발간논문

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During the continuous casting of boron-bearing steel, the corner cracks on the slab are formed by deformation with low strain rate and rapid cooling at the unbending temperature within the range of 800-1,000oC. Especially, the rapid cooling in the corner of slab during the continuous casting leads to as corner cracking. Therefore, in this study, the hot tensile tests applied to the different cooling rates were taken into account in order to study the effect of cooling rate on the hot ductility of boron-bearing steel. The results revealed that increasing cooling rate deteriorate the hot ductility of boron- bearing steel. Rapid decreasing of the hot ductility is caused by formation of a film-like ferrite and precipitate at the austenite grain boundaries. The morphology of the precipitates in the boron-bearing steel was monitored by PTA (Particle Tracking Autoradiography) and TEM, we observed MnS and BN compound and their morphology was quite different depending on the cooling rates. When the cooling rate is increased, rodshape MnS and BN precipitates can be formed along the austenite grain boundaries. It can cause that weakening the boundary region and decreasing the hot ductility of boron-bearing steel.

keyword : boron bearing steel, cooling rate, hot ductility, BN, corner crack

The crystallization kinetics of the Cu43Zr43Al7Be7 bulk metallic glass were studied by differential scanning calorimetry(DSC) in the continuous heating and isothermal annealing modes. Only one major peak could be detected on the DSC traces of Cu43Zr43Al7Be7 bulk amorphous alloy, and the activation energy for crystallization corresponding to the peak determined by the Kissinger method was resulted of 239 kJ/mol. The isothermal kinetic, analyzed by the Johnson-Mehl-Avrami equation yielded values for the Avrami exponents in the range 1.69 to 2.37, which implied a crystallization governed by a three-dimensioned growth. Primary phases were essentially the cubic structure CuZr together with the Cu10Zr7 phase. At higher temperature, the CuZr disappeared while the Cu10Zr7 became predominant. After long term annealing at 731 K, the phases were Cu10Zr7, Cu2ZrAl and Al3Zr5.

keyword : Cu-Zr-Al-Be alloy, bulk metallic glass, crystallization behavior, kinetics

The magnitude of constraint effect A2 value was experimentally estimated by using crack tip opening displacement(CTOD) between elastic and plastic regions near crack tip front for CT specimen with 25.4tᒠSS400 steel. The constraint effect, A2 was dependent on specimen configuration and on the measured positions of CTOD near crack front. A2 should be estimated using the opening displacement calculated within crack front plastic region. If not, it`s not reliable to evaluate of constraint effect at crack growth initiation in this paper.

keyword : CTOD, crack tip opening displacement, constraint effect A2 value, δ 5 method

Thermally sprayed tungsten carbide-based powder coatings are being widely used for a variety of wear resistance applications. The coating deposited by high velocity processes such as high velocity oxy-fuel (HVOF) thermal spraying is known to provide improved wear resistant property. In this study, optimal coating process (OCP) is obtained by the study of coating properties such as surface hardness, porosity, surface roughness and microstructure of 9 coatings prepared by Taguchi program for 3 levels of four spray parameters. The Friction and wear behaviors of HVOF WC-CoCr coating prepared by OCP, electrolytic hard chrome (EHC) plating and Inconel718 (In718) are investigated by reciprocating sliding wear test at 25oC, 450oC. Friction coefficients (FC) of all of the 3 samples are decreased as increasing sliding surface temperature from 25oC to 450oC. FC of WC-CoCr decreases as increasing the surface temperature from 0.33 ± 0.02 at 25oC to 0.26 ± 0.02 at 450oC, showing the lowest FC among the 3 samples. Wear trace (WT) and wear depth (WD) of WC-CoCr are smaller than those of EHC and In718 both at 25oC and 450oC. These show that WC-CoCr is highly recommendable for protective coating on In718 and other metal components.

keyword : HVOF, WC-CoCr, Wear, Thermal spraying, HVOF, Taguchi program

The 2-step low temperature plasma processes (the combined carburizing and post-nitriding) were carried out for improving both the surface hardness and corrosion resistance of AISI 316L stainless steel. The effects of processing time and temperature on the surface properties during nitriding step were investigated. The expanded austenite (γN) was formed on all of the treated surface. The thickness of γN was increased up to about 20 ?m and the thickness of entire hardened layer was determined to be about 40 ?m. The surface hardness reached up to 1,200 HV0.1 which is about 5 times higher than that of untreated sample (250 HV0.1). The thickness of γN and concentration of N on the surface were increased with increasing processing time and temperature. The corrosion resistance in 2-step low temperature plasma processed austenitic stainless steels was enhanced more than that in the untreated austenitic stainless steels due to a high concentration of Non the surface.

keyword : austenitic stainless steel, Low temperature plasma carburizing, Low temperature plasma nitriding, expanded austenite

