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Rails are subjected to repeated stresses due to wheel-rail contact during train service. Rails under stress conditions undergo microstructural changes, and these cause degradations of the structural integrity and lifetime of rails. In this study, three different rails (newly-manufactured rail, newly-manufactured headhardened rail, and worn (used) rail) were compared to examine the effects of heat treatment and repeated wheel-rail contact stress on the microstructure and mechanical behavior of continuous welded rail. The crystal structure, constituent phase distribution, tensile property, and hardness were investigated at various locations along a cross section of the rails. All three rails consisted of a mixture of BCC and FCC crystal structures as a majority phase with a very small amount of cementite (Fe3C) as a minor phase. Rietveld analysis revealed that the weighted fractions of the BCC crystal structure were approximately 74%, 64%, and 85% for the new rail, head-hardened rail, and worn rail, respectively. While the web and foot areas of the three rails showed no significant differences in mechanical properties, the railheads of the three rails revealed much higher yield strength, tensile strength, and hardness. The highest tensile strength and hardness were measured at the railhead in the head-hardened rail, and were attributed to the evolution of the bainite phase, generated by additional heat treatment. The higher mechanical strength of the railhead of the worn rail is thought to have resulted from a combination of work hardening and smaller lamellar spacing of the pearlite phase, induced by repeated wear processes during train operation. (Received September 30, 2019; Accepted November 4, 2019)

keyword : continuous welded rail, microstructure, mechanical property, X-ray diffraction, crystal structure

4YSZ is generally used in fuel cells, oxygen sensors, hip and knee joint replacements and thermal barriers because of it excellent properties, including low density, high biocompatibility, good resistance against corrosion, hard phase and high melting point, and high ionic conductivity. However, 4YSZ with large grain has low resistance to abrasion and wear because of its low fracture toughness and low hardness at room temperature. The hardness and fracture toughness of 4YSZ can be improved by the addition of a second hard phase to form a nanostructured composite. Nanostructured materials have received considerable attention because they possess a high hardness and fracture toughness. Recently, nano-crystalline YSZ powder was fabricated by several methods. However, nanosized sintered YSZ has not been obtained due to fast grain growth during the conventional sintering method. In this study, nanostructured 4YSZ-BN composites with nearly full density were achieved using pulsed current activated sintering for 80 sec at a pressure of 80 Mpa. The addition of hexagonal BN to 4YSZ caused the nano scale structure to be retained in the ceramic by deterring grain growth. The crystalline size of the 4YSZ was reduced by the addition of BN. The addition of BN to 4YSZ greatly improved its mechanical properties (hardness and fracture toughness). (Received October 14, 2019; Accepted November 16, 2019)

keyword : sintering, nanomaterials, bn, fracture toughness, hardness

Fe-36%Ni steels have a much lower thermal expansion coefficient than stainless steels and aluminum alloys for low temperature. They also have moderately high strength and good toughness at cryogenic temperatures because of its stable austenite phase. These excellent low temperature characteristics make this alloy attractive for cryogenic applications such as the cargo containment system in liquefied natural gas carriers, pipes for low temperature and airplane rudder units. Generally, Fe-36%Ni steels are welded using Tungsten Inert Gas (TIG) Welding, Plasma Arc Welding (PAW) and Resistance Seam Welding. Lately white spots have been observed on the autogenous TIG welds of cryogenic Fe-36%Ni steels without any flux and wire in the cargo containment of LNG carriers. But white spot formation has not been studied yet. This paper covers the analysis results of the white spots formation on the autogenous TIG welds of cryogenic Fe- 36%Ni steels. The thickness of the white spots was approximately 6 μm and they were just stuck on the surface of the weld. Based on microstructural and qualitative analysis, the white spots were oxide aggregates (Al-Ca-Mg-Si-O), formed as a net structure in the center of TIG weld surface. The formation of white spots is caused by the excessive addition of minor elements such as Ca, Al, Mg in cryogenic Fe-36%Ni steels. The white spots on the TIG weld of cryogenic Fe-36%Ni steels were bigger than the weld spots on the TIG weld of stainless steel 316L, because of the lower solubility of Ca and Al in the austenite in the TIG Welds. (Received August 27, 2019; Accepted November 5, 2019)

keyword : Fe-36%Ni steel, invar alloy, cryogenic steel, TIG, white spot, slag spot, slag island, heat to heat variation

