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화염급냉 표면처리된 Cu-8.8Al-4.5Ni-4.5Fe 합금의 미세구조 분석 및 내마모성에 관한 연구
이민구 ( M. K. Lee ),홍성모 ( S. M. Hong ),김광호 ( G. H. Kim ),김경호 ( K. H. Kim ),김홍회,( W. W. Kim ) 한국열처리공학회 2004 熱處理工學會誌 Vol.17 No.6
N/A The flame quenching process has been employed to modify the surfaces of commercial marine propeller material, aluminum bronze alloy (Cu-8.8AI-5Ni-5Fe), and the microstructure, hardness and wear properties of the flame-quenched layers have been studied. The thermal history was accurately monitored during the process with respect to both the designed maximum surface temperature and holding time. The XRD and EDX analysis have shown that at temperatures above T_(β), the microstructure consisting of α+κ phases changed into the α+ß` martensite due to an eutectoid reaction of α+κ→ß, and a martensitic transformation of ß→ß`. The ß` martensite phase formed showed a face-centered cubic (FCC) crystal structure with the typical twinned structure. The hardness of the flame-quenched layer having the α+ß` structure was similar to that of the α+ß structure and depended sensitively on the size and distribution of hard κ and ß` phases with depth from the surface. As a result of the sliding wear test, the wear resistance of the flame-quenched layer was markedly enhanced with the formation of the ß` martensite.
부양가스증발응축법으로 제조된 Ti-Ni 합금 나노분말의 특성 연구
한병선,엄영랑,이민구,김길무,이창규,Han, B.S.,Uhm, Y.R.,Lee, M.K.,Kim, G.M.,Rhee, C.K. 한국분말야금학회 2006 한국분말재료학회지 (KPMI) Vol.13 No.6
The Ti-Ni alloy nanopowders were synthesized by a levitational gas condensation (LGC) by using a micron powder feeding system and their particulate properties were investigated by x-ray diffraction (XRD), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) method. The starting Ti and Ni micron powders $150{\mu}m$ were incorporated into the micron powder feeding system. An ingot type of the Ti-Ni ahoy was used as a seed material for the levitation and evaporation reactions. The collected powders were finally passivated by oxidation. The x-ray diffraction experiments have shown that the synthesized powders were completely alloyed with Ti and Ni and comprised of two different cubic and monoclinic crystalline phases. The TEM results showed that the produced powders were very fine and uniform with a spherical particle size of 18 to 32nm. The typical thickness of a passivated oxide layer on the particle surface was about 2 to 3 nm. The specific surface area of the Ti-Ni alloy nanopowders was $60m^2/g$ based on BET method.
자기펄스 압축성형법에 의한 다이아몬드 공구용 세그먼트 분말 성형
윤종수,이정구,이민구,이창규,박문석,홍순직,Yun, J.S.,Lee, J.G.,Lee, M.K.,Rhee, C.K.,Park, M.S.,Hong, S.J. 한국분말야금학회 2008 한국분말재료학회지 (KPMI) Vol.15 No.5
This article presents the successful consolidation of the mixed Co and Diamond powders for a drilling segment by the combined application of magnetic pulsed compaction (MPC) and subsequent sintering, and their properties were analyzed. Homogeneous hardness (Hv 220) and density (97%) of sintered bulks fabricated by MPC were obtained by the new technique, where higher pressure has been employed for short period of time than that of general process. A fine microstructure and homogeneous hardness in the consolidated bulk were observed without cracks. Relatively higher drilling speed of 9.61 cm/min and life time of 6.55 m were found to the MPCed specimens, whereas the value of the specimens fabricated by general process was 11.71 cm/min and 7.96 m, respectively. A substantial improvement of mechanical properties of segment was achieved through this study.
화염경화 표면처리 공정에 의한 12Cr 강의 잔류응력 거동
이민구(M. K. Lee),김광호(G. H. Kim),김경호(K. H. Kim),김흥회(W. W. Kim) 한국표면공학회 2004 한국표면공학회지 Vol.37 No.4
The residual stresses on the surfaces of low carbon 12Cr steels used as a nuclear steam turbine blade material have been studied by controlling the flame hardening surface treatments. The temperature cycles on the surfaces of 12Cr steel were controlled precisely as a function of both the surface temperature and cooling rate. The final residual stress state generated by flame hardening was dominated by two opposite competitive contributions; one is tensile stress due to phase transformation and the other is compressive stress due to thermal contraction on cooling. The optimum processing temperatures required for the desirable residual stress and hardness were in the range of 850℃ to 960℃ on the basis of the specification of GE power engineering. It was also observed that the high residual tensile stress generated by flame hardening induced the cracks on the surfaces, especially across the prior austenite grain boundaries, and the material failure virtually, which might limit practical use of the surface engineered parts by flame hardening.