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강전연,윤재철,김호영,김병환,최중호,양상선,유지훈,김용진,Kang, Jun-Yun,Yun, Jaecheol,Kim, Hoyoung,Kim, Byunghwan,Choe, Jungho,Yang, Sangsun,Yu, Ji-Hun,Kim, Yong-Jin 한국분말야금학회 2017 한국분말재료학회지 (KPMI) Vol.24 No.3
A cold-work tool steel powder is used to fabricate 3-dimensional objects by selective laser melting using a high-pressure gas atomization process. The spherical powder particles form continuous carbide networks among the austenite matrix and its decomposition products. The carbides comprise Nb-rich MC and Mo-rich $M_2C$. In the SLM process, the process parameters such as the laser power (90 W), layer thickness ($25{\mu}m$), and hatch spacing ($80{\mu}m$) are kept fixed, while the scan speed is changed from 50 mm/s to 4000 mm/s. At a low scan speed of 50 mm/s, spherical cavities develop due to over melting, while they are substantially reduced on increasing the speed to 2000 mm/s. The carbide network spacing decreases with increasing speed. At an excessively high speed of 4000 mm/s, long and irregularly shaped cavities are developed due to incomplete melting. The influence of the scan pattern is examined, for which $1{\times}1 mm^2$ blocks constituting a processing layer are irradiated in a random sequence. This island-type pattern exhibits the same effect as that of a low scan speed. Post processing of an object using hot isostatic pressing leads to a great reduction in the porosity but causes coarsening of the microstructure.
강전연,박성원,박준영,박성준,송이화,박성택,김광련,오경원 한국소성∙가공학회 2019 소성가공 : 한국소성가공학회지 Vol.28 No.5
In this study, the development of annealing textures in cold rolled and annealed tantalum sheets was analyzed using electron backscatter diffraction. At 900˚C, the textures of the recrystallized grains in the partially and completely recrystallized microstructures displayed significant similarities. The average diameter of the recrystallized grains with γ-fiber orientations exceeded that of grains with different orientations, and the average growth rates were unrelated to the orientations after an initial stage of recrystallization. Additional cold rolling and annealing was done for controlled initial microstructures and textures inherited from various processes of prior cold rolling and annealing. This second cycle of the process resulted in stronger textures with major γ-fiber orientations as a result of the enhanced γ-fiber orientations in the preceding textures. A coarse-grained prior microstructure resulted in a weaker annealing texture than a fine grained one regardless of the stronger previous texture, which was occasioned by the sub-structures of the minor orientations at local deformation inhomogeneities such as sharp in-grain shear bands.
강전연,박준영,김호영,김병환,Kang, Jun-Yun,Park, Jun-Young,Kim, Hoyoung,Kim, Byunghwan 한국열처리공학회 2017 熱處理工學會誌 Vol.30 No.5
The microstructure of a high-carbon and high-chromium cast steel (HK700) for cold-work die inserts was analyzed by advanced scanning electron microscopy. A continuous network of primary $M_7C_3$ carbide was developed among austenitic matrix after casting. A small amount of $M_2C$ was added to the carbide network owing to the enrichment of Mo and W during the solidification. After quenching in which the austenitization was performed at $1030^{\circ}C$ and double tempering at $520^{\circ}C$, the network structure of $M_7C_3$ was preserved while most of the matrix was transformed to martensite because of additional carbide precipitation. The $M_2C$ in the as-cast microstructure was also transformed to $M_6C$ due to its instability. The continuous network of coarse carbides owing to the absence of hot-working had little influence on the hardness after quenching and tempering, whereas it resulted in severe brittleness upon flexural loading.
Effect of tungsten on the oxidation of alumina-forming austenitic stainless steel
강전연,하헌영,김성대,박준영,장민호,이태호 한국현미경학회 2019 Applied microscopy Vol.49 No.6
As more W replaced Mo in alumina-forming austenitic stainless steels, weight gain by oxidation decreased after 336 h at 1053 K. Electron microscopy revealed slower growth of scale in the presence of more numerous second phases by W addition. The retardation of oxidation was attributed to the necessary partitioning of W in front of the metal-oxide interface. The W-rich second phases interacted with growing oxides and finally transformed to fine particles of metallic W alloy within the scale.
