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최병학,최원열,심종헌,박찬희,강주희,김승언,현용택,Choe, Byung Hak,Choi, Won-Youl,Shim, Jong Heon,Park, Chan Hee,Kang, Joo-Hee,Kim, Seung Eon,Hyun, Yong Taek 한국재료학회 2015 한국재료학회지 Vol.25 No.8
Microstructural analysis of a (${\alpha}+{\beta}$) Ti alloy was investigated to consider phase transformation in each step of the thermo-mechanical process using by SEM and TEM EDS. The TAF (Ti-6Al-4Fe) alloy was thermo-mechanically treated with solid solution at $880^{\circ}C$, rolling at $880^{\circ}C$ and annealing at $800^{\circ}C$. In the STQ state, the TAF microstructure was composed of a normal hcp ${\alpha}$ and metastable ${\beta}$ phase. In a rolled state, it was composed of fine B2 precipitates in an ${\alpha}$ phase, which had high Fe segregation and a coherent relationship with the ${\beta}$ matrix. Finally, in the annealing state, the fine B2 precipitates had disappeared in the ${\alpha}$ phase and had gone to the boundary of the ${\alpha}$ and ${\beta}$ phase. On the other hand, in a lower rolling temperature of $704^{\circ}C$, the B2 precipitates were more coarse in both the ${\alpha}$ and the boundary of ${\alpha}$ and ${\beta}$ phase. We concluded that microstructural change affects the mechanical properties of formability including rolling defects and cracks.
최병학,이범규,장현수,전우일,박용성,임재균,이진희,박찬성,김진표,Choe, Byung Hak,Lee, Bum Gyu,Jang, Hyeon Su,Jeon, Woo Il,Park, Yong Sung,Lim, Jae Kyun,Lee, Jin Hee,Park, Chan Sung,Kim, Jin Pyo 한국재료학회 2017 한국재료학회지 Vol.27 No.3
The aim of this study is to consider the effect hydrogen on dezincification behavior of Cu-Zn alloys. The investigations include microstructural observations with scanning electron microscope and chemical composition analysis with energy dispersive spectrometer. The dezincification layer was found to occur in high pressure hydrogen atmosphere, not in air atmosphere. In addition, the layers penetrated into the inner side along the grain boundaries in the case of hydrogen condition. The shape of the dezincification layers was porous because of Zn dissolution from the ${\alpha}$ or ${\beta}$ phase. In the case of stress corrosion cracks formed in the Cu-Zn microstructure, the dezincification phenomenon with porous voids was also accompanied by grain boundary cracking.
토양 속 박테리아가 지하매설 X65 배관의 미생물 부식 거동에 미치는 영향
최병학,한성희,김대현,김우식,김철만,최광수,Choe, Byung Hak,Han, Sung Hee,Kim, Dae Hyun,Kim, Woosik,Kim, Cheolman,Choi, Kwang Su 한국재료학회 2022 한국재료학회지 Vol.32 No.3
Microbiologically Influenced Corrosion (MIC) occurring in underground buried pipes of API 5L X65 steel was investigated. MIC is a corrosion phenomenon caused by microorganisms in soil; it affects steel materials in wet atmosphere. The microstructure and mechanical properties resulting from MIC were analyzed by OM, SEM/EDS, and mapping. Corrosion of pipe cross section was composed of ① surface film, ② iron oxide, and ③ surface/internal microbial corrosive by-product similar to surface corrosion pattern. The surface film is an area where concentrations of C/O components are on average 65 %/16 %; the main components of Fe Oxide were measured and found to be 48Fe-42O. The MIC area is divided into surface and inner areas, where high concentrations of N of 6 %/5 % are detected, respectively, in addition to the C/O component. The high concentration of C/O components observed on pipe surfaces and cross sections is considered to be MIC due to the various bacteria present. It is assumed that this is related to the heat-shrinkable sheet, which is a corrosion-resistant coating layer that becomes the MIC by-product component. The MIC generated on the pipe surface and cross section is inferred to have a high concentration of N components. High concentrations of N components occur frequently on surface and inner regions; these regions were investigated and Na/Mg/Ca basic substances were found to have accumulated as well. Therefore, it is presumed that the corrosion of buried pipes is due to the MIC of the NRB (nitrate reducing bacteria) reaction in the soil.
IN738LC 초내열합금 정밀 주조의 주조 및 열처리 미세조직에 구성되는 성분 편석 현상
최병학,한성희,김대현,안종기,이재현,Choe, Byung Hak,Han, Sung Hee,Kim, Dae Hyun,Ahn, Jong Kee,Lee, Jae Hyun 한국재료학회 2021 한국재료학회지 Vol.31 No.7
The effect of solidification rate on micro-segregation in investment casting of IN738LC superalloy was studied. In Ni-based superalloys, the micro-segregation of solute atoms is formed due to limited diffusion during cast and solidification. The microstructure of cast Ni-based superalloys is largely divided into dendrite core of initial solidification and interdendrite of final solidification. In particular, mosaic shaped eutectic γ/γ' and carbides are formed in the interdendrite of the final solidification region in some cases. The micro-segregation phenomena formed in regions of dendrite core and interdendrite including eutectic γ/γ' and carbides were analyzed using OM, SEM/EDS and micro Vickers hardness. As a result of analysis, the lack of (Cr, W) and the accumulation of Ti were measured in the eutectic γ/γ', and the accumulation of (Cr, Mo) and the lack of Ti were measured in the interdendrite between dendrite and eutectic. Carbides formed in interdendritic region were composed of (Ti, W, Mo, C). The segregation applied to each microstructure is mainly due to the formation of γ' with Ni<sub>3</sub>(Al,Ti) composition. The Ni accumulation accompanied by Cr depletion, and the Ti accumulated in the eutectic region as a γ' forming elements. The Mo tends to diffuse out from the dendrite core to the interdendrite, and the W diffuse out from the interdendrite to the dendrite core. Therefore, the accumulation of Mo in the interdendrite and the deficiency of W occur in the eutectic region located in the interdendrite. Heat treatment makes the degree of the micro-segregation decrease due to the diffusion during solid solution. This study could be applied to the heat treatment technology for the micro-segregation control in cast Ni-based superalloys.
