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The effects of Aluminum and Copper addition to the cast iron in hypoeutectic composition range were studied on the shape of graphite, the structure of matrix and the mechanical properties. The results obtained are as follows: 1) When the addition was in the form of Al-Cu (50 to 50) alloy, the shape of graphite was uniformed flake graphite (A Type on A.S.T.M). The tendency of such phenomena was more distinct at the 2.0 wt.% addition of Al-Cu alloy. 2) The addition of Aluminum and copper strengthened the matrix structure. 3) The tensile strength and hardness were increased by the addition of either Al or Cu and Al-Cu alloy. In the case of Al-Cu alloy addition, the tensile strength was increased by 20% and 7% compared with that when either Al or Cu was added, respectively.
Mechanical alloying of Al-8wt % Fe mixed powder has been studied. The effects of process control agent(stearic acid), the ratio of grinding media/powder and mechanical alloying times on the microstructure were investigated. Differential Scanning Calorimetry(DSC) measurements indicated that most of process control agent was dispersed through the body of each mechanically alloyed powder particle during milling. After 1000 minutes milling, mechanically alloyed particles were shown the character of steady state. At steady state the particles have a uniform size distribution, a saturation hardness, an equiaxed shape, a composite structure and a chemical homogeneity. The intermetallic compound phase of Al₃Fe was identified by Xray diffraction analysis.
Recrystaltization at the surface of specimen was observed during heat treatment due to lathe machining or shot peening. In order to investigate the effect of surface recrystallization on creep properties, creep test was conducted at 1050℃ under a stress of 123MPa. Creep rupture lives were compared among DS specimen without surface recrystallized layer, specimens with surface recrystallized layer of 80∼100㎛ and 160-180㎛. Creep rupture time of DS specimen without surface recrystallized layer was longer than DS specimen with surface recrystallized layer (160-180㎛), which was reduced more than the specimen with surface recrystallized layer(80-100㎛). Microcracks were formed around γ´ denuded zone.
Precipitation structure, physical and mechanical properties have been examined in Al-Cu-Li-Mg-Ag-Zr(Cl) and Al-Li-Cu-Zr(2090) alloys using hardness and density measurement, tensile test, transmission electron microscopy. And, in an attempt to assess the composition of precipitate, energy dispersive X-rey microanalysis was performed using the scanning transmission electron microscopy technique. The tensile strength 630MPa, density 2.69g /㎤, hardness number H_RB 96 and specific strength extremely exceeding that of 2090 and 7075-T6 alloy are achived. In the near peak-aged condition, finer and homogeneous distribution of θ' and T₁, phases than that of an 2090 alloy were precipitated and due to small amount of Mg addition, S' phase in Al-Cu-Mg alloy was precipitated. Energy dispersive X-ray spectra microanalysis was performed on the relatively coarse black particle obserbed in the matrix and matrix phase. The result of EDXS microanalysis, the X-ray spectrum from the matrix reveals Al and Cu peak but black particle reveals Al, Cu and Ag peak which does not reveal from the matrix.
The microstructure and the fracture toughness of the weld-heat-affected zone of a ductile cast iron haute been investigated with a thermal cycle simulator to simulate the actual welding process. The matrix structure of the heat-affected zone transformed to martensite, mixture of martensite and pearlite, pearlite, and pearlite including small amount of ferrite, in turn, wish increasing cooling time from 800℃ to 500℃. The use of a proper preheat temperature prevented the formation of martensite. The welding condition for this involved a preheat temperature of 200℃ and a heat input of 30 KJ/㎝. Also, a post-weld-heat treatment of 675℃ for 2 hours was effective in reducing the hardness of ductile cast iron weldments by eliminating mariensite. The secondary graphite formed due to martensite decomposition at that title. An instrumented Charpy impact test has been carried out on a welded and post-weld-heat-treated ductile cast irons, respectively. Results showed that the dynamic fracture toughness increased slightly with higher preheat temperature and heat input. The post-weld-heat treatment improved the dynamic fracture toughness of the weldments to three times that of the as-welded condition.
