• International Journal of Korean Welding Society
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• v.3 no.2
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• pp.24-28
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• 2003

This study was aimed to evaluate the quality of flash welded joints and optimize the welding process for flash butt welding of 780MPa grade high strength steel. And then the relationship between the welding process variables and the joint quality would be established. The effect of process variables between flashing and upsetting process was elucidated. Microstructure observation of the joint indicated that the decarburized band was mainly changed with upsetting process. Width of HAZ was also related to the upsetting conditions rather than the flashing conditions. Generally maximum hardness at HAZ was correlated with Ceq of steel and the empirical relationship was obtained to estimate the HAZ properties. Tensile elongation at the joint was usually decreased with increasing the initial clamping distance. Investigation of fracture surface after tensile and bending tests reveal that the origin of cracking at the joint was oxide inclusions composed of $SiO_2$, MnO, $Al_2O_3$, and/or FeO. The amount of inclusions was dependent on the composition ratio of Mn/Si in steel. If this ratio was above 4, the amount of inclusions was low and then the resistance to cracking at the joint was enough to maintain the joint performance. It was obtained that the flashing process influenced the conditions for the energy input to establish uniform or non­uniform molten layer, while the upsetting conditions influenced the joint strength. Heat input variable during flashing process was also discussed with the joint properties.

• Journal of the Korean Society for Heat Treatment
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• v.36 no.3
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• pp.145-152
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• 2023

β titanium alloys containing β stabilizing elements such as V, Nb, Ta, Mo and Fe are widely used etc, due to their excellent specific strength, corrosion resistance, fatigue strength and easy formability. New metastable β titanium alloys are developed containing low-cost elements (Mo and Fe) in this study. Fe element is a strong β-stabilizer which can affect the mechanical and electrochemical properties of Ti-5Mo-xFe (x = 1, 4 wt%) alloys. These properties were analyzed in connection with microstructure and phase distribution. Ti-5Mo-4Fe alloy showed higher compression yield stress and maximum stress than Ti-5Mo-1Fe alloy due to solid-solution hardening and grain refinement hardening effect. As Fe element increased, Fe oxide formation and reduction of ${\bar{Bo}}$ (bond order) value affect the decrease of corrosion resistance. Ti-5Mo-xFe alloys were more excellent than Ti-6Al-4V ELI alloy.

• Journal of the Korean Society for Heat Treatment
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• v.36 no.4
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• pp.230-236
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• 2023

The current study deals with the effect of cooling rate and temperature for annealing on medium-carbon Cr-Mo alloy steel, especially for cold heading quality wire rod, to derive the optimum micro-structures for plastic deformation. This is to optimize the spheroidization heat treatment conditions for softening the material. Heat treatment was performed under seven different conditions at a temperature between A_c1 and A_c3, mostly within 720℃ to 760℃, and the main variables at this time were temperature, retention time and cooling rate. Microstructure and phase changes were observed for each test condition, and it was confirmed that they were greatly affected by the cooling rate. It was also confirmed that the cooling rate was changed in the range of 0.1℃/min to 5℃/min and affected by phase deformation and spheroidization fraction. The larger the spheroidization fraction, the lower the hardness, which is associated with the increasing connection of ferrite phases.

• Computers and Concrete
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• v.31 no.2
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• pp.97-110
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• 2023

This study investigates experimentally the impact of magnetized water (MW) on the fresh and hardened characteristics of concrete. Five types of MW are produced using magnetic fields of 1.4 and 1.6 Tesla for treating water with 100, 150, and 250 cycles. The concrete properties are assessed using the slump test, compressive strength test, scanning electron microscopy (SEM) analysis, energy dispersive X-ray analysis (EDX), and Fourier transform infrared spectrophotometry (FTIR). Furthermore, the chemical-physical characteristics of tap water (TW) and MW are evaluated. The results showed the magnetic field intensity has a significant impact on the magnetization effect; the best magnetizing conditions were found when TW was exposed successively to magnetic fields of 1.6 T and 1.4 T for 150 cycles. In addition, 150 MW cycles can be used to improve the compressive strength and workability of concrete by 40% and 17%, respectively. pH, total dissolved solids, and electrical conductivity improved by 15%, 17%, and 7%, respectively, when using MW. Additionally, MW can be used to enhance cement hydration chemical processes and made concrete's structure denser.

• Journal of the Korean Society for Heat Treatment
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• v.37 no.5
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• pp.221-227
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• 2024

The damping capacities in the strain-amplitude dependent and strain-amplitude independent regions were comparatively investigated for pure Mg and Mg-X%Al solid solutions (X : 1, 2 at%) to clarify the role of Al solute in the damping properties of Mg-Al binary solid solution. In order to rule out the effect of grain size on damping capacity, grain sizes of the samples were adjusted to an almost similar level by changing the heat-treatment or solution treatment times at 683 K (12 h, 24 h and 32 h for pure Mg, Mg-1%Al and Mg-2%Al alloys, respectively). The damping capacities of the heat-treated pure Mg and Mg-X%Al solid solutions exhibited a decreasing tendency with an increase in Al concentration both in the strain-amplitude dependent and strain-amplitude independent regions. The observed damping trends depending on strain-amplitude were analyzed and discussed in association with decreasing length between weak pinning points (Al solutes) in Granato-Lücke model.

