• Journal of Ocean Engineering and Technology
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• v.28 no.4
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• pp.306-313
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• 2014

Super duplex stainless steels (sDSS) are excellent for use under severely corrosive conditions such as offshore and marine applications like pipelines and flanges. sDSS has better mechanical properties and corrosion resistance than the standard duplex stainless steel (DSS) but it is easier for a sigma phase to appear, which depresses the mechanical property and corrosion resistance, compared to DSS, because sDSS has a higher alloy element than DSS. In addition, sDSS has a feeble ductile-brittle transition temperature (DBTT) because it has a 50% ferrite microstructure. In the actual operating environment, sDSS would be thermally affected by welding and a sub-zero temperature environment. This study analyzed how precipitated sDSS behaves at a sub-zero temperature through annealing heat treatment and a sub-zero tensile test. Six types of specimens with annealing times of up to 60 min were tested in a sub-zero chamber. According to the experimental results, an increase in the annealing time reduced the elongation of sDSS, and a decrease in the tensile test temperature raises the flow stress and tensile stress. In particular, the elongation of specimens annealed for 15 min and 30 min was clearly lowered with a decrease in the tensile test temperature because of the increasing sigma phase fraction ratio.

• The Journal of Korean Academy of Prosthodontics
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• v.22 no.1
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• pp.95-108
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• 1984

Even though the tarnishing and corrosion problems characteristic with dental silver-palladium alloy are not yet fully solved, it is recently widely used because of its low cost. However the effects of major alloying elements on the various properties of this system are not fully understood. The object of this research is to clarify the effects of In and Zn additives on the corrosion and tarnishing resistances and precipitation hardening behavior of this sytem, using electrodynamic polarization, immersion, and Vicker's microhardness test and X-ray diffraction and electron probe micro analysis methods. The obtained results were as follows: I. As indium content is increased, both the corrosion resistance in Cl-solution and microhardness are also increased while the tarnishing resistance is decreased. 2. As Zinc content is increased, the corrosion resistance is decreased, but tarnishing resistance is increased 3. At 70Ag-25Pd-2.5Zn-2.5In composition, the precipitation harding behavior was mot significant. The optimum aging temperature was $450^{\circ}C$ and the time was 2 hrs. The resulting specimen of this work carried 180VHN. 4. Under the heat treatment, the changes in the mechanical property are due to the changes in the shape and composition of dendrite matrix, namely, it is because of the precipitation hardening behavior which has been proved by electron probe micro analysis and optical microscopic finding.

• The Journal of Advanced Prosthodontics
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• v.8 no.3
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• pp.224-228
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• 2016

PURPOSE. This study aimed to investigate whether dentin surface preparation with diamond rotary instruments of different grit sizes affects the shear bond strength of resin-bonded restorations. MATERIALS AND METHODS. The buccal enamel of 60 maxillary central incisors was removed with a low speed diamond saw and wet ground with silicon carbide papers. The polished surfaces of the teeth were prepared with four groups of rotary diamond burs with super-coarse (SC), coarse (C), medium (M), and fine (F) grit sizes. Following surface preparation, 60 restorations were casted with nickel-chromium alloy and bonded with Panavia cement. To assess the shear bond strength, the samples were mounted on a universal testing machine and an axial load was applied along the cement-restoration interface at the crosshead speed of 0.5 mm/min. The acquired data was analyzed with one way ANOVA and Tukey post hoc test (${\alpha}=.05$). RESULTS. The $mean{\pm}SD$ shear bond strengths (in MPa) of the study groups were $17.75{\pm}1.41$ for SC, $13.82{\pm}1.13$ for C, $10.40{\pm}1.45$ for M, and $7.13{\pm}1.18$ for F. Statistical analysis revealed the significant difference among the study groups such that the value for group SC was significantly higher than that for group F (P<.001). CONCLUSION. Dentin surface roughness created by diamond burs of different grit sizes considerably influences the shear bond strength of resin bonded restorations.

