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A study has been made to investigate the effects of silicon(Si) addition on the oxidation properties of Fe-14Cr-1Al ferritic stainless steels at 1173K. Four steels containing different amount of Si (0.5, 1, 2, 5 wt% Si) were arc melted and homogenized at 1473K for 24 hours and oxidized at 1173K. Both the isothermal and cyclic oxidation tests revealed that the addition of Si increased the oxidation resistance of Fe-14Cr-1Al alloy. With 0.5w%Si, Cr oxide was formed as surface oxide, and Si, Al oxides were formed internally below Cr oxide layer. With 1w%Si, Al oxide was mixed with Cr oxides on the surface. However, with Si content higher than 2w%, a stable Al₂O₃ layer was formed rapidly. From theoretical evaluation using ChemSage^(TM) program, the Al activity in Fe-Al-5Si steel is about 5 times higher than that in Fe-1Al alloy at 1173K, which is the most probable reason for the rapid formation of Al oxide in the high Si steels.

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The effects of the accumulated annealing parameter on the corrosion characteristics in the Nb-containing Zr alloys have been investigated. Static autoclave corrosion test was carried out in 360℃ water, 360℃ 70ppm LiOH, and 400℃ steam on the two Zr alloys (Zr-0.4Nb-0.8Sn-xFeCrMn, Zr-0.2Nb-1.1Sn-xFeCrMn), which were manufactured to give the three given three different accumulated annealing parameters. The corrosion resistance decreased with increasing the accumulated annealing parameter (ΣA) under the corrosion conditions. The microstructure changes with increasing the accumulating annealing parameter were observed by TEM. The corrosion characteristics with increasing the accumulated annealing parameter was mainly analyzed by observing the matrix microstructure and precipitate. The size of precipitates increased with increasing the accumulated annealing parameter. From the XRD analysis of the oxide, it was revealed that the relative fraction of tetragonal ZrO₂ also decreased gradually with increasing the accumulated annealing parameter. Conclusively, it could be thought that the controlling of the precipitates size to be small by the decrease of the accumulated annealing parameter gave rise to improve the corrosion resistance in Nb-containing Zr alloys.

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Flow accelerated galvanic corrosion characteristics of a carbon steel coupled to stainless steel were investigated in deaerated alkaline-chloride solutions as a function of flow velocities, pH and temperatures. The electrochemical properties of specimens were investigated by potentiodynamic test and galvanic corrosion test using RCE(Rotating Cylinder Electrode). Carbon steel showed active behavior in the deaerated alkaline-chloride solution. The galvanic current density of carbon steel increased with increasing flow velocity and temperature, but decreased with increasing pH. Flow velocity had a small effect on the galvanic current density at 25℃, whereas the flow velocity increased galvanic current density significantly at 50℃ and 75℃. This might be due to the increased solubility of magnetite at the higher temperatures.

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InGaAs layers with a thickness of 3㎛ are grown on undoped GaAs substrates which have [001] exact and 2°off toward [101] direction surfaces. The refractive indices of the samples with various crystallographic directions have been measured by means of variable azimuthal angle ellipsometry technique. InGaAs layers have shown twofold symmetric anisotropy with a maximum at [11￣1] and a minimum at [110] directions for 6328Å laser light. The Bragg angle difference ΔΘ and the full width at half maximum(FWHM) have also been measured by double crystal X-ray diffraction along [11￣0] and [11￣0] directions. The results indicate that the anisotropy of the refractive index as well as the FWHM are closely related to the direction of the misfit dislocation formed by the lattice mismatch between the InGaAs layers and the substrate. We introduce a simple formula which represents the critical layer thickness of InGaAa/GaAs single layer as a function of In contents from the geometric form of 60° type dislocation. The results agree well with the experimental measurements.

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200㎛ thick Ta thin sheets and Fe-C-(Si) alloys with different chemical compositions, and interfacial reaction was investigated. The growth of carbide layer formed at the interface were cast-bonded after isothermal heat treatment at 1173K, 1223K, 1273K and 1323K for various times was investigated. The carbide formed at the interface was TaC, its chemical composition was 91.5wt.%Ta-6.6wt.%C-1.9wt.%Fe, and its hardness was ranging from 1820 to 1970Hv. The thickness of TaC layer was increased linearly in proportional to the heat treating time. Therefore, it was suggested that the growth of TaC layer was controlled by the interfacial reaction. The growth rate constant of TaC layer was slightly increased with increase of carbon content when the silicon content is similar in the cast irons. However, as silicon content increases with in case of almost same amount of carbon content, the growth of TaC layer was greatly retarded. The calculated activation energy for the growth of TaC layer was varied in the range of 278.1∼329.1kJ/㏖ with the compositions of cast irons. The activation energy was shown to be a tendency to increase with an increase of silicon content in cast iron.

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Effects of Sn and Ni in the Cd-free Ag-Cu-Zn based brazing alloys on the microstructure, physical and mechanical properties of the brazed joints were investigated by differential scanning calorimetry(DSC), scanning electron microscope(SEM) equipped with energy-dispersive spectrometer(EDS), X-ray diffractometer(XRD) and shear strength test. As the Ni is added 1∼2wt% in the brazing alloys, the grain size of brazing alloys decreased and microhardness increased from Hv120 up to Hv140. Wetting depth measurement test showed that the wettability of Cd-free 30wt%Ag-35Cu-32Zn-2Sn-1Ni alloy was better than that of 35wt%Ag-24Cu-22Zn-19Cd alloy. However the increase in Ni addition from 1wt% to 2wt%, reduced the wettability. The maximum shear strength obtained from the brazed specimen with Cd-free 30wt%Ag-34Cu-32Zn-2Sn-2Ni alloy was measured to be approximately 270㎫.

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The press hardening process, which is the hot pressing of steel parts using cold dies, can utilize both ease of shaping and high strength due to the hardening effect of rapid quenching during the pressing. In this study, a thermo-elastoplastic analysis of the press hardening process using the finite element method was performed in order to investigate the deformation behaviour and temperature history during the process and the mechanical properties of the pressed parts. The effects of friction on the thermal-deformation behaviour of steel sheets are investigated. We also carried out experiments of temperature measurements during pressing and quenching of a car bumper part to obtain the experimental heat transfer coefficients for comparison with the predictions. It was found that the strength of the press hardened boron steel sheet can increase up to about Hv 430 due to the rapidly quenched microstructure consisting of martensite. The finite element analysis can produce useful information for the process optimisation.

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