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The effects of cooling after solution heat treatment on the microstructure and mechanical properties of light-weight steels were investigated using Fe-30Mn-xAl-0.9C alloys containing 9.0-12.8 wt% Al. Lab-scale specimens (thickness: 10 mm) of the alloys were cooled by different cooling rates from -337 to -0.053 ℃/s using water, air, and furnace cooling. As the cooling rate decreased, hardness of the alloys increased due to precipitation of κ-carbides. However, reduction of the cooling rate induced the growth of intergranular κ-carbides. This resulted in the decrease of impact absorbed energy of the alloys at room temperature, and exhibited intergranular brittle fracture behavior. The 12.8 wt% Al alloy cooled at the slowest cooling rate in particular showed the formation of β-Mn. To estimate the cooling rates of large-scale slabs of light-weight steels, finite element simulations were conducted. The cooling rates at the center of the slab under air and water cooling (free convection) were calculated to be -0.049 and -0.15 ℃/s, respectively. The results indicated that water cooling could prevent the excessive formation of κ-carbides/β-Mn during fabrication of large-scale slabs of alloys containing an Al content lower than 10.5 wt%. (Received September 1, 2017; Accepted September 20, 2017)

keyword : light-weight steel, microstructure, cooling rate, κ-carbide, β-Mn

The authors report the fabrication of a flexible nickel brazing foil using powdered nickel alloy filler (BNi-2) mixed with an organic binder. The organic binder was composed of a polyacrylic acid polymer, glycol, carbon tetrachloride, and water as a dispersion medium for the powder. The brazing paste so formed was then tape-cast on a polymer foil with different polymer to paste (dispersant) ratios, of □ 9:1, 8:2 and 7:3 by weight, and was dried in a low-temperature oven. The filler pyrolysis temperature, paste spreading, permeability, and wettability were examined. The thermal analysis results showed that the filler paste decomposition temperature was in the range of 463-498 ℃, while the tape-cast brazing foil had a pyrolysis temperature of □334.36 ℃. The resulting flexible self-adhesive foil was used to braze steel foils. The AISI 304 steel joint microstructure and joint tensile shear tests were also performed. It was found that the brazing foil produced □1 wt% of residue after melting. The microstructural analysis showed a uniform distribution of a Cr-rich in Ni-rich matrix at a polymer to dispersant ratio of 8:2. It is suggested that the wettability of the brazing foil on AISI 304 steel will be maximum, and a higher joint strength can be obtained when the polymer to dispersant ratio is kept at 8:2. (Received May 29. 2017; Accepted September 14, 2017)

keyword : joining, brazing, stainless steel, adhesives, thermal analysis, flexible

In this research, we studied the effects of Ni, Ti, and B on the elastic modulus of hypereutectic Al-Si alloys and successfully synthesized 100 GPa Al casting alloys by designing various alloys phases. Generally, Al-Si based alloys are known in the automotive industry to be good casting alloys with high wear resistance, low thermal-expansion coefficient, good corrosion resistance at a wide range of temperatures, especially for cylinder blocks, cylinder heads, pistons and valve lifters, but the low elastic modulus of the alloys remaines a weakness. Previously, the elastic modulus of eutectic Al-Si casting alloys was enhanced by 10 GPa by strengthening phases with added Ti and B. However, elastic modulus was not enhanced by increasing Si content in the hypereutectic Al-Si alloys, since Si only reacts with the Ti containing strengthening phase. We overcame this problem by adding Ni to Ti and B containing hypereutectic Al-Si alloys. The effects of the strengthening phases produced by the addition of Ni, such as AlNi, AlNiCu, on the elastic modulus were studied, and we observed a two-fold enhancement in the wear resistance of the Ni, Ti, and B containing hypereutectic Al-Si alloys. The rate of strengthening by phases in the alloys was qualitatively analyzed by EPMA (Electron probe micro-analyzer), SEM (Scanning Electron microscopy), etc, and their mechanical properties were quantitatively evaluated by elastic modulus measurement and pin-on tester. (Received July 5, 2017; Accepted September 13, 2017)