Hot stamping is the most efficient manufacturing process to produce high strength automotive parts with reduced springback. Recently, 30MnB5 steel has been developed for hot stamping steel which can provide above 1700 MPa. However, resistance spot weldability of this steel is insufficient due to the need for a large amount of carbon addition. In this study, the effect of the hot stamping heat treatment temperature on resistance spot weldability of Al-10% Si coated 30MnB5 was evaluated to improve the resistance spot weldability of 30MnB5. In terms of nugget diameter, weldable current range, tensile shear strength and cross tension strength, the specimen heat treated at 850 ℃ achieved better performance than the specimen heat treated at 950 ℃. There was a coating microstructure difference between the specimens heat treated at 850 ℃ and 950 ℃ whereas both specimens had similar microstructure and mechanical properties of their substrates. The Fe-rich layer of the specimen heat treated at 950 ℃ was twice as thick as the specimen heat treated at 850 ℃ because the diffusion rate was much higher at 950 ℃. The thick Fe-rich layer deteriorated the resistance spot weldability due to high electrical resistivity during resistance spot welding. (Received August 22, 2019; Accepted October 6, 2019)

keyword : hot stamping, Al-10%Si coating, 30MnB5, resistance spot welding, dynamic resistance

Fe(III) can be easily extracted from weak acidic solutions by organophosphorus extractants, but stripping a large amount of Fe(III) from the loaded extractants is very difficult. The extraction and stripping of Fe(III) from 2 pH chloride and sulfate solutions was performed with ionic liquids(ILs) and organophosphorus acidic extractants, and the behaviors compared. As ionic liquids, two succinimides ([N88SA][C4min] and ([N88SA][C4Py]) were used, and bi-functional ionic liquids (Bif-ILs) synthesized from Aliquat336 and the organophosphorus acids Cyanex272, D2EHPA, and PC88A were employed. The extraction of Fe(III) by the ILs and extractants employed in this work was in the following order: organophosphorus extractants > Bif-ILs > succinimide ILs. The extraction of Fe(III) by succinimide ILs and Bif-ILs was higher from the chloride solution than from the sulfate one, while the nature of the medium had little effect on the extraction of Fe(III) by the organophosphorus extractants. FT-IR spectra confirmed the ion exchange mechanism of Fe(III) by the succinimide ILs. There was little difference in the stripping of Fe(III) by H₂SO₄ from the loaded Bif-ILs and the organophosphorus acids. However, stripping of Fe(III) from the loaded Bif- ILs using HCl did not readily occur. Fe(III) was readily stripped from the loaded [N88SA][C4min] and [N88SA][C4Py], and the HCl solution was more effective for stripping than H₂SO₄. [N88SA][C4min] is considered a promising new extractant for the recovery of Fe(III) based on its Fe(III) extraction and stripping behavior from weak acidic solution. (Received September 18, 2019; Accepted October 29, 2019)

keyword : ionic liquids, succinimide, extraction, stripping, iron(III)

With the expanding development of flexible optoelectronic devices, the need for high-performance (i.e., high conductivity, high transmittance and flexibility) transparent conductive electrodes has grown. In this study, the critical role of thickness for each of the layers in ZnO/Cu/ZnO transparent conductive electrodes (TCE) for optoelectronic devices has been investigated. The Ag layer deposited on the bottom ZnO exhibited sheet resistance lower than 10 Ω/Sq. at a Ag thickness of 8 nm. It was then fixed with a top and bottom ZnO layer, whose thicknesses were independently varied to achieve the optimized transmittance of visible light. The variation in thickness of the top and bottom ZnO layers, which serve as anti-reflection layers as well as anti-oxidation layers, resulted in a significant modulation of the transmittance behavior. In particular, the relatively low transmittance in the wavelength range of 400-500 nm was shown to be strongly affected, requiring further thickness optimization. The optimized thicknesses for the top and bottom ZnO layers were found to be 30 and 40 nm, respectively, with the corresponding average visible light transmittance of 85% and peak transmittance of 95%. According to the Haccke figure of merit, the value for the optimized ZnO/ Ag/ZnO electrode was 0.021, which surpasses the reported values of Cu-based electrodes. (Received September 18, 2019; Accepted October 28, 2019)

