• 한국재료학회지
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• 제27권11호
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• pp.636-642
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• 2017

In the present study the microstructure of low-carbon steels fabricated by controlled rolling and accelerated cooling processes was characterized and identified based on various microstructure analysis methods including optical and scanning electron microscopy, and electron backscatter diffraction(EBSD). Although low-carbon steels are usually composed of ${\alpha}-ferrite$ and cementite($Fe_3C$) phases, they can have complex microstructures consisting of ferrites with different size, morphology, and dislocation density, and secondary phases dependent on rolling and accelerated cooling conditions. The microstructure of low-carbon steels investigated in this study was basically classified into polygonal ferrite, acicular ferrite, granular bainite, and bainitic ferrite based on the inverse pole figure, image quality, grain boundary, kernel average misorientation(KAM), and grain orientation spread(GOS) maps, obtained from EBSD analysis. From these results, it can be said that the EBSD analysis provides a valuable tool to identify and quantify the complex microstructure of low-carbon steels fabricated by controlled rolling and accelerated cooling processes.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2010년도 춘계학술발표대회 초록집
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• pp.71-71
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• 2010

SBD (Strain-based design) of pipe lines have gained world-wide attention in recent years. The present research aims to evaluate the fracture characteristics of API (America Petroleum Institute) SBD X100 girth weldment that typically applied for cold climate and deep water offshore, with the focus on the influence of heat input changing with 6kJ/cm and 10kJ/cm from GMAW (Gas Metal Arc Welding). At a low heat input at 6kJ/cm, the weld metal had Multi-phase matrix (Acicular ferrite + Banite + Martensite) that could fill up both fracture toughness and strength as reported previously. Also, the weld metal exhibited 859MPa YS (Yield strength), 108J impact toughness at $-40^{\circ}C$ and 0.52mm CTOD (Crack Tip Open Displacement) at $-10^{\circ}C$. These results can be satisfied with the requirement of API SBD X100 girth weldment and Alaska pipe line project.

• 한국산업융합학회 논문집
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• 제23권4_1호
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• pp.531-539
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• 2020

When injection mold is repeatedly used for mass production, fatigue phenomenon due to cyclic stress may occur. The surface and interior of structure might be damaged due to cyclic stress or strain. The objective of this study was to analyze failure of SCM435 high-tension bolts connecting upper and lower parts of a three-stage injection molding machine. These bolts have to undergo an accurate heat treatment to prevent the formation of chromium carbide and the action of dynamic stresses. Bolts were fractured by cyclic bending stress in the observation of ratchet marks and beach marks. Damaged specimen showed an acicular microstructure. Impurity was observed. Chromium carbide was observed near the crack origin. Both shape parameters of the Vickers hardness were similar. However, the scale parameter of the damaged specimen was about 20% smaller than that of the as-received specimen. Much degradation occurred in the damaged specimen. Bolts should undergo an accurate heat treatment to prevent the formation of chromium carbide. They must prevent the action of dynamic stresses. Bolts need accurate tightening and accuracy of heat treatment and screws need compression residual stress due to peening.

• 한국세라믹학회지
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• 제36권5호
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• pp.547-554
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• 1999

The self-reinforced Si3N4 ceramics were prepared by pressureless-sintering using ${\beta}$-Si3N4 whiskers as a seed. Effects of ${\beta}$-Si3N4 whiskers on microstructure and mechanical properties and the ${\alpha}$ to ${\beta}$ phase transition of Si3N4 were investigated. The self-reinforced Si3N4 ceramics were densified(relative density$\geq$98%) by pressureless-sintering (1800$^{\circ}C$ 2h) using 8mol% Y2O3 and 6mol% Al2O3 as sintering aids and 5 vol% ${\beta}$-Si3N4 whiskers within self-reinforced Si3N4 ceramic seemed to hinder the densification owing to their acicular shapes but accelerated the ${\alpha}$ to ${\beta}$ phase transition because they acted as pre-existing nuclei. It was found that the more ${\beta}$-Si3N4 nucei the faster ${\alpha}$ to ${\beta}$ phase transition.

• Geomechanics and Engineering
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• 제20권3호
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• pp.233-241
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• 2020

Alkali-silica reaction (ASR) is among one of the most important damaging mechanisms in concrete, depending primarily on aggregates which contain reactive minerals. However, expansion in concrete may not directly relate to the reactive minerals. This study aims to investigate the influence of ASR and the expansion of mortar bars depending on aggregate type containing various components such as quartz, clay minerals (montmorillonite and kaolinite) and micas (muscovite and biotite). In this study, the accelerated mortar bar tests (AMBT) were performed in two conditions (mortar bars in the same and sole NaOH solutions). Petrographic thin section studies, X-ray diffraction (XRD) analysis (Rietveld method), scanning electron microscopy (SEM) and chemical analyses were carried out. This study showed that quartzite bars led to increase in expansion values of mortar bars in diabase-1 and andesite when these were in the same NaOH solution. However, three samples (basalt, quartzite and claystone) were found having ASR expansion based on the AMBT when the special molds were used for each sample. SEM study revealed that samples which exhibit highest expansions according to AMBT had a generally rough surface and acicular microstructures in or around the micro-cracks. Basalt and quartzite showed more variable in major oxides than those of other samples based on the chemical analyses, SEM studies and AMBT. This study revealed that the highest expansions were observed to source not only from reactive aggregates but also from alteration products (silicification, chloritization, sericitization and argillisation), phyllosilicates (muscovite, biotite and vermiculite) and clays (montmorillonite and kaolinite).