• Archives of Metallurgy and Materials
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• v.65 no.3
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• pp.1001-1004
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• 2020

We investigated the austenite stability and mechanical properties in FeMnNiC alloy fabricated by spark plasma sintering. The addition of Mn, Ni, and C, which are known austenite stabilizing elements, increases its stability to a stable phase existing above 910℃ in pure iron; as a result, austenitic microstructure can be observed at room temperature, depending on the amounts of Mn, Ni, and C added. Depending on austenite stability and the volume fraction of austenite at a given temperature, strain-induced martensite transformation during plastic deformation may occur. Both stability and the volume fraction of austenite can be controlled by several factors, including chemical composition, grain size, dislocation density, and so on. The present study investigated the effect of carbon addition on austenite stability in FeMnNi alloys containing different Mn and Ni contents. Microstructural features and mechanical properties were analyzed with regard to austenite stability.

• Archives of Metallurgy and Materials
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• v.65 no.3
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• pp.1019-1022
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• 2020

In the past few years, overhead copper transmission lines have been replaced by lightweight aluminum transmission lines to minimize the cost and prevent the sagging of heavier copper transmission lines. High strength aluminum alloys are used as the core of the overhead transmission lines because of the low strength of the conductor line. However, alloying copper with aluminum causes a reduction in electrical conductivity due to the solid solution of each component. Therefore, in this study, the authors attempt to study the effect of various Al/Cu ratios (9:1, 7:3, 5:5) to obtain a high strength Al-Cu alloy without a significant loss in its conductivity through powder metallurgy. Low-temperature extrusion of Al/Cu powder was done at 350℃ to minimize the alloying reactions. The as-extruded microstructure was analyzed and various phases (Cu9Al₄, CuAl₂) were determined. The tensile strength and electrical conductivity of different mixing ratios of Al and Cu powders were studied. The results suggest that the tensile strength of samples is improved considerably while the conductivity falls slightly but lies within the limits of applications.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.24 no.2
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• pp.185-191
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• 2011

Plastic gradient from geometrically necessary dislocation(GND) can affect material behavior significantly. In this research, mechanical behavior of polycrystalline solid is investigated using the finite element method incorporating plastic gradient from long range dislocation or GND effect. Plastic gradient effect is implemented in the analysis model by considering a long range strain term as well as elastic and plastic terms in the multiplicative decomposition. In the model, gradient hardness coefficient and length parameter are used to evaluate the effect of the long range strains and sensitive study is conducted for the parameters. It is confirmed that the GND amplifies hardening response of polycrystals compared with the single crystal.

• Journal of Korea Foundry Society
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• v.37 no.5
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• pp.148-156
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• 2017

The effects of minor additives on the casting properties of AC4A aluminum alloys were investigated. Measurements of the cooling curve and microstructure observations were conducted to analyze the effects of Ti-B and Sr minor elements during the solidification process. A fine grain size and an increase in the crystallization temperature for the ${\alpha}-Al$ solution were evident after the addition of 0.1wt% Al-5%Ti-1%B additive. The modification effect of the eutectic $Mg_2Si$ phase with the addition of 0.05% Al-10%Sr additive was prominent. A fine eutectic $Mg_2Si$ phase and a decrease in the growth temperature of the eutectic $Mg_2Si$ phase were evident. Fluidity, shrinkage and solidification-cracking tests were conducted to evaluate the castability of the alloy. The combined addition of Al-5%Ti-1%B and Al-10%Sr additives showed the maximum filling length owing to the effect of the fine ${\alpha}-Al$ grains. The macro-shrinkage ratio increased, while the micro-shrinkage ratio decreased with the combined addition of Al-5%Ti-1%B and Al-10%Sr additives. The macro-shrinkage ratio was nearly identical, while the micro-shrinkage ratio increased with the addition of the Al-10%Sr additive. The tendency of the occurrence of solidification cracking decreased owing to the effect of the fine ${\alpha}-Al$ grains and the modification of the $Mg_2Si$ phase with the combined addition of Al-5%Ti-1%B and Al-10%Sr additives.

• Restorative Dentistry and Endodontics
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• v.20 no.2
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• pp.549-576
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• 1995

Bacteria are one of the most important causative agents of the pulpal and periapical diseases. Streptococci are one of the most frequently isolated facultative anarerobic bacteria in the infected root canals. Bacterial cell wall components have a direct effect in the pathogenesis of the pulpal and periapical infections. Hyaluronidase produced by bacteria has been implicated in dissemination of the diseases. The purpose of this study was to evaluate the effect of cell wall extract of streptococci on the L929 cells using inverted microscope and the transmission electron microscopy (TEM). Hyaluronidase production of streptococcal strains were investigated to determine the correlation between the severity of cell damage and the activity of enzymes. Bacterial cell wall extracts of S. sanguis, S. mitis and S. uberis isolated from infected root canals and ATCC type strains of S. mutans (ATCC 10449) and E. faecalis (ATCC 19433) were prepared by sonication and confirmed with SDS-PAGE. Silver stain of SDS-PAGE of sonic extract was efficient at $100{\mu}g$/ml concentration of cell wall protein, while Coomasie blue stain was efficient at $100{\mu}g$/ml concentration. Inverted microscope showed that sonic extract-treated L929 cells were round and detached from the substratum while others lost their fibroblastic shapes. Transmission electron microscopic examination revealed that streptococcal extracts induced death of L929 cells. Sonic extracts of streptococci had variable effect on microstructure of L929 cells. significant chromatin condensation was observed in the nucleus of the cells. Disappearance of cell surface microvilli and nuclear fragments with dense chromatin were observed. The cell nucleus had an irregular shape and numerous large vacuoles were seen in the cytoplasm and some breaks of the cell membrane could be seen. Cell organelles were in various stages of destruction and cristae of mitochondria were disoriented or disappeared. Eighteen strains of streptococci did not produce hyaluronidase.