keyword : elastic modulus, Al alloy, reinforcement, Ni addition, wear resistance

The effect of pre-aging treatment on the microstructure and mechanical properties of Al-0.6Mg-1.2Si aluminum alloy sheets fabricated by twin roll casting (TRC) was investigated. It was observed that two kinds of nanoclusters, cluster(1) and cluster(2), play an important role in the bake-hardening properties of Al-Mg-Si aluminum alloy. The hardness of the TRC Al-0.6Mg-1.2Si alloy sheets was continuously increased up to 48 h by natural-aging, and the highest yield strength value was about 146 MPa. The yield strength was 280 MPa after bake-hardening at 180 ℃ for 30 min immediately after quenching, whereas the yield strength was 172 MPa when natural-aging proceeded for about 1 week before bake-hardening. This result is due to fact that cluster(1) is formed during natural-aging at room temperature and has detrimental effects on bake-hardening. On the other hand, when pre-aged at 150 ℃ for 10 min, cluster(2) predominantly formed and promoted the precipitation of β″ phase. The yield strength was 256 MPa after bake-hardening, which indicates that the pre-aging treatment is an effective method, and also depresses the negative effects of natural-aging and promotes β″ precipitation. The Al-Mg-Si aluminum alloy sheets fabricated by twin roll casting method exhibited mechanical properties equal to or higher than those of the direct-chill (DC) casting sheets despite the simplified manufacturing process. (Received July 10, 2017; Accepted September 15, 2017)

keyword : aluminium, twin roll casting, bake-hardening, yield strength, precipitation

This study investigated the microstructure and the ambient-elevated temperature compression and elevated temperature creep properties of Fe-14Cr ferritic ODS steel manufactured by mechanical alloying (high energy ball milling) and hot extrusion processes. Initial microstructural observation of the as-extruded material identified elongated grains in the direction of extrusion; however, some recrystallization and grain growth were observed in the heat-treated material. The average grain sizes of the as-extruded material and heat-treated material were 0.4 and 1.1 μm, respectively. In addition, the heat treatment resulted in the increase in number density and the decrease in size of the nanocluster. A room temperature compression tests confirmed the heat-treated materials (1457 MPa) had relatively higher yield than the as-extruded material (1377 MPa), and a general tendency was observed that the difference in yield strength decreased as temperature increased. Furthermore, elevated temperature compressive creep tests confirmed that the heat-treated material had relatively greater creep resistance compared to the as-extruded material at all stress levels. (Received July 14, 2017; Accepted September 18, 2017)

keyword : mechanical alloying, oxide dispersion strengthening, microstructure, high temperature compression, high temperature creep

The effects of Nb on grain refinement and the pitting corrosion resistance of Febalance18Cr10 Mn0.3C0.3NxNb (x = 0.1 and 0.2 wt%) high interstitial alloys were investigated through microstructure analysis and potentiodynamic polarization tests. The resistance to pitting corrosion of the two alloys, covered with a passive film and an air-formed oxide films, was also examined. As the Nb content increased from 0.1 to 0.2 wt%, the grain size decreased, the volume fraction of Nb(C,N) particles increased, and the pitting potential was elevated. In addition, the two alloys covered with the passive film exhibited higher pitting potentials than the alloys with the air-formed oxide films. The physico-chemical and electronic properties of the passive film formed in an aqueous environment, and air-formed oxide of the two alloys were examined using X-ray photoelectron spectroscopy and Mott-Schottky analysis. As a result, it was revealed that the defect density level in the passive film was the dominant factor in determining the pitting corrosion resistance of the specimens. Nb addition was found to promote the formation of a protective passive film with low defect density, which resulted in the increase in the pitting potential. Moreover, the fact that the point defect density of the passive film was smaller than the air-formed oxide films was observed in both alloys, which can explain the higher pitting corrosion resistance of the passivated specimen than the specimen covered with an air-formed oxide film. (Received July 5, 2017; Accepted September 25, 2017)

keyword : high interstitial alloys, Nb, grain refinement, pitting corrosion, Mott-Schottky analysis

To improve the hot-workability of free-machining austenitic stainless steel, we studied the effects of homogenization treatment, Mn/S ratio and the retained δ-ferrite on the hot-workability of 303 austenitic stainless steels. The reduction of area was measured as a function of homogenization temperature in the range of 1160-1250 ℃ and 1-50 s^-1 using a Gleeble 3800. The hot-workability of the 303-series austenitic stainless steels reached a maximum value at a homogenization temperature of ~1240 ℃. The high temperature (e.g. ~1250 ℃) hot-workability increased with the Mn/S ratio and reached a constant value above ~7.0. At the same time, medium temperature (e.g. ~900 ℃) hot-workability increased linearly with the Mn/S ratio. The fraction of δ-ferrite decreased with increasing Mn/S ratio in STS 303-series VIM ingots. The retained δ-ferrite was found to seriously deteriorate hot-workability at above a critical volume fraction. The critical value was determined to be ~0.06. The Cr content in the (Mn, Cr) S compounds decreased with increasing Mn/S ratio. The hardness of (Mn, Cr) S increased with increasing Cr content. The hot ductility deteriorated with increasing Cr content within MnS. The evaluation of the harmful effects of Pb_eq. on the hot workability of a STS 303 series billet was ineffective. (Received August 4, 2017; Accepted August 30, 2017)

