발간논문

Home > KJMM 논문 > 발간논문

SUH660 is a precipitation hardening stainless steel which has high strength at moderately high temperature. In this study, the effect of a secondary aging-treatment after the standard 720 °C heat-treatment was investigated. The Rockwell C hardness test with detailed TEM observerations was performed to study the steel’s hardening mechanism. The additional secondary treatment noticeably increased hardness and tensile strength. In particular, a secondary aging treatment at 630 °C for 24 hrs resulted in the maximum hardness of 35.5 HRC, which is higher than the hardness of a standard heat-treated sample by around 4 HRC. A transmission election microscopy study revealed the size distribution of γ′ and h precipitates after standard aging treatment. In contrast, the  precipitate was not oberserved after the 630°C secondary aging-treatment. Contrary to expectation, the γ’’ precipitate was not observed regardless of aging-treatment. The mechanism of the hardening process is discussed.

keyword : SUH660, superalloy, precipitation hardening, aging treatment, hardness

The effect of a small amount of added Mg on the microstructure. Fe-Al inhibition layer, corrosion resistance and welding properties of hot-dip galvanized and galvannealed steel sheets were investigated. Both the microstructure and distribution of Mg were measured by scanning election microscopy, transmission electron microscopy, glow discharge spectrometer and electron probe micro analyzer. A thin Mg layer was inhomogeneously precipitated between the Zn-coating and Al inhibition layer. During the annealing process around 470-510°C, the Mg layer was completely diffused into the inside and/or extreme surface of the Zn coating. The corrosion resistance of galvannealed steel sheets produced in a commercial galvanizing line was significantly improved by adding a small amount of Mg into the molten zinc pot. Fe content, the formation of the Fe-Al inhibition layer and weldability were also influenced by the Mg addition.

keyword : Mg adding, corrosion resistance, galvanized and galvannealed coatings

In this work, both experimental and numerical studies were conducted to investigate the densification behavior of stainless steel 316L (STS 316L) powders during hot isostatic pressing (HIP), and to characterize the mechanical properties of HIPed specimens. The HIP experiments were conducted with gas atomized STS 316L powders with spherical particle shaped under controlled pressure and temperature conditions. The mechanical properties of HIPed samples were determined on a series of tensile tests, and the results were compared to a reference STS 316L sample prepared by the conventional process, i.e., extrusion and annealing process. Corresponding microstructures before and after tensile tests were observed using scanning electron microscopy and their relationships to the mechanical properties were addressed. Furthermore, a finite element simulation based on the power-law creep model was carried out to predict the density distribution and overall shape change of the STS316L power compact during HIP process, which agreed well with the experimental results.

keyword : metals, hot isostatic pressing(HIP), densification, mechanical properties, computer simulation

This study conducted an analysis of the carbothermal reduction of oxidized WC/Co hardmetal scrap with solid carbon under an argon and hydrogen gas atmosphere. The WC/Co hardmetal scrap oxide consisted of WO₃ and CoWO₄. WO₃ was reduced before CoWO₄ by the solid carbon regardless of gas atmosphere. The simple oxide WO₃ transformed to a tungsten carbide phase without the formation of tungsten under argon atmosphere. However, WO₃ reduced to tungsten and then tungsten carbide was formed under the hydrogen atmosphere. Further, the complex oxide CoWO₄ transformed to tungsten carbide and cobalt phases via a Co-W-C system complex carbide phase under the argon atmosphere. However, in contrast to the reaction under the argon atmosphere, under the hydrogen atmosphere an additional phase transformation route was observed, i.e., the formation of tungsten carbide and cobalt phases from CoWO₄ through the formation of tungsten oxide and cobalt-tungsten alloy phases. The tungsten carbide formation under argon hydrogen atmospheres occurred at about 950°C and 850°C, respectively.

keyword : powder, tungsten carbide, hardmetal, carbothermal reduction, phase transformation

Cordierite was synthesized using multi-component materials based on a tal0alumina-clay system. The cordierite sintered at 1360°C showed a high relative density of 98.8% and a low thermal expansion coefficient of 1.59×10-6/°C. To study the effect of adding alkaline-earth minerals on the cordierite properties, petalite, potash-feldspar, and dicalciu, phosphate were added to the synthesized cordierite. In the case of 9 wt% petalite or potash-feldspar addition, the cordierite was more densified; however, the thermal expansion coefficient and the pyroplasticity index were increased. In particular, a 5 μm thick self-glazed coating was formed with the addition of 9 wt% potash-feldspar. In the case of adding dicalcium phosphate, a glass phase was formed at low temperature and gas bubbles formed at high temperature above 1320 °C. The cordierite synthesized using multi-component materials is expected to be employed a as materials for high thermal shock, dense-microstructure flameware.

