발간논문

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Microstructural evolutions attributed to the change of Cr (or Mo) content and the heat treatment procedure have been studied to optimize the mechanical properties for pressure vessel applications. The relationship between microstructures and mechanical properties of Cr-Mo steels is established through laboratory scale experiments using 35 kg ingots that were hot-rolled and heat treated in various conditions. As the Cr or Mo content increased, the average grain size of ferrite decreased, while the volume fraction of pearlite increased, which resulted in the increase of tensile strength of the specimen. The experimental results were expected with continuous cooling curves calculated by a commercial program (JMatPro). Temper embrittlement after heat treatment occurred in high Cr (or Mo) steels.

keyword : alloys, tempering, microstructure, tensile test, CCT

Carbon diffusion of ultra-low carbon steel treated at 880 ℃ during 10, 30, 60, and 120 minutes was calculated by the finite element method and investigated by experimental tools (optical microscopy, EPMA and Micro Vickers). Results of experiments and finite element analysis were compared. Carbon diffusion according to optical microscopy, EPMA and the Micro Vickers test increased as the time treated at 880 ℃ increased. Such phenomena are explained by finite element analysis.

keyword : solid quenching, carbon diffusion, ultra-low carbon steel, finite element analysis

Medium carbon steel was aluminized with various silicon and magnesium content for selected dipping times. Addition of Si made the shape of the intermetallic layers less irregular than that of the hot-dip aluminizing in pure aluminum. As more Si was added, the thickness of the intermetallic layer became smaller. When Mg was added, the shape of the intermetallic layer was more irregular than that of the Al-Si hot-dip, but less irregular than that of the Al hot-dip. The thickness of the intermetallic layer in hot-dip aluminizing with Mg addition was generally thinner than that obtained in hot-dip aluminizing with pure Al but thicker than that of hot-dip aluminizing with Si addition. The apparent activation energy obtained for the pure Al hot-dip and 1 at% Si-1 at% Mg-Al hot-dip was 82.82 kJ mol.1 and 153.61 kJ mol.1, respectively.

keyword : hot-dip aluminizing, intermetallics, surface modification, oxidation, optical microscopy

The effect of silicon and magnesium addition in a molten Al bath on the morphology and corrosion resistance of the Al coating layer of steel formed during hot-dip aluminizing was investigated at the temperatures of 700, 800, and 900 ℃ for the dipping time of 1-15 minutes. The thickness of the intermetallic layer decreased with an increasing Mg or Si content within a range of 0-1 at%. Addition of Si made the intermetallic layer thin whereas addition of Mg compensated for the decrease of the thickness significantly. The shape of the intermetallic layers became less irregular than that of the hot-dip aluminizing in pure aluminum. The results of immersion corrosion tests and electrochemical corrosion tests in a solution of 3 wt% NaCl showed that the corrosion resistance of the Al-1% Si-1% Mg coated steel had increased significantly over the Al coated steel. The open circuit potential changed from .0.56324 V (vs. SCE) to .0.48872 V (vs. SCE) and the corrosion current density decreased from 9.9991 μA/cm2 to 2.1160 μA/cm2. A salt fog spray test on these specimens also showed hot-dip aluminizing with Si and Mg addition increased the corrosion resistance significantly.

keyword : hot-dip aluminizing, intermetallics, surface modification, corrosion, optical microscopy

Titanium and titanium alloys have been used extensively as bone-implant materials due to their high strength-to-weight ratio, good biocompatibility and excellent corrosion resistance. In this work, we investigated the surface characteristics of nanotubes formed on Ti25Nb alloys in a 0.1M NaF aqueous electrolyte containing HF using electrochemical methods. The constant voltages were widely applied to a range from 20 to 80 V. As a result, the oxide layers formed with nanotube structures in which the nanotubes increased in a linear diameter and length. The TiNb nanotube morphology was strongly influenced by the fluoride concentration, applied potential and reaction time.