keyword : hot workability, STS303, austenitic stainless steel, free-machining stainless steel, Mn/S ratio, delta-ferrite

Since strength and ductility have a trade-off relationship in Al alloys, controlling these properties using heat treatments requires careful attention to the dissolution and evolution of the reinforcing phases. In this study, the effects of solution and aging treatments on the microstructure and mechanical properties of Al-5.2Si-3.4Cu-0.28Mg alloy were investigated. 1-step and 2-step solution treatments were conducted at 495 and 520 ℃ with various holding times, which were determined by thermal analysis. At 495 ℃, the eutectic θ(Al₂Cu) phases were quickly dissolved while block-shaped θ phases remained, even until 13 hours. During the 2-step treatment at 520 ℃, these remaining phases were melted both in the grains and boundaries. Although all of the Q(Al₅Cu₂Mg₈Si₆) phases were completely dissolved in the 2-step treatment, the hardness of the 2-step treated specimen was lower than the 1-step treated one. After solution treatments at 495 ℃/9 hours, where the highest hardness was observed, artificial aging treatments were conducted at 160 and 200 ℃ with various times. Yield and ultimate tensile strengths showed similar values at the peak aging regardless of the temperature, while higher hardness (117%) and elongation (281%) were obtained at 160 than 200 ℃. Based on the observed results, the correlations between the microstructures and mechanical properties by the heat treatments were investigated. (Received July 5, 2017; Accepted September 18, 2017)

keyword : Al alloy, solution treatment, aging, microstructure, mechanical properties

In the iron ore sintering process, a thermal imbalance can occur in the sinter feed bed which degrades the quality of the sintered ores. In order to solve this thermal imbalance, it is necessary to control the sizes of the sintering feed charging through the chute by designing an appropriate chute shape, and to improve particle size segregation. In this study, a discrete element method(DEM) simulation was applied to the sinter feed charging process to investigate charging characteristics, such as the escape velocity of particles, size segregation intensity and porosity in the sinter feed bed, depending on the shape of the chute. A charging analysis using three types of chute found that horizontal velocity, segregation intensity and porosity in the lowest position in the bed were the highest when a curved chute was used. Therefore, it seems that a curved chute would be effective for solving the thermal imbalance caused by segregation charging. However, in order to use a curved chute, it is necessary to find a way to maintain uniform permeability in the sinter feed bed. (Received May 23, 2017; Accepted August 30, 2017)

keyword : powder processing, segregation, computer simulation, sintering process, DEM

The boron alkali molybdate glass system was investigated as lead-free low-temperature melting sealing materials, with different boron oxide contents between 0 and 8 mol%. The glasses were characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), infrared spectroscopy (IR), and by densities. Their chemical properties were measured by their dissolution rates (DR) and by Product Consistency Test (PCT). As the boron oxide contents were increased, the glasses maintained their low temperature properties (Tg under 175 ℃) and new network formers of [BO₃] triangular units and [BO₄] tetrahedral units appeared. The results confirmed that boron oxide doped molybdate glass is a candidate low-temperature sealing material. The introduction of small amounts of B₂O₃ causes a change in the amorphous network’s main structural units from molybdate units to a mixture of molybdate and borate units. The glasses possess MoO₄, MoO₆, BO₃ and BO₄ groups as basic structural units. In this study, the amount of MoO₃ was much higher than that of B₂O₃, so the boroxol ring structure occurred much less frequently than molybdates as a network former. But addition of B₂O₃ leads to an increase in the number of bridging oxygens and strengthens the glass network. As a result, the overall glass system becomes more rigid, and chemical durability is improved. (Received May 8, 2017; Accepted July 31, 2017)

keyword : lead-free low-melting sealing glass, molybdenum, boron oxide, glass structure