발간논문

Home > KJMM 논문 > 발간논문

Austenitic stainless steel SS304 is vulnerable to Cl atmosphere SCC (stress corrosion crack). In this study, SCC phenomena related to stress and corrosion composition were analyzed to identify the mechanism for SCC initiation and propagation in SS304. The microstructure and mechanical properties resulting from crack propagation were analyzed by OM, SEM/EDS and micro Vickers hardness tests. The abnormal phase transformation induced by the SCC was analyzed by TEM and diffraction. As a result of these analyses, the shape of SCC was observed to form a branched type crack, which was related to etch pit patterns on the etched surface due to the austenitic fcc (face centered cubic) lattice slip. In addition, the high concentration accumulation of Cl and S components at the SCC site, observed by SEM/EDS, indicated that the SCC was affected by the corrosive atmosphere. The SCC crack propagation was accompanied by hardening, which is believed to be associated with the mechanism of hydrogen embrittlement. High resolution TEM analysis found abnormal satellite diffraction points in the SCC high hardness region. This means that a superlattice phase with high hardness values is formed near the SCC region. And the HIC (hydrogen induced crack) effect, a kind of hydrogen embrittlement, was also influenced by the hardened superlattice phase. It is assumed that the SCC and HIC are similar phenomena produced in the same stress and corrosive atmosphere by superlattice phase transformation.

keyword : SS304 stainless steel, stress corrosion crack, Cl and S, superlattice phase, hydrogen induced crack

The short range ordering (SRO) reaction of the X-750 Ni-base alloy used as a garter spring material for CANDU reactors was systematically investigated through differential scanning calorimeter (DSC) analysis. Water quenching (WQ) after solution annealing (SA) and 20% cold rolled (CR) X-750 were prepared and these samples were subjected to ordering treatment at 475 ℃ for up to 2,160 hours. The WQ and CR specimens showed an exothermic reaction due to the release of entropy formed by the WQ and CR processes, respectively, whereas the ordered WQ and CR specimens exhibited endothermic reactions. The exothermic reaction from the WQ and CR specimens means the SRO reaction occurred. In the WQ specimen, two exothermic reactions at 577 ℃ and 671 ℃ were observed, which corresponds to SRO formation of Ni₂Cr and CrFe, respectively. The critical temperature and thermal activation energy for these specimens were measured by varying the heating rate. The lattice variation with ordering time at 475 ℃ up to 2,160 hours was measured by XRD using CuKa X-ray. A 0.03-0.09% lattice contraction of the (200) plane occurred, depending on prior treatment condition. We discuss whether a lattice contraction due to SRO reaction may cause voids, providing formation sites for He bubbles in the X-750 garter spring exposed to the operating environment of the CANDU reactor.

keyword : Ni-base alloy, X-750, short range ordering, exothermic reaction, entropy

Hydrogen diffusion and sulfide stress cracking of simulated heat-affected zone (HAZ) of A516- 65 grade steel were examined using an electrochemical permeation technique, glycerin volumetric method, and constant loading method. HAZ samples were fabricated using a metal thermal cycle simulator with a welding heat input of 20, 35, and 50 kJ/cm. The fractions of bainite and martensite-austenite (M-A) constituent in coarse-grained HAZ (CGHAZ) and intercritical HAZ (ICHAZ) obtained by a simulated thermal cycle with a low heat input (20 kJ/cm) were higher than those with a higher heat input. These fractions contributed to the increase in the reversible hydrogen trap density (N_[H]rev) and reversibly trapped hydrogen concentrations (C_rev). Although CGHAZ had higher N_[H]rev and C_rev meaning that it is more likely to be vulnerable to brittle failure by hydrogen, actual fracture by sulfide stress cracking (SSC) occurred in ICHAZ composed of a mixture of soft ferrite/pearlite, and hard bainite and M-A. The hydrogen diffusion/ trapping parameters, which were obtained from the electrochemical permeation or glycerin method, cannot be directly indicative of the resistance to SSC of the steel in a H₂S environment. The susceptibility to SSC was more influenced by the level of M-A-localization and localized corrosion attack, acting as a stress intensifier under a tensile load.

