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급속응고한 Fe-3.7wt% C-Cr 계 합금분말 압출재의 미세조직과 기계적 성질
나형용,김동익 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.12
The purpose of this work was to develop an engineering materials having a high volume fraction of carbide. For this purpose, rapidly solidified Fe-3.7wt%C-(1∼4)wt%Cr alloy powder was made using high pressure gas atomizer and followed by hot extrusion. In this study, the microstructures and mechanical properties of extruded bars were discussed. The microstructures of extruded bars were found to be the mixture of carbide and fine pearlite. The average carbide particle size was about 2㎛ and the pearlite lamellar spacing was about 0.1㎛. The tensile strength increased with increasing extrusion ratio and reached 85∼90㎏/㎟ when extrusion rate was 13. Toughness was 4∼5㎏/㎟ when extrusion race was 13. Toughness was 4∼5㎏ m/㎠ and hardness was 55∼60 HRc.
급속응고한 Al-Fe-Mo-Si 계 합금분말 압출재의 고온 성질에 관한 연구
나형용,최주태 대한금속재료학회(대한금속학회) 1993 대한금속·재료학회지 Vol.31 No.3
The microstructure and properties of the rapidly solidified Al-8Fe-2Mo-1.5Si alloy for elevated temperature applications were investigated. The effects of Mo on thermal stability and Si on fracture toughness through the observation of microstructure, hardness test and tensile test were described. The mechanical properties of extrudates with the variation of extrusion ratio and size distribution of powder were investigated. Hardness of smaller powder(50㎛) was superior to that of larger powder(150㎛). The extrudate consolidated with finer powder size distribution (10∼100㎛) yields higher strength than that with a coarse size distribution(100∼200㎛). The tensile strength of 2nd extruded bar which was consolidated with finer powders was 520MPa at 25℃, 310MPa at 275℃. The deterioration of mechanical properties at elevated temperature was due to the decomposition of Zone A and metastable phase Al_6Fe into stable phase Al₃Fe and the growth of needle-like Al₃Fe precipitates. Quarternary Mo and Si containing alloys showed uniform dispersion of fine cuboidal-polygonal precipitates, bcc Al_(12)(Fe,Mo)₃Si phase which displayed a remarkable resistance to coarsening.
흑연형태 (黑鉛形態) 에 따른 주철의 Melting Morphology 에 관한 연구
나형용,임창희 대한금속재료학회(대한금속학회) 1982 대한금속·재료학회지 Vol.20 No.12
Many studies on the solidification process of cast iron were reported, but the studios on the melting morphology of cast iron are rare. Therefore, in this work the melting has been done by rapid melting method and melting morphology has been examined, according to graphite shape in cast iron. The main results are summarized as follows: 1. In white cast iron of Fe-C-Si system, melt appears first along the interface between temper carbon and austenite matrix, after the minute temper carbon which was decomposed from Fe₃C diffuses to austenite to eutectic composition. 2. In either hypo or hyper-eutectic gray cast iron, it is observed that the melt appears first at the boundary of eutectic cell. In case of the eutectic gray cast iron, the vicinity of all graphites melts at the same time. 3. In rapidly heated and water quenched specimen of hypereutectic gray cast iron, the crystallization of fine graphites near kish graphite leads to the conclusion that kish graphite does not dissolve easily. 4. In case of nodular cast iron, the boundary between austenite shell and austenite matrix melts first and nodular graphite dissolves very slowly.
스트립캐스팅한 구상흑연주철박판의 합금원소 및 열처리에 따른 미세조직과 기계적 성질의 변화
나형용,이기락 한국주조공학회 2000 한국주조공학회지 Vol.20 No.2
Strip casting process is a new technology that makes a near net shape thin strip directly from molten metal. With this process, a large amount of energy and casting cost could be decreased from the abbreviation of reheating and/or hot rolling process. Ductile cast iron which has spheroidal graphite in the matrix is the most commercial and industrial material, because of its supreme strength, toughness, and wear resistance etc. But it cannot be produced to the thin strip owing to difficulty in rolling of ductile cast iron. In this study, ductile cast iron strips are produced by the twin roll strip caster, with different chemical compositions of C, Si, and Mn contents. And then heat-treated, microstructures and mechanical properties are examined. The microstructures of as-cast strip are that of white cast iron which consists of the mixture of cementite and pearlite, but the equiaxed crystal zone of the pearlite or segregation zone of cementite exists in the center region of the strip thickness, which cannot be observed in the rapidly solidified metallic mold cast specimens. This structure is supposed to be formed from the thermal distribution of strip and the rolling force. Comparing with the structures of each strips after heat treatment, increasing Si content makes smaller spheroidal graphite and more compact in the matrix, furthermore the less of Mn content makes the ferrite matrix be obtained clearer and easier. As a result of the tensile test of graphitization heat-treated strips, the yield strengths are about 250㎫, the tensile strengths are about 430-500㎫, and the elongations are about 10∼13%. In the case of the strip which has the smaller and more compact spheroidal graphite in the ferrite matrix, the higher tensile strength and better drawability could be obtained.
Al-Li-Cu-Mg-Zr 계 합금분말 압출재의 기계적 성질에 미치는 석출물의 영향
나형용,이동수,이성의 대한금속재료학회(대한금속학회) 1994 대한금속·재료학회지 Vol.32 No.12
The changes of microstructures and mechanical properties of Al-Li-Cu-Mg-Zr P/M alloy with various composition and extrusion variables were observed. Increasing Li content, yield stress increases but tensile stress decreases because of increasing intermetallic phases and δ phase formation. As the extrusion temperature get higher from 573K to 773K, the larger size of subgrain is obtained. Besides in case of extrustion at 773K, composite precipitates are precipitated in the matrix. The mechanical properties of extruded bars-Y.S. (570MPa), E.L. (2.6%)-were obtained by the finer microstructure. The reason is considered that composite precipitates derived cross slip or Orowan by pass of dislocation and Zr acted as grain refiner to suppress recrystallization.