• Advances in materials Research
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• v.4 no.1
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• pp.1-12
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• 2015

Low carbon steel of composition 0.05C - 0.18 Mn - 0.012 Si is intercritically annealed at temperatures $750^{\circ}C$, $775^{\circ}C$ and $800^{\circ}C$. The equilibrated alloys of different amounts of austenite with varying carbon contents are quenched in iced water. The same alloys are subcritically annealed at $675^{\circ}C$ and $700^{\circ}C$ for varying periods of times; the subcritically annealed alloy samples are quenched in iced water. Optical, scanning electron and transmission electron microscopy are carried out for all the samples. The dislocation structure, its distribution and density present in the above prepared duplex ferrite martensite steels are studied. The martensites are found to be highly dislocated due to lattice invariant deformation. At the same time ferrite adjoining the martensite areas also exhibits quite a high dislocation density. The high dislocation density is favorable for strain ageing and hence bakes hardenability. EDS analyses were carried out for both martensite and ferrite phases; it is found that the degree of supersaturation in ferrite together with carbon content in martensite varies with the process parameters. The microhardness test results show that the hardness values of different phases differ appreciably with process parameters. The microstructures and the corresponding microanalyses reveal that differently processed steels contain phases of varying compositions and different distribution.

• Korean Journal of Materials Research
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• v.25 no.10
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• pp.559-565
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• 2015

The ductile-brittle transition behavior of two austenitic Fe-18Cr-10Mn-N-C alloys with different grain sizes was investigated in this study. The alloys exhibited a ductile-brittle transition behavior because of an unusual brittle fracture at low temperatures unlike conventional austenitic alloys. The alloy specimens with a smaller grain size had a higher yield and tensile strengths than those with a larger grain size due to grain refinement strengthening. However, a decrease in the grain size deteriorated the low-temperature toughness by increasing the ductile-brittle transition temperature because nitrogen or carbon could enhance the effectiveness of the grain boundaries to overcome the thermal energy. It could be explained by the temperature dependence of the yield stress based on low-temperature tensile tests. In order to improve both the strength and toughness of austenitic Fe-Cr-Mn-N-C alloys with different chemical compositions and grain sizes, more systematic studies are required to understand the effect of the grain size on the mechanical properties in relation to the temperature sensitivity of yield and fracture stresses.

• Journal of Welding and Joining
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• v.16 no.1
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• pp.125-133
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• 1998

The effect of volume fraction of Cr carbide phase (Cr CVF) on the low stress abrasion resistance in the chromium-carbide-type high Cr white iron hardfacing weld deposits has been investigated. In order to examine Cr CVF, a series of alloys with varying Cr CVF by changing chromium and carbon contents and the ratio of Cr/C were employed. The alloys were deposited once or twice on a mild steel plate using the self-shielding flux cored arc welding process. The low stress abrasion resistance of the alloys against sands was measured by the Dry Sand/Rubber Wheel Abrasion Test (RWAT). It was shown that hardness and abrasion resistance increased with increasing Cr CVF within the whole test range (Cr CVF : 0.23-0.64). Both primary Cr carbide and eutectic Cr carbide were particularly effective in resisting wear due to their high hardness.

• Corrosion Science and Technology
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• v.17 no.2
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• pp.68-73
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• 2018

Substitution of hip and knee joints by CoCr alloys is in great demand due to their high wear resistance and good biocompatibility. Understanding of tribocorrosion in joint replacements requires study of variables such as coefficient of friction and the choice of a proper corrosive medium in wear-corrosion tests carried out in the lab. The objective of this study was to characterize tribocorrosion behaviour of CoCr alloy with low (LCCoCr) and high carbon (HCCoCr) contents in several corrosive media: NaCl, Phosphate Buffer Solution (PBS), and PBS with hyaluronic acid (PBS-HA). Tribocorrosion tests were carried out on a pin-on-disk tribometer with an integrated electrochemical cell. A normal load of 5N was applied on the alumina ball counterpart at a rotation rate of 120 rpm. Coefficient of friction (COF) was measured and tribocorrosion behaviour was characterized by in situ application of electrochemical techniques. HCCoCr alloy immersed in PBS-HA showed the best tribocorrosion behaviour with the lowest COF. In this case, in situ measurement of corrosion potential and the impedance data under wear corrosion process showed an active state while passive film was continuously destroyed without possibility of regeneration.

• Journal of Korea Foundry Society
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• v.18 no.4
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• pp.383-388
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• 1998

Effects of $CaCN_2$ addition on the grain refinement in the AM60 magnesium ingots were investigated. The effects of the $CaCN_2$ are estimated with different inoculation temperatures, inoculation contents, and holding time to find out the optimum condition. AM60 alloy was melted in the low carbon steel crucible by cylindrical electric furnace under an argon atmosphere. The melting and casting apparatus is specially designed for magnesium alloys. The grain size of AM60 magnesium alloy decreased significantly with an increase in $CaCN_2$ content and, at 0.8 wt% $CaCN_2$ or more, grain size becomes constant at about $85 {\mu}m$. The optimum condition was obtained in the 0.8 wt% $CaCN_2$ for holding molten metal of 30 min. at the temperature of $710^{\circ}C$. The tensile properties of AM60 magnesium alloys were improved due to grain refinement by addition of $CaCN_2$. In the optimum condition, the yield strength, tensile strength and elongation were ${\sigma}_{0.2}=107 MPa$, ${\sigma}_{T.S}=234 MPa$ and e=14.2%. The variation of stress with strain obeyed the relationship of the ${\sigma}=K{\varepsilon}^n$. The strain-hardening exponent, n and strength coefficient, K obtained in the 0.8 wt% $CaCN_2$ added AM 60 magnesium alloy were n=0.21 and K=390 MPa.