• Transactions of the Korean hydrogen and new energy society
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• v.17 no.4
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• pp.389-394
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• 2006

Mg and Mg-based alloys are promising hydrogen storage alloys for renewable clean energy applications. It is a lightweight and low cost material with high hydrogen storage capacity. However, commercial applications of the Mg hydride are currently hindered by its high absorption/desorption temperature, and very slow reaction kinetics. In this work, we aim to study the absorption properties of the $Mg_2Ni$-5mass% Nb composite prepared by mechanical alloying under hydrogen. The absorption capacity of the sample is found to be about 3.0 wt.% at T=573 K and P=1.0 MPa. The absorption characteristics observed have been compared with those of the prepared $Mg_2Ni$.

• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.1
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• pp.18-26
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• 2000

Boiler high-temperature pipelines such as main steam pipe, header and steam drum in fossil power plants are degraded by creep damage due to severe operating conditions such as high temperature and high pressure for an extended period time. Conventional measurement techniques(replica method, electric resistance method, and hardness test method) for measuring creep damage have such disadvantages as complex preparation and measurement procedures, too many control parameters. And also these techniques have low practicality and applied only to component surfaces with good accessibility. In this paper, artificial creep degradation test and ultrasonic measurement for their creep degraded specimens(Cr-Mo alloy steels) were carried out for the purpose of evaluation for creep damage. Absolute measuring method of quantitative ultrasonic measurement for material degradation was established, and long term creep degradation tests using life prediction formula were carried out. As a result of ultrasonic tests for crept specimens. we conformed that both the sound velocity decreased and attenuation coefficient linearly increased in proportion to the Increase of creep life fraction($\Phi$c).

• Journal of Korean Vacuum Science & Technology
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• v.4 no.1
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• pp.23-26
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• 2000

We have fabricated by metal organic chemical vapor deposition (MOCVD) In$\_$0.13/Ga$\_$0.87/N/GaN multiple quantum well (MQW) with thickness as thin as 10 A and barriers also of th same width on (0001) sapphire substrate. We have investigated this thin MQW by steady-state and time-resolved photoluminescence(PL) in picosecond time scale in a wide temperature range from 10 to 290 K. In the PL at 10 K, we observed a broad peak at 3.134 eV which was attributed to the quantum well emission of InGaN. The full width at half maximum (FWHM) of this peak was 129 meV at 10 K and its broadening at low temperatures was considered to be due to compositional fluctuations and interfacial disorder in the alloy. The narrow width of the quantum well was mainly responsible for the broadening of the emission linewidth. We also observed an intense and sharp peak at 3.471 eV of GaN barrier. From the temperature dependent PL measurements, the activation energy of the InGaN quantum well emision peak was estimated to be 69 meV. The lifetime of the quantum well emission was found to be 720 ps at 10 K, which was explained in terms of the exciton localization arising from potential fluctuations.

• Korean Journal of Materials Research
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• v.13 no.1
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• pp.36-42
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• 2003

The oxidation behaviors of Y-Cr bilayer deposited on ferritic steel by magnetron-sputtering for application of the Fe-Cr alloys as interconnectors of planar-type solid oxide fuel cells (SOFCs) were studied. After oxidation at $800^{\circ}C$ for 40 hours, the major phase of $Y_2$$O_3$and the minor phase of $YCrO_3$, $Mn_{1.5}$ $Cr_{1.5}$ $O_4$and Cr$_2$SiO$_4$were formed in the Y/Cr bilayered samples, while the major phase of Cr$_2$O$_3$and the minor phase of $Y_2$$O_3$were formed as the major phase in the Cr/Y bilayered samples. The Log(ASR/T) that expresses electric resistance of the Y/Cr coated specimen with nonconducting $_Y2$$O_3$oxide showed high value of -2.80 Ω$\textrm{cm}^2$$K^{-1}$ / and that of the Cr/Y coated specimen with conducting $Cr_2$$O_3$oxide appeared to be -4.11 Ω$\textrm{cm}^2$$^{K}$ . The electric resistance of the Y/Cr coated specimen was largely increased due to the formation of high resistance oxide scales. However, the Cr/Y coated specimen did not show any increase in the electric resistance and had the long-term stability of oxidation because there was no formation of the secondary phases with low conductivity.