1. 緖論 高珪素-Al 合金은 鑄物用 Al合金材로서 熱膨脹係數가 적고 耐磨耗性도 우수하여 그 用途가 넓고 重要한 合金이다. 그러나 이 合金의 實用化에 있어서의 결함은 初晶珪素의 粗大化가 기계적性質이나 機械加工性에 큰 영향을 준다는 點이다. 그러므로 初晶珪素의 微細化에 對한 硏究는 多角度로 行하여지고 있으나, 熱處理가 機械加工性에 미치는 影響에 對해서는 거의 없다. 本 實驗에서는 熱處理가 機械加工性에 미치는 영향을 鑄造材와 熱處理材로 區分하여, 切削速度, 切削깊이를 변화시켜서 微細化處理, 熱處理와 機械加工性과의 相互關係를 組織, 機械的性質, 표면거칠기, chip 형태 등으로 조사 검토하였다. 2. 實驗 方法 使用된 原料는 純 Al(ALCOA 製 97.7%)와 金屬 Si(98.0%)를 高周波 誘導爐에서 12번 흑연도가니를 使用하여 Al-Si 母合金(Si 30% 目標)를 製造하였다. 그것을 電氣爐에서 4번 흑연도가니를 使用하여 再溶解시킨후, 純 Al을 첨가하여 Al-Si合金(Si 20% 目標)를 製造했다. 初晶微細化는 所定의 微細化劑를 첨가하여 微細化處理를 行한 後, 직경 20 ㎜ø, 길이 200 ㎜의 金型을 200℃정도로 豫熱하여 溶湯을 鑄入하여 試片을 製造한다. 熱處理는 上記 試片을 515℃∼520℃에서 8시간, 176∼178℃에서 16시간 熱處理를 行하였다. 組織검사 및 硬度측정用 試片은 試片底面에서 15 ㎜되는 곳을 절단하여 측정했다. 機械加工性은 試片底面에서 20 ㎜ 되는 곳부터 平 bite를 使用하여 表面으로부터 1 ㎜ 두께로 切削한 後, 직각 bite를 使用하여, 폭 2 ㎜씩, 切削速度 및 切削깊이를 變化시키면서 切削하여 chip를 채취한다. 포면거칠기 측정은 폭 10 ㎜씩을, 切削速度를 變化시키면서 切削한 後 표면거칠기를 측정한다. 3. 實驗結果 및 考察 3. 1. 初晶Si에 미치는 熱處理의 影響 금속현미경 組織相으로는 큰 차이가 없어 Scaning으로 관찰하였다. 熱處理는 初晶 Si과 eutectic組織인 기지조직과의 境界(boundary)부근을 變化시키며 그로 因하여 機械加工性이 향상되었다. 3.2. 기지조직에 미치는 熱處理의 영향 기지조직에 대한 熱處理效果는 금속현미경 관찰로서도 뚜렷이 나타났다. Photo 1.에서 보는 것같이 熱處理하므로서 기지조직 內의 共晶 Si이 재결정하여 침상모양의 共晶 Si이 구상형태로 되었으며, 기지조직을 균질화하여 기계가공성, 특히 표면거칠기를 현저하게 향상시킨다. 3.3. 切削速度에 對하여 切削速道에 對한 표면거칠기는 切削速度에 따라 향상되었고, 熱處理로 더욱 향상시킨다. 微細化效果도 또한 표면거칠기를 향상시키나 微細化劑에 따라서는 감소시키는 것도 있다. 3.4. 切削 깊이에 對하여 高珪素-Al合金의 加工에 있어서 bite의 마모현상은 무시되나, 過多한 切削깊이는 build-up edge의 형성이 두드러져 적당한 절삭깊이의 설정이 요구된디. 熱處理나 微細化處理에 의한 영향은 절삭깊이가 적을때는 뚜렷이 나타나나 切削깊이가 클 때는 별 차이가 없다. 이것은 build-up-edge의 형성으로 切削性을 저하시키기 때문이다. 4. 結論 1) 熱處理는 기지조직內의 共晶 Si를 구상형태로 만든다. 2) 高珪素-Al合金의 切削速度는 클수록 표면거칠기가 향상된다. 3) 高珪素-Al合金의 切削깊이는 적을수록 양호하다.