• Transactions on Electrical and Electronic Materials
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• 제1권3호
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• pp.23-28
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• 2000

A block-oxide layer was formed on the surface of Cu-based leadframe by chamical oxidation method in order to enhance the adhesion strength between Cu-based leadframe and epoxy molding compound (EMC) Using sandwiched double cantilever beam (SDCB) specimens, the adesion strength was measured in terms of interfacial fracture toughness, G$\sub$IC//Results showed that the black-oxide layer was composed of two kinds of layers: pebble-like Cu$_2$O layer and acicular CuO layer, At the initial stage of oxidation the Cu$_2$O layer was preferentially formed and thickened up to around 200 nm whithin 1 minute of the oxidation time. Then the CuO layer started to from atop of the Cu$_2$O layer and thickened up to around 1300 nm until 20 minutes. As soon as the CuO layer formed, the thickness of Cu$_2$O layer began to reduce and finally reached to around 150 nm. The pre-cleaned and the Cu$_2$O coated leadframes showed almost no adhesion of EMC, however, as the CuO precipitates appeared and became continuous, G$\sub$IC/ increased up to around 80 J/㎡. Further oxidation raised G$\sub$IC/ up. to around 100 J/㎡.

• 한국주조공학회지
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• 제13권2호
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• pp.187-193
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• 1993

In this study, it was examined the relationship between the microstructure, fatigue properties, mechanical properties and retained austenite volume of Mo-Ni ADI corresponding to various austempering temperatures. When the austempering temperature is increased to $370^{\circ}C$, acicular bainite structure was found to be transformed to feathery bainite structure. But at the austempering temperature of $420^{\circ}C$, the dissolved bainite lath was showned. Up to the austempering temperature of $370^{\circ}C$, the volume of retained austenite was increased. However at the austempering temperature of $420^{\circ}C$ a large amount of retained austenite was decreased. In this study, the retained austenite volume was determined by XRD(X-ray diffraction). It was observed that the optimum fatigue properties can be obtained at the condition of austempering temperature $370^{\circ}C$. Under the such conditions, fatigue limit determined as the value of 290 MPa, tensile strength 877MPa elongation 6%, hardness 285(BHN), impact values(CVN) 9.2J and retained austenite volume 30.3%, respectively.

• Journal of Welding and Joining
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• 제27권6호
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• pp.25-30
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• 2009

SBD (Strain-based design) of pipe lines have gained world-wide attention in recent years. The present research aims to evaluate the fracture characteristics of API (America Petroleum Institute) SBD X100 girth weldment that typically applied for cold climate and deep water offshore, with the focus on the influence of heat input changing with 6kJ/cm and 10kJ/cm from GMAW (Gas Metal Arc Welding). At a low heat input at 6kJ/cm, the weld metal had Multi-phase matrix (Acicular ferrite + Banite + Martensite) that could fill up both fracture toughness and strength as reported previously. Also, the weld metal exhibited 859MPa YS (Yield strength), 108J impact toughness at $-40^{\circ}C$ and 0.52mm CTOD (Crack Tip Open Displacement) at $-10^{\circ}C$. These results can be satisfied with the requirement of API SBD X100 girth weldment and Alaska pipe line project.

• Corrosion Science and Technology
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• 제20권1호
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• pp.37-43
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• 2021

Additively manufactured (AM) Ti-6Al-4V alloys exhibit a dominant acicular martensite phase (α'), which is characterized by an unstable energy state and highly localized corrosion susceptibility. Electrochemical critical localized corrosion temperature (E-CLCT, ISO 22910: 2020) and electrochemical critical localized corrosion potential (E-CLCP, ISO AWI 4631: 2021) were measured to analyze the localized corrosion resistance of the AM Ti-6Al-4V alloy. Although E-CLCP was measured under mild corrosive conditions such as human body, the validity of evaluating localized corrosion resistance of AM titanium alloys was demonstrated by comparison with E-CLCT. However, the mechanisms of resistance to localized corrosion on the as-received and heat-treated AM Ti-6Al-4V alloys under E-CLCT and E-CLCP differ at various temperatures because of differences in properties under localized corrosion and repassivation. The E-CLCT is mainly measured for initiation of localized corrosion on the AM titanium alloys based on temperature, whereas the E-CLCP yields repassivation potential of re-generated passive films of AM titanium alloys after breaking down.

• Metals and materials international
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• 제24권6호
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• pp.1221-1231
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• 2018

The effect of strain aging on tensile behavior and properties of API X60, X70, and X80 pipeline steels was investigated in this study. The API X60, X70, and X80 pipeline steels were fabricated by varying alloying elements and thermomechanical processing conditions. Although all the steels exhibited complex microstructure consisting of polygonal ferrite (PF), acicular ferrite, granular bainite (GB), bainitic ferrite (BF), and secondary phases, they had different fractions of microstructures depending on the alloying elements and thermomechanical processing conditions. The tensile test results revealed that yielding behavior steadily changed from continuous-type to discontinuous-type as aging temperature increases after 1% pre-strain. After pre-strain and thermal aging treatment in all the steels, the yield and tensile strengths, and yield ratio were increased, while the uniform elongation and work hardening exponent were decreased. In the case of the X80 steel, particularly, the decrease in uniform elongation was relatively small due to many mobile dislocations in PF, and the increase in yield ratio was the lowest because a large amount of harder microstructures such as GB, BF, and coarse secondary phases effectively enhanced work hardening.