keyword : cordierite, flameware, thermal expansion, densification, thermal shock

In this study, the effect of cold drawing and heat treatment on the microstructure of Invar36 alloy wire was investigated. Invar36 alloy wire is used as a transmission line core material, and is required to have high strength. The diameter of the Invar36 alloy wire specimens were reduced from 16 mm to 4.3 mm after three cold drawing and two heat treatment processed, thereby increasing tensile strength. Specimens were taken after each of the cold drawing and heat treatment processes, and their microstructure and tensile properties were analyzed. The Invar36 alloy γ-(Fe, Ni) phase matrix before the cold drawing and heat treatment processes. After the cold drawing processes, {220} and {200} textures were mainly achieved. After the heat treatment processes, a {220} recrystallization γ-(Fe, Ni)phase was formed with fine carbides. The recrystallization γ-(Fe, Ni) phase grains had low dislocation density, so they probably accommodated a large amount of deformation during the cold drawing processes.

keyword : Invar36 alloy, wire, cold drawing, heat treatment, microstructure

During the selective extraction of Pd(Ⅱ) by LIX 63 from 6 M HCI solutions containing platinum group metals, an oxidation reduction reaction occurs between the LIX 63 and Ir(Ⅳ). Sind the reduced Ir(Ⅲ) cannot be extracted by solvating and amine extractants, the oxidation-reduction reaction has a significant effect on the separation of Pt(Ⅳ), Ir(Ⅳ) and Rh(Ⅲ). Therefore, the effect of HCI concentration on the reduction of Ir(Ⅳ) during the extraction with LIX was investigated at 3 and 6 M HCI solutions. The extraction behavior of Iridium by Aliquat 336 from the Pd(Ⅱ) free raffinate showed that the percentage of iridium extraction rapidly decreased when HCI concentration was increased from 3 to 6 M, indicating that more Ir(Ⅳ) was reduced to Ir(Ⅲ). Extraction schemes for the separation of Pt(Ⅳ), iridium and Rh(Ⅲ) by Alquat 336 from 3 and 6 M HCI solutions were investigated.

keyword : platinum, iridium, rhodium, LIX 63, separation

Ti-doped In₂O₃ (TIO) thin films were deposited on glass substrates by RF magnetron sputtering. The films were then annealed at 100, 200 or 300 °C for 30 min to investigate the effects of the annealing temperature on the films’ structural, electrical and optical properties. The films annealed at 200 °C and above were polycrystalline in phase, and their electrical resistivity decreased to as low as 7.5×10-4 Ω cm at the annealing temperature of 300°C. The films’optical transmittance in the visible wavelength region also improved from 77.7% to 81.2% when the annealing temperature was increased. The TIO films’ figures of merit were evaluated, showing that the TIO films annealed at 300 °C had better optical and electrical performance than the other films prepared using lower-temperature or no annealing.

keyword : thin film, sputtering, electrical properties, x-ray diffraction, figure of merit

Si/SiOx composite anodes were prepared using a facile solution reaction-based method. Their initial capacity was significantly improved to 1.3 times that of SiOx by incorporating 3 wt% silicon nanoparticles into SiOx. The capacity retention of the Si/SiOx anodes after 50 charge-discharge cycles was significantly enhanced, to 99%, compared with 8% retention for the pristine silicon anode. These improved results were achieved by combining the advantages of silicon, for high capacity, with SiOx, for cyclic stability. Our current results suggest that this simple solution reaction-based approach is a promising method for fabrication low cost Si/SiOx composite anode materials for lithium ion batteries, which exhibit high electrochemical performance, of more than 1000 mAh/g capacity, and cyclic stability.

keyword : energy storage materials, sol-gel, electrical properties, electrical, lithium ion battery

The effect of added WO₃ on the catalytic activity and surface characteristics of the MnOx-TiO₂ system was investigated for the low-temperature selective catalytic reduction of NOx with NH₃. Based on physio-chemical characterizations, the specific surface areas of the WO₃-MnOx-TiO₂ catalysts increased from 76.12m²/g to 90.29m²/g as the WO₃ content increased, and crystallinity of the anatase TiO₂ phase decreased. The NOx conversion efficiency of the WO₃-MnOx-TiO₂ catalysts with WO₃ contents was over 70% at 200 °C, and their efficiency was over 90% at 300 °C. The results of Fourier transform infrared spectroscopy indicated that the 1438 and 1632 cm-1 values, which were assigned to Lewis and Brønsted acid sites, increased with the increase in WO₃ content. The enhanced catalytic acidity originated from changes to the physical and chemical structures on the surface. MmOx formed on the TiO₂ surface of the 15 wt% WO₃-doped MnOx-TiO₂, and amorphous state Mn2O3 (Mn3+) and WO3 (W6+) existed on the surface, which led to the improved Lewis and Brønsted acid sites, respectively.

keyword : nanostructured materials, sol-gel, surface, raman spectroscopy, catalytic acidity