Effects of top coat morphology and thickness on thermal fatigue behavior of thermal barrier coatings (TBC) were investigated in this study. Thermal fatigue tests were conducted on three coating specimens with different top coat morphology and thickness, and then the test data were compared via microstructures, cycles to failure, and fracture surfaces. In the air plasma spray specimens (APS1, APS2), top coat were 200 and 300 ?m respectively. The thickness of top coat was about 700 ?m in the perpendicular cracked specimen (PCS). Under thermal fatigue condition at 1,100oC, the cycles to top coat failure of APS1, APS2, and PCS were 350, 560 and 480 cycles, respectively. The cracks were initiated at the interface of top coat and thermally grown oxide (TGO) and propagated into TGO or top coat as the number of thermal fatigue cycles increased. For the PCS specimen, additive cracks were initiated and propagated at the starting points of perpendicular cracks in the top coat. Also, the thickness of TGO and the decrease of aluminium concentration in bond coat do not affect the cycles to failure.

keyword : thermal barrier coatings, TBC, thermal fatigue, air plasma spray, thermally grown oxide, TGO

The formation of anodic oxide film of titanium (Ti) was studied at a variety of electrolyte concentrations and current density to clarify their effects on morphology, microstructure and composition of Ti oxide layer. For the analysis of the Ti oxide films, a scanning electron microscopy (SEM), X-ray diffractometer (XRD), and X-ray photoelectron spectroscopy (XPS) were used. The results showed that the concentration of phosphoric acid played a crucial role in the crystalline structure of the Ti oxide layer while the current density gave a critical effect on the thickness and diameter of its pore. In particular, the crystalline anatase phase with a thickness larger than 2 ?m, which is quite desirable for a dental implant application, could be readily prepared at the phosphoric acid concentration of 0.5 M and current density higher than 2.0 A/dm2.

keyword : Ti oxide Layer, anodizing, current density, electrolyte, microstructure

Prism sheet of LCD BLU which depends on supply from Japan and U.S.A was studied by using Si anisotropic etching and injection molding technologies. First, the prism sheet was patterned on Si wafer through photolithography, and the best conditions of Si etching were determined through etching Si wafer with TMAH to obtain straight optimized zigzag patterns, and a cross pattern to provide light diffusion and concurrent focusing. The etch rate of TMAH was concluded to be constant for 25wt%-70oC condition. Ni stamp of prism sheet was made by electrodeposition using patterned Si wafer, normal or fast H/C(Heating/Cooling) injections were carried out to fabricate prism sheet. It was known that fast H/C injection could fabricate prism sheet more accurately than normal injection. Zigzag patterns and the cross pattern showed higher transmissivity than the straight patterns because of light diffusion through diagonal direction. The fast H/C injection for zigzag patterns showed lower transmissivity than normal injection because there occurred more light diffusion through precise injection patterns, but the fast H/C injection for straight patterns showed only refraction without diffusion, causing lower transmissivity than normal injection.

keyword : prism sheets, Si anisotropic etching, zigzag patterns, cross pattern, Ni stamp, fast H/C injection molding, transmissivity

To manufacture Al MMCs, in-situ melt mixing process is used because it is free from contamination, and it makes reinforcements homogeneously dispersed. Large eddy simulation method is used to find the optimum melt mixing condition. At the Re 3000, the most suitable mixing is occurred between Al-Ti and Al-B melts. The in-situ formed TiB2 particles has the size varying from 40 nm to 130 nm, due to the increase of cooling rate, and exhibits a homogeneous dispersion. And the interface between reinforcement and matrix is clean. Both hardness and Young`s modulus of this composite are improved with increasing the cooling rate.

keyword : Melt mixing process, Al-TiB2 composite, Large eddy simulation

The objective of this study is to evaluate various machining characteristics of AlN-Hbn machinable ceramics in micro end-milling process for its further application. First, AlN based machinable ceramics with hBN contents in the range of 10 to 20vol% were prepared by hot-pressing. Material properties of the composites, such as relative density, Vickers hardness, flexural strength, Young`s modulus and fracture toughness were measured and compared. Then, micro end-milling experiments were performed to fabricate micro channels using prepared system. During the process, cutting forces, vibrations and AE signals were measured and analyzed using applied sensor system. Machined micro channel shapes and surface roughness were measured using 3D non-contact type surface profiler. From the experimental results, it can be observed that the cutting forces, vibrations and AE signal amplitudes decreased with increasing hBN contents. Also, measured surface roughness and profiles were improved with increasing hBN contents. As a result of this study, optimum machining conditions can be determined to fabricate desired products with AlN-hBN machinable ceramics based on the experimental results of this research.

keyword : AlN-hBN ceramics, machinable ceramics, machinability evaluation, micro end-milling, micro channel machining