keyword : copper, zinc oxide, transparent conductive electrodes, thin films

Partially double-filled skutterudites were prepared via encapsulated melting and hot pressing processes. The skutterudite phase was successfully synthesized through partial double filling of La/Yb and substitution of Co. A small quantity of secondary phase (marcasite FeSb₂) was formed, but its formation was suppressed by increasing Co charge compensation. The marcasite phase was detected in all specimens, which was because charge compensation was not sufficiently performed owing to the partial filling with low valence ions of La³⁺ and Yb^2+/3+. Hot-pressed specimens showed relative densities higher than 97.4% compared to the theoretical density. The calculated lattice constant ranged from 0.9129 to 0.9142 nm. As the contents of La and Co increased, the lattice constants decreased; thus, the filling and substitution were confirmed. The electrical conductivity decreased and the Seebeck coefficient increased with increasing La filling and Co substitution. The thermal conductivity significantly decreased with increasing Co substitution, and was further slightly lowered with increasing La filling. (La_0.75Yb_0.25)_0.8Fe₃CoSb₁₂ yielded a maximum Seebeck coefficient of 168.2 μVK^-1 at 823 K, a maximum power factor of 2.67 mWm^-1K^-2 at 723 K, and the lowest thermal conductivity of 2.09 Wm^-1K^-1 at 323 K. As a result, (La_0.75Yb_0.25)_0.8Fe₃CoSb₁₂ exhibited the highest dimensionless figure of merit, ZT = 0.66 at 823 K. (Received September 16, 2019; Accepted October 21, 2019)

keyword : thermoelectric, skutterudite, partial double filling, charge compensation

CBAPC (chemically bonded aluminum phosphate ceramic) binders have various advantages, including superior corrosion resistance and strength at high temperature. They are also a non-toxic and eco-friendly material. We investigated an inorganic binder based on CBAPC using artificial sand for casting. The CBAPC binders were synthesized with various P/Al molar ratios from 2.0 to 6.0. Specimens were prepared by mixing the artificial sand and CBAPC binder, and then heat-treating at 260 ℃ for 10 min. The prepared CBAPC binders were characterized using TGA/DSC, XRD, FT-IR, and FE-SEM. The artificial sands (Esperal 40, Cps 1) were characterized by XRF, grain size analysis, and FE-SEM. The ultimate flexural strengths obtained for the artificial sands (Esperal 40, Cps 1) were 6.40 and 6.36 MPa, respectively, under the conditions P/Al=4.5, curing temperature=260 ℃, and curing time=10min. The CBAPC_4.5 had higher thermal stability at low temperature than the samples with other P/Al molar ratios. The AlH₂P₃O₁₀ · 2H₂O structure formed at 260 ℃ exhibited high mechanical strength and moisture resistance with improved flexural strength. The effect on the surfaces of the CBAPC_4.5 coated artificial sands depended on the curing temperature and curing time. We confirmed that the P/Al molar ratio and curing conditions strongly affected flexural strength. (Received August 28, 2019; Accepted November 18, 2019)

keyword : artificial sand, casting, CBAPC binder, flexural strength, moisture resistance

The use of locally developed high performance alloys in power plants has become essential in the past ten years. Application of these alloys has been difficult because of a shortage of field demonstration testing, and the lack of lab-based test data. In this work, three types of locally developed heat resistant alloys were demonstration tested in boiler tubes in a power plant. The test procedures included a pre-test, a field demonstration and a post-test. The pre-test revealed the quality of the local tubes was the same level as the originally imported tubes. This suggested the potential outcomes of further demonstration testing. The demonstration was carried out by replacing parts of the original components with newly manufactured test components, made of local and imported tubes. There were no failures in the test components after 13,000h of operation. A portion of the tubes were sampled for a post-test, while the remaining parts were operated in the boiler for an even longer time. The post-test results revealed no significant degradation of the properties of the local tubes. The works no matter set up the properties database, but also set-up the performance and reliability, as well as test procedures for locally developed tubes. (Received September 3, 2019; Accepted October 29, 2019)

keyword : heat resistant alloys, boiler tubes, locally development, demonstration test, properties, microstructure