keyword : A516 steel, heat affected zone, hydrogen embrittlement, hydrogen permeation, corrosion, sulfide stress cracking

To clearly understand the effect of the eutectic structure in the Zn-Al-Mg alloy coating on the corrosion behaviors in chloride containing aqueous environments, a bulk Zn-MgZn₂ sample was fabricated by diffusion bonding process, and a variety of experiments, such as electrochemical polarization and galvanic corrosion measurements, salt spray tests, and cyclic corrosion tests, were carried out. This study revealed that the corrosion potential values of MgZn₂ and Zn samples exposed to a 5 wt.% of NaCl solution were approximately -1.5 and -1 V_SCE, respectively, and the MgZn₂ showed a higher corrosion current density than Zn at the early stage of corrosion. The continued dissolution of MgZn₂ phase from the eutectic structure in the coating layer provides not only sacrificial protection, but also environmental conditions for the co-formation of two types of corrosion products (simonkoleite (Zn₅(OH)₈Cl₂·H₂O) and hydrotalcite (ZnAl₂(OH)₆Cl₂·H₂O)) which have an inhibiting effect on the outer surface. The formation areas and kinetics of the two products on the coating surface were different, and they were greatly dependent upon the types of eutectic structures in the coating layer. Based on the results, a modified anti-corrosion mechanism of Zn-Al-Mg alloy coated steel sheet in chloride containing aqueous environment was proposed.

keyword : hot dip galvanizing, Zn-Al-Mg alloy coatings, Zn-MgZn_{2 eutectic structure, corrosion, corrosion product}

The vacuum carburizing behavior of AISI 4115 steel according to acetylene flow rates (1, 5, 10, 20, 30, 40, and 50 sccm) and temperatures (900, 950, and 1000˚C) was quantitatively investigated through microstructure, carbon concentration and hardness analyses. As the acetylene flow rate increased, the penetration rate of carbon into the surface of the samples (surface-carbon flux) increased, resulting in an increase in the solute carbon concentration at the surface up to the solubility of carbon in austenite. At the same time, the amount of carbon diffusion into the interior of the specimen and overall carburizing depth increased. When the carbon concentration in the surface of the carburized specimen exceeded the solubility of carbon in the austenite above a certain flow rate, cementite precipitated in the austenite grain boundaries. Accordingly, the surface hardness decreased and the amount of carbon diffusion into the specimen became constant as the solute carbon concentration in the austenite matrix at the surface of the specimen became constant. As a result, the overall solute carbon concentration and the effective case depth no longer increased. Meanwhile, the tendency of vacuum carburizing behavior according to temperature was the same, but as temperature increased, the surface-carbon flux and rate of carbon diffusion into the interior of the sample increased, resulting in increased carburizing depth. The appropriate flow rates (10, 20, and 30 sccm) and the corresponding surface-carbon fluxes (62.3, 135.7, and 216.7 g/m²h) were derived according to carburizing temperatures (900, 950, and 1000˚C), respectively.

keyword : vacuum carburizing, acetylene flow rate, surface-carbon flux, carbon concentration, hardness, carbides

We fabricated a transparent conducting electrode composed of graphene oxide (GO) and silver (Ag) nanofibers. The graphene oxide was spray-coated on the Ag nanofiber film, which was fabricated by electrospinning process. Ag/poly(vinyl alcohol) ink was fabricated in a polymer matrix solution using the solgel method. The sprayed film was sintered at 200 ℃ for 100 min under H₂/Ar atmosphere. The optical transmittance of the transparent electrodes was measured by UV/VIS spectroscopy, and sheet resistance was measured using I-V measurement system. As the amount of GO sprayed on the nanofibers increased, the diameters of the nanofibers increased, therefore, the transmittance of the electrode linearly decreased. However, the conductivity of the electrode increased. This is because the sprayed GO filled the gap between the nanofibers, and GO deposited on the surface of the nanofibers will form more effective electron pathways, resulting in increased conductivity. The GO-Ag nanofiber electrode also exhibited excellent environmental stability, and the sheet resistance of the electrode remained very stable during 30 days testing. The lowest sheet resistance of the transparent electrode was 250 ohm/sq with approximately 83% transparency at a wavelength of 550 nm. This excellent electrical properties and environmental stability might facilitate applications of the GO-Ag nanofiber electrode in optoelectronic devices.

