• Journal of the Korean Society for Heat Treatment
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• v.24 no.3
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• pp.133-139
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• 2011

This study has been investigated for tensile properties with lead-free brass containing 1 wt.% of Bi. And also characteristic of corrosion resistance was analyzed by polarization test. An increase of tempering temperature was found to tend to decrease tensile strength, and percentage of elongation was shown to be the lowest value at $300^{\circ}C$. On the other hand, the elongation was increased with an increase of tempering temperature after $300^{\circ}C$. The change of mechanical properties was closely related with the content and shape of acicular Witmanst$\ddot{a}$tten ${\alpha}$ formed at the interface of ${\beta}$ phase as well as in ${\beta}$ phase. Tensile strength had a tendency to be decreased with an increase of test temperature. The elongation was shown to be the lowest value at around $300^{\circ}C$, while it began to increase as test temperature rose after $300^{\circ}C$. It might be speculated that the reason that elongation was decreased was found to form bismuth film at the interface of ${\alpha}/{\beta}$ phase leading to be easily brittle when loaded by tensile stress. The lead-free brass containing 1 wt.% of Bi had similar characteristic of corrosion resistance with a free-cutting brass with 3.4 wt. % of Pb in spite of higher fraction of ${\beta}$ phase.

• Korean Journal of Materials Research
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• v.9 no.10
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• pp.992-999
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• 1999

Due to the inherently poor adhesion strength of Cu-based leadframe/EMC(Epoxy Molding Compound) interface, popcorn-cracking phenomena of thin plastic packages frequently occur during the solder reflow process. In this study, in order to enhance the adhesion strength of Cu-based leadframe/EMC interface, brown-oxide layer was formed on the leadframe surface by immersing of leadframe sheets in hot alkaline solution, and the adhesion strength of leadframe/EMC interface was measured by using SDCB(Sandwiched Double Cantilever Beam) and SBN(Sandwiched Brazil-Nut) specimens. Results showed that brown oxide treatment of leadframe introduced fine acicular CuO crystals on the leadframe surface and improved the adhesion strength of leadframe/EMC interface. Enhancement of adhesion strength was directly related to the thickening kinetics of oxide layer. This might be due to the mechanical interlocking of fine acicular CuO crystals into EMC.

• Journal of the Korean Ceramic Society
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• v.43 no.9 s.292
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• pp.575-581
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• 2006

To precipitate the complex gel of flux and aluminum hydroxides gel, aqueous solution of the mixture of $Na_2CO_3\;and\;Na_2PO_4{\cdot}12H_2O$ was added with stirring in aqueous solution of the mixture of $Al_2(SO_4)_3{\cdot}14{\sim}18H_2O,\;Na_2SO_4$, and then the complex gel was aged in $0{\sim}30h\;at\;90^{\circ}C$. As aging time passed, the aluminum hydroxides was grown into the acicular AlO(OH) gel. Also, aging time had an effect on physical properties of the AlO(OH) gel and on crystal growth of the flaky ${\alpha}-Al_2O_3$ prepared by molten-salt precipitation. In this study, the complex gel was crystallized in temperature range of $400{\sim}1,200^{\circ}C$ after drying at $110^{\circ}C$, and then it was investigated to effect of aging time on precipitation temperature, size, thickness, morphology and particle size distribution of the flaky ${\alpha}-Al_2O_3$ crystal. As aging time passed, the flaky a${\alpha}-Al_2O_3$ crystal showed a tendency toward an increase in size and thickness as result from an increase in BET surface area and pore volume of the acicular AlO(OH) gel.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.29 no.9
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• pp.552-558
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• 2016

Goethite, ${\alpha}$-FeOOH have various applications such as absorbent, pigment and source for magnetic materials. Goethite particles were synthesized in a two step process, where $Fe(OH)_2$ were synthesized in nitrogen atmosphere using $FeSO_4$ as a raw material in the first process, and after that acicular goethite particles were obtained in an air oxidation process of $Fe(OH)_2$ in highly alkaline aqueous solution. Their phase and microstructure were investigated with XRD and FE-SEM. It was found that the morphology of goethite and the ratio of length-to-width (aspect ratio) of acicular goethite are dependent on the some factors such as R value ($OH^-/Fe^{2+}$), air flow rate and pH conditions. In particular, R value has the strongest influence on the synthesized goethite morphology. It is considered that the optimal value R is 4.5 because X-ray diffraction peaks of goethite have the highest intensity at that value. Morphology of goethite particles was controlled by air flow rates, showing that their size and aspect ratio are getting smaller and decrease, respectively as air flow rate increases. The largest goethite particle obtained is about 1,500 nm in length and 150 nm in diameter.

• Korean Journal of Materials Research
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• v.6 no.8
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• pp.768-778
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• 1996

Iron oxide (hematite, $\alpha$-${Fe}_{2}{O}_{3}$) particles were prepared directly from aqueous solution using a crystal growth controller. Paticles properties and reaction mechanisum of products as a function of basicity, formation process and mechanism of needle-lkie hematite were investigated. hexagonal hermatite particles were formed in teh range below pH 9.0, ellipsoidal or rectangular hematite particles in the range of pH 10.75-11.75 respectively. In the range above pH 12.50, acicular $\alpha$-FeOOH was formed. Basicity of product solution produced in the range of pH 10.7511.75 was increased slightly as compared with basicity of reastants due to hydroxly ion(OH-) formed by dissociation crystal growth controller. Citric acid which is acted as a crystal growth controller was adsorbed in the form of itrate anion(R-COO-) on the ferric hydroxide and exerted important role on the formation to the needle-like $\alpha$-${Fe}_{2}{O}_{3}$ particles in this reaction system.