강전연 ( Jun-yun Kang ),박성원 ( Seongwon Park ),박준영 ( Jun Young Park ),박성준 ( Seong-jun Park ),송이화 ( Yi-hwa Song ),박성택 ( Sung-taek Park ),김광련 ( Gwang-lyeon Kim ),오경원 ( Kyeong-won Oh ) 한국열처리공학회 2018 熱處理工學會誌 Vol.31 No.6
Two different modes of rolling were applied to control the texture development in tantalum sheet. In the conventional uni-directional rolling, the typical rolling textures of a body-centered cubic metal which was primarily composed of <110>//(rolling direction) was developed. In a cross rolling where the specimen was rotated by 90o between each pass, the rotated cube components, i.e. {100}<011> were greatly reinforced. The prediction of lattice rotation by the full-constraint Taylor model showed that the high stability and the symmetry of the rotated cube components caused their strengthening in cross-rolling. The two specimens were heated to 1,100℃ at 9℃/ min and held for 1 hour for annealing, then cooled to room temperature in atmosphere. In spite of the significant difference in the deformation textures, the annealing textures were very similar. They developed strong <111>// (plane normal) components with negligible intensity at the rotated cube components, which was attributed to the negligible capability of the latter components to provide effective recrystallized grains. (Received September 10, 2018; Revised October 11, 2018; Accepted October 17, 2018)
매트릭스(matrix)형 냉간금형강의 기계적 특성에 미치는 NbC 탄화물 첨가의 영향
강전연 ( Jun Yun Kang ),김호영 ( Ho Young Kim ),손동민 ( Dong Min Son ),이재진 ( Jae Jin Lee ),윤효윤 ( Hyo Yun Yun ),이태호 ( Tae Ho Lee ),박성준 ( Seong Jun Park ),박순근 ( Soon Keun Park ) 한국열처리공학회 2015 熱處理工學會誌 Vol.28 No.5
Various amount of NbC carbide was intentionally formed in a matrix-type cold-work tool steel by controlled amount of Nb and C addition. And the effect of NbC addition on the mechanical properties was investigated. Four alloys with different Nb and C contents were cast by vacuum induction melting, then hot forging and spheroidizing annealing were conducted. The machinability of the annealed specimens was examined with 3 different cutting tools. And tensile tests at room temperature were conducted. After quenching and tempering, hardness and impact toughness were measured, while wear resistance was evaluated by disk-on-plate type wear test. The increasing amount of NbC addition resulted in degraded machinability with increased strength, whereas the absence of NbC also led to poor machinability due to high toughness. After quenching and tempering, the additional NbC improved wear resistance with increasing hardness, whereas it deteriorated impact toughness. Therefore, it could be found that a moderate addition of NbC was desirable for the balanced combination of mechanical properties. (Received July 23, 2015; Revised August 5, 2015; Accepted August 14, 2015)
고성능 냉간금형강의 미세조직과 기계적 특성: 레데부라이트(ledeburitic) 및 매트릭스(matrix)형 강종의 비교
강전연 ( Jun Yun Kang ),김호영 ( Ho Young Kim ),손동민 ( Dong Min Son ),이재진 ( Jae Jin Lee ),윤효윤 ( Hyo Yun Yun ),이태호 ( Tae Ho Lee ),박순근 ( Soon Keun Park ) 한국열처리공학회 2015 熱處理工學會誌 Vol.28 No.4
Two types of advanced cold-work tool steels were characterized and compared. A higher-alloyed ledeburitic steel with primary carbides (denoted as 9Cr) and a lower-alloyed steel without primary carbides (5Cr) were fabricated by vacuum induction melting and subsequent hot forging. They were spheroidizing-annealed at 870℃, quenched at 1030℃ and tempered at 180 or 520℃. Their machinability after annealing and hardness, impact toughness, wear resistance after tempering were compared and interpreted in association with their characteristic microstructures. After annealing, 5Cr showed higher resistance to machining due to higher ductility and toughness in spite of lower strength and smaller carbide volume. Owing to smaller carbide volume fraction and the absence of coarse primary carbides, 5Cr showed even better impact toughness although the hardness was lower. The improved toughness of 5Cr resulted in excellent wear resistance, while smaller volume fraction of retained austenite also contributed to it.