keyword : electrospinning, transparent electrode, graphene oxide, silver nanofiber, environmental stability

Magnetic induction and resonance wireless power charging generally operates in a high frequency range, and its efficiency is degraded by the generation of heat at high frequency. This makes it necessary to develop new soft magnetic materials capable of maintaining their characteristics even at high frequency. The purpose of this study is to minimize the heat generated by the eddy currents when charging, and to improve the charging efficiency under an alternating magnetic field. In this study, Fe-Si-B alloys containing Cu, Zr and Ca elements were melt-spun to prepare amorphous ribbons. The amorphous ribbons were heat treated to crystallize nanograins, The structure was analyzed by TEM and EELS, As a result, it was found that Zr was distributed mainly at the grain boundaries after heat treatment, whereas Ca was uniformly distributed only along the grain boundaries. It could be concluded that the Ca and Zr elements effectively suppressed the grain growth, and thus maintained the very fine nanograin structure.

keyword : wireless power charger, nanostructure, soft magnet, EELS, TEM micrograph

Thin copper-nickel foil with multi-nano layers was prepared by pulse-electroforming to develop a high performance electromagnetic shielding material for electronic devices. The pulse electroforming conditions of the aqueous solution chemistry were selected based on the aqueous copper-nickel-sulfur phase diagram and an evaluation of the deposition rate using the finite element method based on the current distribution in front of a cathodic electrode. The thermodynamic stability diagram revealed that the coppernickel multi-nano layers could be formed at pH< 4 and ΔE >1.0 V in a modified sulfide bath. The electro-formed copper-nickel multi-layer was well produced at the pulse plating conditions of -0.2V_SHE, -0.5 mA/cm², and 25 seconds for copper layer and -1.7 V_SHE, -50 mA/cm² and 80 seconds for nickel layer, which was composed of about 25 nm thick copper and about 30 nm thick nickel rich phases, respectively. The average deposition rate of the copper-nickel foil with multi-nano layers was estimated by the finite element method to be about 0.115 mm/sec, which was in good agreement with the real value of the thin multi-nano layered copper-nickel foil. The effectiveness of the electromagnetic shielding of the copper-nickel mesh with multi-nano layers was more than 30% higher than that of copper mesh in the frequency range of 8.2 and 12.5 GHz.

keyword : copper-nickel multi-nano-layers, electroforming, electromagnetic shielding effectiveness

Because of their simplicity and low equipment costs, various oxidation methods can be used to create metal oxides, but they still require long processing times and/or high temperatures. In this study, a new electrochemical oxidation (ECO) method, which oxidizes metal in DI water with a constant flowing current to both the cathode and anode, was developed to overcome the disadvantages of conventional oxidation methods. The mechanism of the ECO method is discussed. Metallic Zn films were oxidized by the ECO method in DI water for 1 h. The DI water temperature was varied from 30 to 90 ˚C to determine the optimal temperature for the ECO process. Increasing the temperature of the DI water led to distinct surface changes in the disk-shaped Zn plate. The intensity of diffraction peaks from the ZnO (002) plane gradually increased from 31.67 to 2806.48. The structural and optical properties of the ZnO thin film were enhanced as the temperature of the DI water approached 90 ˚C, which means that a high-quality ZnO thin film was synthesized using the ECO method at lower temperatures and shorter processing time compared with established oxidation methods. These results confirm the ECO method can be useful for the low-cost fabrication of transparent and flexible optoelectronic devices.

keyword : ZnO, electrochemical oxidation, surface morphology, oxidation mechanism