Journal of Powder Materials

R&D TRends of Thermal Sprayed Coatings: J Korean Powder Metall Inst. 1996;3(2):79-85.

Effects of the Addition of Other Carbides on the Properties of Submicron WC-10% Co Cemented Carbides: J Korean Powder Metall Inst. 1996;3(2):86-90.

Abstract PDF: The effects of added VC, Cr_3C_2 and TaC on the microstructures and properties of submicron WC-10%Co cemented carbides. The relative sintered density of compact was increased by addition of Cr_3C_2 but decreased oppositely by addition of VC or TaC. The growth of WC grains was significantly suppressed by addition of these carbides. The hardness of these alloys was increased by addition of other carbides and showed a maximum value by simultaneously added VC and Cr_3C_2. The transverse rupture strength(T.R.S.) was in- creased by addition of Cr_3C_2, while it was decreased by addition of VC or TaC. The relative sintered density and T.R.S. of these alloys were improved by HIP-treatment. The maximum T.R.S. was 328kg/mm2 in the Wc-10%Co cemented carbide with addition of 0.5%VC.

EffEct of vacuum annealing on an oxidation of milled WC-Co powder: J Korean Powder Metall Inst. 1996;3(2):91-96.

Abstract PDF: The effect of vacuum annealing on the oxidation behavior of milled WC-15%Co powder mixture has been studied. A cobalt component in the milled powder mixture was oxidized preferentially above 175°C in air. The specimens showed a steady increase in weight at 175°C but did constant weight followed by rapid increase in specimen weight at the beginning above 200°C. Oxidation of the milled powder mixture was significantly suppressed by vacuum annealing at 300°C for 10 h. Suppression of oxidation by vacuum annealing and different oxidation behaviors of the milled powder mixture between 175°C and 200°C, were attributed to removal of strain energy stored in the cobalt powder during vacuum annealing or oxidation treatment above 200°C. The role of stored strain energy on oxidation of milled WC-15%Co powder mixture was proved by X-ray diffraction method and differential thermal analysis.

Characteristics of Sintering Densification of Co and Fe+Co Fine Powders: J Korean Powder Metall Inst. 1996;3(2):97-103.

Abstract PDF: The densification of the compacts of pure Co, Fe+50%.Co and Fe+25% Co sintered under H_2 gas or in vacuum was investigated. The effects of AL, Nb, Ti, and V additions on the densification were also studied. The sintered compact of Co was fully-dense when the density of the compact was lower than Dgc. However, above Dgc, it was never fully-dense regardless of sintering atmosphere, temperature, and time. The densification of sintered compacts of Fe-50% Co and Fe-25% Co were always incomplete. While the addition of AL made all compacts fully-dense, the addition of Ti was effective for the compacts of Co and Fe-25% Co. V was effective only for the Fe-25% Co. These results tell us that the particle size of Co powder, the amount of Fe, and the amount of additives forming stable oxides play on important role for the complete densification. Therefore it is desirable to reduce or eliminate the equilibrium pressure of H_2O or CO in isolated pores to obtain a fully-dense sintered compact.

Three Dimensional Finite Element Analysis for Powder Forging Process: J Korean Powder Metall Inst. 1996;3(2):104-111.

Abstract PDF: In order to obtain homogeneous and high quality products in powder compaction forging process, it is very important to control stress, strain, density and density distributions. Therefore, it is necessary to understand quantitatively the elasto-plastic deformation and densification behaviors of porous metals and metal powders. In this study, elasto-plastic finite element method using Lee-Kim's pressure dependent porous material yield function has been used for the analysis of three dimensional indenting process. The analysis predicts deformed geometry, stress, strain and density distribution and load. The calculated load is in good agreement with experimental one. The calculated results do not show axisymmetric distributions because of the edge effect. The core part which is in contact with the indentor and the outer diagonal edge part are in compressive stress states and the middle part is in tensile stress state. As a results, it can be concluded that three dimensional analysis is more realistic than axisymmetric assumption approach.

Studies on Ceramic Powder Fabrication from Rice Phytoliths. II. Obtaining Amorphous Silica Powder by Acid Treatments.: J Korean Powder Metall Inst. 1996;3(2):112-119.

Abstract PDF: Using rice husks pulverized by rotating knife cutter, the raw rice husk powder was solution treated by nitric and hydrochloric acids in order to separate phytolith from organic constituents. Because of the strong resistance of organic components of rice husk to acids, the raw powder had to be boiled in concentrated acids up to 300 min. By boiling in nitric acid for 60~120 min, all organic components were resolved while amorphous silica Powder of about 20 nm in size and of higher than 99.8% in purity was left behind. Inferior to the nitric acid, hydrochloric acid was not able to resolve organic component completely leaving unresolved matter of about 40% by weight even after 300 min of boiling. From the acid treatments and impurity analyses, it is considered that most of metallic elements in rice husks are combined to organic components that are easily soluble to acids.

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