• Korean Journal of Metals and Materials
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• v.50 no.4
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• pp.285-292
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• 2012

Hot ductility behavior of precipitation-hardened low-carbon iron alloys containing 0.02 wt% Ti and 0.05 wt% Nb was characterized by a hot tensile stress test. Carbon (0.05, 0.1, 0.25 wt%) and boron (0.002 wt%) contents were varied to study the effect of precipitates on the high-temperature embrittlement of the alloys in the temperature range of $600{\sim}800^{\circ}C$. Ductility loss was observed at $700^{\circ}C$ for the tested alloys. The cause of the ductility loss was mainly attributed to the carbides and ferrite films formed at the grain boundaries during deformation. Although the carbon content tended to raise the total fraction of Nb (C, N), the precipitates were formed mostly in the grain interior as the precipitation temperature was raised above the deformation temperature by the high carbon content. Hence, carbon in excess suppressed the hot ductility loss. Meanwhile, boron addition improved the hot ductility of the alloys. The improvement is likely due to the boron atoms capturing carbon atoms and thus retarding the carbide formation.

• Journal of the Korean Society for Advanced Composite Structures
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• v.2 no.4
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• pp.1-10
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• 2011

This study investigates the performance of composite (steel-concrete) frame structures through numerical experiments on individual connections. The innovative aspects of this research are in the use of connections between steel beams and concrete-filled tube (CFT)columns that utilize a combination of low-carbon steel and shape memory alloy (SMA) components. In these new connections, the intent is to utilize the recentering provided by super-elastic shape memory alloy tension bars to reduce building damage and residual drift after a major earthquake. The low-carbon steel components provide excellent energy dissipation. The analysis and design of these structures is complicated because the connections cannot be modeled as being simply pins or full fixity ones they are partial restraint (PR). A refined finite element (FE) model with sophisticated three dimensional (3D) solid elements was developed to conduct numerical experiments on PR-CFT joints to obtain the global behavior of the connection. Based on behavioral information obtained from these FE tests, simplified connection models were formulated by using joint elements with spring components. The behavior of entire frames under cyclic loads was conducted and compared with the monotonic behavior obtained from the 3D FE simulations. Good agreement was found between the simple and sophisticated models, verifying the robustness of the approach.

• Journal of the Microelectronics and Packaging Society
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• v.26 no.4
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• pp.163-169
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• 2019

Sn-Pb solder alloys in electronics rapidly has been replaced to Pb free solder alloys because of various environmental regulations such as restriction of hazardous substances directive (RoHS), European Union waste electrical, waste electrical and electronic equipment (WEEE), registration evaluation authorization and of chemicals (REACH) etc. Because Sn58%Bi (in wt.%) solder alloy has low melting point and higher mechanical properties than that of Sn-Pb solder, it has been studied to manufacture electronic components. However, the reliability of Sn58%Bi solder could be lowered because of the brittleness of Bi element included in the solder alloy. Therefore, we observed the microstructures of Sn58%Bi composite solders with various contents of Sn-decorated multiwalled carbon nanotube (Sn-MWCNT) particles and evaluated bonding strength of the FR-4 components assembled with Sn58%Bi composite solder. Also, microstructures and bonding strengths of the Sn58%Bi composite solder joints were evaluated with the number of reflows from 1 to 7 times, respectively. Bonding strengths and fracture energies of the Sn58%Bi composite solder joints were measured by die shear test. Microstructures and fracture modes were observed with scanning electron microscope (SEM). Microstructures in the Sn58%Bi composite solder joints were finer than that of only Sn58%Bi solder joint. Bonding strength and fracture energy of Sn58%Bi composite solder including 0.1 wt.% of Sn-decorated MWCNT particles increased up to 20.4% and 15.4% at 5 times in reflow, respectively.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.534-537
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• 2008

In recent years, the dissimilar metal, Alloy 82/182 welds used to connect stainless steel piping and low alloy steel or carbon steel components in nuclear reactor piping system have experienced cracking due to primary water stress corrosion(PWSCC). It is well known that one reason of the cracking is the residual stress by the weld. But, it is difficult to estimate exactly weld residual stress due to many parameters of welding. In this paper, the analysis of 3 FEM models made by ABAQUS Code is performed to estimate exactly the weld residual stress on the dissimilar metal weld. 3 FEM models are Butt model, Repair model and Overlay model and are the plane.strain 2D model. The thermal analysis and the stress analysis are performed on each model and the residual stresses on each model were calculated and compared respectively. Also, the specimen of Butt model was made and the residual stresses were measured by X-Ray method and Hole Drilling Technique. These results were compared with the FEM result of Butt model.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.4 no.2
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• pp.50-56
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• 2008

The tubes in heat exchanger are typically made of copper alloy, stainless steel, carbon steel, titanium alloy material. Type-439 ferritic stainless steel is ferromagnetic material, and furnish higher heat transfer rates than austenitic stainless steels and higher resistance to corrosion-induced flaws. Ferritic stainless steel can be found in low-pressure(LP) feedwater heaters and moisture separator reheaters(MSRs) in turbine system. LP feedwater heaters generally utilize thin wall Type-439 stainless steel tubing, whereas MSRs typically employ a heavier wall tubing with integral fins. Service-induced damage can occur on the O.D(outside diameter) surface of Type-439 ferritic stainless steel tubing which is employed for MSRs tubing, and the most typical damage mechanism is vibration-induced tube-to-TSP(tube support plate) wear and fatigue cracking. The wear has been reported that occurs mainly on the OD surface. Accordingly, in this study, we have evaluated the flaw sizing capability of magnetic saturation eddy current technique using magnetic saturation probe and flawed specimen.