• Journal of the Korean Society for Nondestructive Testing
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• v.22 no.1
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• pp.65-73
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• 2002

Artificial aging was performed to simulate the microstructural degradation in 2.25Cr-1Mo steel arising from long time exposure at $540^{\circ}C$. Microstructural analysis (mean equivalent size, number of carbides per unit area) and measurement of mechanical properties(UTS, Vickers hardness) and magnetic properties(coercivity, remanence) were performed. By comparing these results, the relationship between magnetic properties and microstructural changes with artificial aging was clarified. The carbides were classified as rod, globular and acicular type in terms of morphology. The fine acicular carbides were found to diminish drastically in the initial stage of aging. The magnetic coercivity and remanence were observed to decrease rapidly in the initial about 920 hours of aging time and then decrease slowly afterwards. Linear correlations between the mechanical properties and magnetic properties such as correlations remanence were found.

• Korean Journal of Metals and Materials
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• v.48 no.7
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• pp.615-624
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• 2010

This study is concerned with tensile and Charpy impact properties of high-strength bainitic steels fabricated by controlled rolling process. Six kinds of steels were fabricated by varying finish rolling temperature, start cooling temperature, and cooling rate, and their microstructures and tensile and Charpy impact properties were investigated. Their effective grain sizes were also characterized by the electron backscatter diffraction analysis. The microstructures of the steels rolled in the single phase region were most similar to those of the steels rolled in the two phase region. The steels cooled from $700{^{\circ}C}$ were composed mainly of granular bainites, while those cooled from $600{^{\circ}C}$ contained a number of bainitic ferrites, which resulted in the decrease in ductility and upper shelf energy in spite of the increase in strength. In the steels cooling from $600^{\circ}C$, fine acicular ferrites were well formed when the cooling rate was slow, which led to the best combination of high ductility, high upper shelf energy, and low energy transition temperature according to the decrease in the overall effective grain size due to the presence of acicular ferrites having smaller effective grain size.

• Korean Journal of Metals and Materials
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• v.50 no.1
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• pp.52-58
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• 2012

Hydrogen-delayed fracture (HDF) was analyzed from the deposited weld metals of 600-MPa and 800-MPa flux-cored arc (FCA) welding wires, and then from the diffusible hydrogen behavior of the weld zone. Two types of deposited weld metal, that is, rutile weld metal and alkali weld metal, were used for each strength level. Constant loading test (CLT) and thermal desorption spectrometry (TDS) analysis were conducted on the hydrogen pre-charged specimens electrochemically for 72 h. The effects of microstructures such as acicular ferrite, grain-boundary ferrite, and low-temperature-transformation phase on the time-to-failure and amount of diffusible hydrogen were analyzed. The fracture time for hydrogen-purged specimens in the constant loading tests decreased as the grain size of acicular ferrite decreased. The major trapping site for diffusible hydrogen was the grain boundary, as determined by calculating the activation energies for hydrogen detrapping. As the strength was increased and alkali weld metal was used, the resistance to HDF decreased.

• Korean Journal of Metals and Materials
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• v.48 no.9
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• pp.798-806
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• 2010

The effects of alloying elements and the cooling condition on the microstructure, tensile properties, and Charpy impact properties of high-strength bainitic steel plates fabricated by a controlled rolling process were investigated in the present study. Eight kinds of steel plates were fabricated by varying C, Cr, and Nb additions under two different cooling rates, and their microstructures and tensile and Charpy impact properties were evaluated. The microstructures present in the steels increased in the order of granular bainite, acicular ferrite, bainitic ferrite, and martensite as the carbon equivalent or cooling rate increased, which resulted in a decrease in the ductility and Charpy absorbed energy. The steels containing a considerable amount of bainitic ferrite or martensite showed very high strengths, together with good ductility and Charpy absorbed energy. In order to achieve the best combination of strength, ductility, and Charpy absorbed energy, granular bainite and acicular ferrite were properly included in the high-strength bainitic steels by controlling the carbon equivalent and cooling rate, while about 50 vol.% of bainitic ferrite or martensite was maintained to maintain the high strength.

• Korean Journal of Metals and Materials
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• v.46 no.12
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• pp.788-799
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• 2008

This study is concerned with the effects of Mo, Cr, and V addition on tensile and Charpy impact properties of API X80 linepipe steels. Four kinds of steels were processed by varying Mo, Cr, and V additions, and their microstructures and tensile and Charpy impact properties were investigated. Since the addition of Mo and V promoted to form fine acicular ferrite and granular bainite, while prohibiting the coarsening of granular bainite, it increased the strength and upper shelf energy, and decreased the energy transition temperature. The Cr addition promoted the formation of coarse granular bainite and secondary phases such as martensite-austenite constituents, thereby leading to the increased effective grain size, energy transition temperature, and strength and to the decreased upper shelf energy. The steel containing 0.3wt.% Mo and 0.06wt.% V without Cr had the highest upper shelf energy and the lowest energy transition temperature because its microstructure was composed of fine acicular ferrite and granular bainite, together with a small amount of hard secondary phases, while its tensile properties maintained excellent.