keyword : anodization, Ti oxide, TiNb, nanotubes, electrochemistry

This study investigated the effect of the hydrogen charging time under the electro-chemical hydrogen charging condition on hydrogen embrittlement of hot-rolled and then air-cooled API X70 steel. Tensile and V-notch Charpy impact test specimens were held for 0 h, 2 h, 4 h, and 10 h in the electrolyte pot during electro-chemical hydrogen charging processes and then both of tensile and impact tests were completed at room temperature. The microstructure consisted of fine acicular ferrite, coarse polygonal ferrite and pearlite. The yield and tensile strength decreased slightly and elongation decreased rapidly up to 2 h holding time and then decreased slowly with the holding time. The V-notch Charpy impact value decreased continuously up to 4 h holding time and then increased slightly up to 10 h. The morphology of the fracture surface changed from a ductile type to a brittle type with the hydrogen charging time. Secondary cracks were observed in the hydrogen charged specimens. No external cracks were formed at the specimen surface of the tensile tested with 0 h holding time, but many external cracks were observed at the surface of the hydrogen charged specimens. Cracks were observed in the nominal and parallel directions to the tensile direction at the longitudinal cross-section region near the tensile fractured surface with the 2 h held specimen.

keyword : embrittlement, electro-chemical hydrogen charging, alloys, mechanical properties, scanning electron microscopy

Nanopowders of TiO2 and Al were prepared from a high energy ball milling method to fabricate sintered 3Ti-Al2O3 composites with nanostructures. A dense nanocrystalline 3Ti-2Al2O3 composite was consolidated by a pulsed current activated sintering method from the mechanically activated powder. Consolidation of dense 3Ti-Al2O3 composites with a relative density of up to 99% was accomplished under the combined contribution of a pulsed current and a mechanical pressure of 60 MPa. The hardness and fracture toughness of the fabricated composites were 1207 kg/mm2 and 7 MPa￥m1/2, respectively.

keyword : composite, 3Ti-2Al2O3, sintering, nanomaterials, mechanical properties

The precipitation and sensitization characteristics in AISI436 weld metal were investigated in different chemical composition ranges of Ti and Nb content. We manufactured four welding wires made of 0-0.2 wt% of Ti and 0-1.0 wt% of Nb and did flux cored arc welding. After heat treatment at 900 ℃ for 20 hours, we made a Double Loop Electrochemical Potentiokinetic Reactivation (DL-EPR) test, Electron Backscattering Diffraction and SEM. The DL-EPR test revealed that as the amount of addition of Ti and Nb rose, the degree of sensitization fell. The microstructure became more refined, and Cr carbide formed at the grain boundary that had no addition of Ti and Nb. Furthermore, in the specimen with the addition of Ti, Nb, the Ti, Nb carbide and nitride were precipitated in the intergranular boundary, and the laves phase was precipitated at the grain boundary.

keyword : alloys, welding, precipitation, electron backscattering diffraction(EBSD), stainless steel

Cu(In,Ga)Se2 (CIS/CIGS) absorber films were economically synthesized using metal chloride (CuCl2, InCl3, GaCl3, and SeCl₄) precursors in acetone solution with no addition of organic binders for doctor bladed deposition. Heat treatment was applied to CIS/CIGS precursors from 100 to 500 ℃ and the crystallinity and surface morphologies were investigated by FESEM, XRD, etc. It was observed that CIS crystals began to form at 300 ℃ and as the metal chloride concentration in the precursor solution became higher and the heat treatment temperature increased, then the CIS crystal size grew larger and the crystallinity was better. In addition, with the substitution of Ga for In, the (112) main peak in the CIS absorber layer shifted to a higher angle due to a decrease in the lattice parameter and its energy band gap increased. The best quality CIS film was obtained from a CIS concentration of 0.4 M / 0.4 M / 0.8 M with heat treatment of 500 ℃ for 30 minutes.

keyword : solar cells, chemical synthesis, crystallization, X-ray diffraction, CIS / IGS

We studied the effect on physical properties of CuInSe2 (CIS) thin films by change of the Se content in Cu-In-Se precursors. The three kinds of precursors with different Se content were deposited by a hybrid sputtering system which was equipped with the Selenium Knudsen cell (Se K-cell). The precursors were heated in an Se atmosphere for 15 minutes at 425 ℃, which produced crystalline CIS films. We characterized the crystalline CIS films using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), and a Hall effect measurement system. The size of the island in the CIS films was reduced with an increasing Se content in the Cu-In-Se precursors. We found that the island structure in the CIS films disappeared with precursors of Se content over 19 at%. We were able to make a highly adhesive CIS film to Mo back contacts by using precursors with Se content of 19 at%. The solar cells fabricated with the CIS films using precursors with an Se content of 19 at% achieved 8.2% efficiency.

keyword : solar cells, sputtering, interfaces, X-ray diffraction, CuInSe2 film