최병학,이범규,장현수,박찬성,김진표,박남규,김청인,김보미,Choe, Byung Hak,Lee, Bum Gyu,Jang, Hyeon Su,Park, Chan Sung,Kim, Jin Pyo,Park, Nam Gyu,Kim, Cheong In,Kim, Bo Mi 한국재료학회 2017 한국재료학회지 Vol.27 No.8
This study investigated the surface pit corrosion of SS420J2 stainless steel accompanied by intergranular crack. To reveal the causes of surface pits and cracks, OM, SEM, and TEM analyses of the microstructures of the utilized SS420J2 were performed, as was simulated heat treatment. The intergranular cracks were found to have been induced by a grain boundary carbide of $(Cr,Fe)_{23}C_6$, which was identified by SEM/EDS and TEM diffraction analyses. The mechanism of grain boundary sensitization occurred at the position of the carbide, followed by its occurrence at the Cr depleted zone. The grain boundary carbide of $(Cr,Fe)_{23}C_6$ type precipitated during air cooling condition after a $1038^{\circ}C$ solid solution treatment. The carbide precipitate formation also accelerated at the band structure formed by cold working. Therefore, using manufacturing processes of cooling and cold working, it is difficult to protect SS420J2 stainless steel against surface pit corrosion. Several counter plans to fight pit corrosion by sensitization were suggested, involving alloying and manufacturing processes.
최병학,이범규,심종헌,고형순,조남철,이재성,박경균,김유찬,Choe, Byung Hak,Lee, Bum Gyu,Shim, Jong Hun,Go, Hyung Soon,Jo, Nam Chul,Lee, Jae Sung,Park, Kyung Gyun,Kim, Yu Chan 한국재료학회 2016 한국재료학회지 Vol.26 No.10
The aim of this paper is to consider the effect of the manufacturing processes on corrosion and centerline cracking of ancient bronze spoons. The ancient bronze spoons in question were made by several steps of forging, in reheated condition with cast ingots. The manufacturing method is similar to that of the modern spoons. The investigations include observations from light and scanning electron microscopes of the microstructure in terms of the crack propagation. Cracks in the centerline are caused by solute segregation in the center-line region; this solute is solidified in the final stage of bronze spoon manufacture. Centerline cracking is also caused by ${\alpha}$ phase segregation, accompanied by forged overlapping along the longitudinal direction of the spoons. A vertical stripe with cracks along the centerline of the spoon's width is formed by folding in the wrought process. The overlapping area causes crack propagation with severe corrosion on the spoon surfaces over a period of a thousand years. The failure mechanisms of ancient bronze spoons may be similar to that of modern spoons, and the estimation of the failure mechanisms of ancient spoons can be appropriate to determine failure causes for such modern spoons.
최병학,심종헌,김승언,현용택,박찬희,강주희,이용태,김영욱,Choe, Byung Hak,Shim, Jong Heon,Kim, Seung Eon,Hyun, Yong Taek,Park, Chan Hee,Kang, Joo-Hee,Lee, Yong Tai,Kim, Young Ouk 한국재료학회 2015 한국재료학회지 Vol.25 No.8
Metastable phase characteristics of beta Ti alloys were investigated to consider the relationship of the microstructure and diffraction pattern in TEM. TEM analysis showed that the microstructure was mottled as a modulated structure, and the diffraction pattern was composed of spot streaks between the main spots of a stable beta phase with a specific lattice relationship. The modulated structure may be induced by short distance slip or atom movement during a very short interval of solution treated and quenched (STQ) materials. The athermal ${\omega}$ phase, which could be precipitated at low temperature aging, is also analysed by the metastable phase. The metastable phases including athermal ${\omega}$ phase had a common characteristic of hardened and brittle behavior because the dislocation slip was restricted by a super lattice effect due to short distance atom movement at the metastable state.
Ti-6Al-4V 레이저 클래딩부에 대한 미세조직 분석
최병학(Byung Hak Choe),이유철(You-Cheol Lee),조재훈(Che-Hoon Cho) 대한용접·접합학회 2021 대한용접·접합학회지 Vol.39 No.3
The microstructure and phase configuration of Ti-6Al-4V laser cladding were investigated in this study. The cladding microstructures were divided into the base metal of the substrate and heat-affected zone (HAZ), and the weld metal of the weld. However, depending on the location, the microstructure shapes were different, owing to heat transfer effects. The final solidification area of the weld metal had a fine structure because of the rapid cooling on contact with the air layer rather than at the center. HAZ also exhibited a relatively fine structure, owing to the rapid cooling of the substrate contact. All microstructure compositions consisted of a lamellar structure of a lean V-content α phase and a rich V-content β phase. The lamellar spacing in the HAZ was 0.5 μm, which was narrower than the 1.5 μm weld metal. However, the average hardness of the HAZ was lower than that of the 500 HV weld metal because of the low alloy content in the HAZ area, as it was welded on a pure Ti substrate. The alloy content of the weld metal welded using Ti-6Al-4V constituent electrodes was high; therefore, the estimated hardness value was high despite the coarse microstructure. In the weld area, cracks were formed in the HAZ because of the pores generated through the separation of oxides and weld residual stresses. The microstructure, phase configuration, and defects of Ti-6Al-4V laser cladding were identified, and the cladding is useful for the cladding process.