• Composites Research
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• v.14 no.5
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• pp.1-11
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• 2001

The particulates reinforced metal matrix composites(PMMC) have a number of interesting mechanical properties. including high strength and good resistance to wear at high temperature and low thermal expansion. The equipment structure to obtain the homogeneous distribution in composites are proposed for the continuous pouring of reinforcement at the desired temperature. The particulates reinforced metal matrix composites(A357/SiCp) were fabricated by the process of the combined stirring method with the various fabrication process. The combined stirring method to niform distribution of particle is consisted of two stirring force both electro-magnetic stirring generated from induction heating and mechanical stirring with graphite stirrer. PMMC billets were fabricated with the volume fractions ranged from 0% to 20% and particle sizes ranged from 14${\mu}{\textrm}{m}$ to 25${\mu}{\textrm}{m}$. It is important to cont the size of primary $\alpha$-Al solid particles because it could become the cause of the particle pushing or capture phenomena from the fact that secondary dendrite arm spacing size depends on the cooling rate during the solidification in hypoeutectic Al-Si alloy. Therefore, the effect of primary $\alpha$-Al on the reinforcement distribution in matrix alloys has been investigated. The microstructure of PMMC fabracated with various volume fractions(0%, 10%, and 20%) and particle size were observed.

• Journal of Advanced Marine Engineering and Technology
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• v.29 no.8
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• pp.891-898
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• 2005

Inertia welding is a solid-state welding process in which butt welds in materials are made in bar and in ring form at the joint race, and energy required lot welding is obtained from a rotating flywheel. The stored energy is converted to frictional heat at the interface under axial load. The quality of the welded joint depends on many parameters, including axial force, initial revolution speed and energy amount of upset. working time, and residual stresses in the joint. Inertia welding was conducted to make the large exhaust valve spindle for low speed marine diesel engine. superalloy Nimonic 80A for valve head of 540mm and high alloy SNCrW for valve stem of 115mm. Due to different material characteristics such as, thermal conductivity and flow stress. on the two sides of the weld interface, modeling is crucial in determining the optimal weld geometry and Parameters. FE simulation was performed by the commercial code DEFORM-2D. A good agreement between the Predicted and actual welded shape is observed. It is expected that modeling will significantly reduce the number of experimental trials needed to determine the weld parameters. especially for welds for which are very expensive materials or large shaft. Many kinds of tests, including macro and microstructure observation, chemical composition tensile , hardness and fatigue test , are conducted to evaluate the qualify of welded joints. Based on the results of the tests it can be concluded that the inertia welding joints of the superalloy exhaust valve spindle are better properties than the material specification of SNCrW.

• Journal of the Korean Society for Heat Treatment
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• v.28 no.6
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• pp.300-309
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• 2015

The aim of the present study was to predict the variations in microstructure and deformation occurring during gas carburizing and quenching processes of a SCM420H planetary gear in a real production environment using the finite element method (FEM). The motivation for the present study came from the fact that previous FEM simulations have a limitation of the application to the real heat treatment process because they were performed with material properties provided by commercial programs and heat transfer coefficients (HTC) measured from laboratory conditions. Therefore, for the present simulation, many experimentally measured material properties were employed; phase transformation kinetics, thermal expansion coefficients, heat capacity, heat conductivity and HTC. Particularly, the HTCs were obtained by converting the cooling curves measured with a STS304 gear without phase transformations using an oil bath with an agitator in a real heat treatment factory. The FEM simulation was successfully conducted using the aforementioned material properties and HTC, and then the predicted results were well verified with experimental data, such as the cooling rate, microstructure, hardness profile and distortion.