HK-40 has been the predominant alloy for reformer or cracking tubes in petrochemical plants. The material degradation of HK-40 tubes in using high temperature is due to microstructural changes, especially about carbide behavior and crack formation. The structure of HK-40 is characterized by a skeletal eutectic carbide network and a huge amount of secondary carbides in an austenitic matrix. Carbides on grain boundary form continuous plate-like shape in the as-cast condition, and coalesce progressively to increase discontinuity of itself during creep. Cavities initiate at this grain boundary and cracking is caused by grain boundary sliding of carbides denuded area. The secondary carbides in matrix explicitly affect crack growth behavior and stress-rupture elongation. But the carbides precipitated in grain boundary at the first stage of heat treatment improve rupture time and strain by the grain boundary strengthening.
The effect of the graphite change and the structure of matrix makes a great difference in the properties of cast iron. Therefore the method to improve the properties of cast iron by inoculation treatment which adds Fe-Si, Ca-Si alloy just before the injection, has been studied and is being studied in these days. In this study, the increase of tensile strength by the effect of inoculation and solid solution strengthening, when the small amount of Al and Cu known as the element of graphitization was added, was studied. That is, the author increased the strength through refinement and homogenization of graphite which is caused by the addition of the graphitization elements, and through the change of the structure of matrix. The results obtained in this study were as follows; 1) The tensile strength showed an increase when the small amount of Al, Cu and Al-Cu alloy known as the graphitization elements were added just before the pouring. 2) In the case of Al addition, the cast iron with 0.75% Al showed 22% more increase in tensile strength in comparison with that without addition. 3) In the case of Cu addition, the cast iron with 1.2% Cu showed a maximum tensile strength, 37㎏/㎟, and 36% more increase in tensile strength in comparison with that without addition. 4) In the case of Al-Cu (1:1) alloy addition, the cast iron with 2.0% Al-Cu alloy showed a maximum tensile strength, and 48% more increase in tensile strength in comparison with that without addition.
In this study, CV graphite cast iron was made from adding small amount of A1 after making an incomplete spheroidal graphite by adding small amount of Fesi-Mg alloy. As a result, it was found that CV ratio and graphite length were increased as increasing amount of Al addition and holding time, but graphite width was decreased, Also, it was found that brinell hardness value 170-190 HB and tensile strength value 38-40 ㎏/㎟ of the CV graphite cast iron were closer to those values of the spheroidal than those values of the flake graphite cast iron, but thermal diffusivity value 7.8×10^(-6)㎡/sec of the CV graphite cast iron was closer to that value of the flake than that value of the spheroidal graphite cast iron.
Al₂O₃has been joined to STS304 stainless steel by a brazing method using Al-10 wt% Si insert-metal. The joining has been performed in a vacuum hot press under 4 mPa vacuum varying the conditions such as temperature(850-953 K), holding time(0-3.6 ksec) and applied contact pressure(0-9. 8 ㎫). With use of SEM, EDX and XRD observation of the morphology and microstructure for a bonded interface and the analyses for existing elements and phases have been carried out to investigate possible reaction(s) at the bonded interfaces. No reaction at the interface between Al₂O₃and insert-metal has occured but a formation of reaction layer between STS304 and insert-metal layer has been observed which is composed of Al, Si, Fe and Cr. The thickness of reaction layer increases with a joining temperature and a holding time. In the insert-metal layer the acicular eutectic Si-phase and the new ternary compound of (Al, Fe, Si) exist in the Al-matrix after the joining. The strength of joined body measured by a tensile test has been varied with the joining conditions. The maximum average strength of 32 ㎫ has been obtained from the joints which were joined at 873 K for 1.8 ksec under a contact pressure of 4. 9 ㎫.