahn) 대한기계학회 2008 大韓機械學會論文集A Vol.32 No.11

Crack-healing behavior of Si₃N₄ composite ceramics has been studied as functions of heat-treatment temperature and amount of additive SiO₂ colloidal. Results showed that optimum amount of additive SiO₂ colloidal and coating of SiO₂ 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 SiO₂ colloidal. Crack healing strength was far the better cracked specimen with SiO₂ 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 SiO₂ colloidal 0.0 wt.% without SiO₂ colloidal coating recovered to the value of the smooth specimens at room temperature for the investigated crack sizes 100 ㎛. But that with SiO₂ colloidal coating increase up to 140 %. The amount of optimum additive SiO₂ colloidal was 1.3 wt.% and crack healed bending strength with SiO₂ colloidal coating increase up to 160 % to smooth specimen of additive SiO₂ 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 Si₃N₄ composite ceramics.

레이저 TWB된 이종접합강의 내식 특성

남기우(Ki Woo Nam),박상현(Sang Hyun Park),이규현(Kyu Hyun Lee),이문용(Mun Yong Lee) 대한기계학회 2011 대한기계학회 춘추학술대회 Vol.2011 No.10

센터 필라에 적용하기 위하여 이종재를 레이저 TWB하여 경량화 및 안전성을 확보하고, 친환경 방청 소재를 적용하여 모재 및 용접재의 내식성을 평가하였다. 모재에서 Al+Si 도금되어 있는 SABC1470은 18시간 후에 적녹이 발생하였으며, 아연 도금되어 있는 SPFH590은 6시간 후에 백청이 발생하였다. 한편, 무도금한 SPFC980은 30분 경과 전에 적녹이 발생하였다. 용접재에서 LR-0727(1)용액이 LR-0727(2)보다 우수한 내식성을 나타내었다. 72시간의 염수분무 시험 후에 SABC1470+SPFH590 용접시험편은 950℃에서 열처리한 시험편이 850 ℃에서 열처리한 시험편보다 내식성이 매우 우수하였으며, SABC1470+SPFC980 용접시험편은 850 ℃에서 열처리한 시험편의 내식성이 우수하였다. The different materials secure the light weight and safety by laser TWB for center pillar, and as-received specimen and TWB specimen were evaluated the corrosion resistance with environment-friendly materials. In salt spray test, as-received materials, SABC1470 with Al+Si plating had red rust after 18 hours, SPFH590 with Zn plating had rust after 6 hours. But, SPFC980 without plating had red rust in 30 minute. In salt spray test in 72 hours, the corrosion resistance of SABC1470+SPFH590 TWB specimen with heat treatment at 950 ℃ was more excellent than that of 850 ℃. SABC1470+SPFC980 TWB specimen was excellent in 850 ℃.

SiO₂ 나노 콜로이달 1.3 wt. %가 첨가된 Si3N4세라믹스의 치유거동

남기우(Ki-Woo Nam),박상현(Sang-Hyun Park),김종순(Jong-Soon Kim) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.5

This study analyzed the crack healing behavior of Si₃N₄ composite ceramics (SSTS-1) with SiO₂ 1.3 wt.% nano colloidal added and compared with that of Si₃N₄ composite ceramics (SSTS-2) with SiO₂0.0 wt.% nano colloidal added. Cracks of about 100 - 500 ㎛ were obtained from single and multi- Vickers indenter using the load of 24.5 - 98N. All specimens carried out heat treatment in an air atmosphere for one hour at temperatures of 1,273 K after implementing hydraulic pressure coating on the specimens surface with cracks. In this study, the SSTS-2 specimens with Si02 1.3 wt.% nano colloidal added exhibited totally higher strength values than that of the SSTS-1 specimens with SiO₂ 0.0 wt.% nano colloidal added in each condition.

표면균열재의 피로균열 관통거동에 따른 어코스틱에미션

남기우(Ki-Woo Nam),김선진(Seon-Jin Kim),이건찬(Kun-Chan Lee),오정환(Jung-Hwan Oh),오세규(Sae-Kyoo Oh),이주석(Joo-Suk Lee) 한국해양공학회 1997 韓國海洋工學會誌 Vol.11 No.3

Crack penetration behavior by fatigue crack propagation and measurements of AE before-and-after crack penetration were examined using SS41 steel plate. Experimental crack shape of SU type was in good agreement with calculated shape rather than S type. Crack propagation behavior on the front surface appears not to change markedly after penetration. However, crack growth on the back surface appears to accelerate as reported by author. As a crack propagates, AE occurred heavily just before penetration. Then, it decreases and crack is penetrating. A transition from plane strain to plane stress was observed by fractographic study. At this time, separation of fracture surface was shown which affects AE occurrence.