발간논문

Home > KJMM 논문 > 발간논문

In the present study, the correlation of microstructural factors with yield ratio and uniform elongation was investigated in API X80 linepipe steels with different microstructures by varying thermo-mechanical control process conditions. The grain size and microstructure fraction of the API X80 linepipe steels were quantitatively measured by EBSD analysis and then tensile tests were carried out on them. Although all the steels showed complex microstructures of polygonal ferrite (PF), and bainitic microstructures such as acicular ferrite (AF), granular bainite (GB), and bainitic ferrite (BF), they had different grain sizes and microstructure fractions. The D steel with the finest grain size showed the highest yield strength due to grain refinement strengthening even though it had a relatively high fraction of PF with lower yield strength. The yield ratio usually increases with decreasing grain size because grain refinement typically produces an increase in yield strength and a decrease in work hardening due to the accumulation of dislocations at grain boundaries. On the other hand, uniform elongation increases with increasing PF fraction because an increase in the PF fraction eases slip and thus reduces plastic instability caused by accumulated dislocations at grain boundaries. Consequently, the microstructural factors affecting the yield ratio, uniform elongation, and work hardening exponent of the API X80 linepipe steels are different, and the work hardening exponent is considered to be a parameter affected by stress and strain simultaneously. Therefore, it was confirmed that there is no general correlation between yield ratios, uniform elongations, and work hardening exponents in the API X80 linepipe steels investigated in this study, and previously reported API linepipe steels. (Received February 22, 2018 ; Accepted April 9, 2018)

keyword : linepipe steels, yield ratio, uniform elongation, grain size, microstructure fraction, work hardening exponent

A Ni-Cr-B-Si-C coating layer was manufactured on a plate of Fe base alloy using a laser cladding process, and the microstructural and mechanical properties of the laser cladded coating layer and a Ni electroplating layer (used as a reference material) were investigated. To identify the microstructural and mechanical properties, scanning electron microscopy (SEM), x-ray diffraction (XRD), room and high temperature hardness, and wear tests were performed. The microstructural observation results confirmed that the layer was mainly composed of γ-Ni phases, Ni₄B₃, Cr₇C₃, and Cr₅B₃. The hardness of the laser cladded coating layer and the Ni electroplating layer were measured to be 950 and 154 HV respectively. Hardness measurements from room temperature up to a high temperature of 700 ℃ indicated that hardness decreased as temperature increased, but the hardness of the laser cladded coating layer was higher in all temperature conditions. Room temperature wear results showed that the wear loss of the laser cladded coating layer was 1/5 of the wear level of the Ni electroplating layer. (Received January 25, 2018; Accepted March 20, 2018)

keyword : laser cladding, Ni-Cr-B-Si-C, microstructure, high temperature hardness, wear

AISI M4 and CPM15V are used as tool steels since they have high hardness and wear resistance. This study is concerned with die and mold hardfacing using laser melting deposition with M4 and CPM15V powders. For this purpose, the two types of metal powders were deposited on SKD11 base metal, and the microstructures, hardness and wear resistance of the deposited materials were compared. During processing, substrate preheating was used to reduce the degree of thermal deformation and prevent crack formation. Based on the preliminary experiments, the process conditions were established. As a result of observing of the microstructure, columnar and cellular dendrites were observed in the M4 deposited layer. Using EDS analysis, the carbides precipitated along grain boundaries were identified to be M₆C/M₂₃C₆ carbides with a high content of molybdenum and tungsten. On the other hand, no dendritic microstructures were observed in the CPM15V deposited layer, and metastable austenite, martensite, and M₈C₇ carbides were formed. A large amount of fine carbide was found to have a high content of vanadium and chromium. The M4 and CPM 15V deposited regions had average hardnesses of 815 and 897 HV, respectively. The ball-on-disc test results showed that the deposited CPM15V had better wear resistance than that of the M4 deposited. These results are attributed to high martensite fraction, and the types and sizes of carbides of the deposited CPM15V. (Received February 12, 2018; Accepted April 16, 2018)

keyword : AISI M4, CPM15V, laser melting deposition, hardness, wear

To fabricate a film with the highest electrical conductivity through liquid-phase reaction sintering below 200 ℃ using composite pastes containing both Sn-58Bi and Cu particles as a filler material, the effects of process parameters, such as the mixing ratio of the fillers, reaction temperature, and reaction time, were evaluated and the parameters were optimized. The lowest sheet resistance of 0.0278 Ω/□ was obtained when the composite paste of Sn-58Bi/40 wt% Cu was sintered at 190 ℃ for 180 s. The reduced resistance was attributed to the increase in the portion of acicular Cu₆Sn₅ phases. The phases were formed by the dissolution and precipitation of Cu, and the three-dimensional linkage of irregular polygonal Cu₆Sn₅ phases using the acicular Cu₆Sn₅ through a Bi phase of the highest electrical resistivity. The reaction time, which was prolonged up to 180 s during the reaction at 190 ℃, maximized the number of acicular Cu₆Sn₅ and irregular polygonal Cu₆Sn₅ phases, through the formation of fine Cu₆Sn₅ phases by splitting behavior of coarse Cu₆Sn₅ phases. (Received March 14, 2018; Accepted April 26, 2018)

keyword : Sn-58Bi, composite paste, reaction sintering, sheet resistance, acicular Cu₆Sn₅

In this study, the extrusion behavior of Cu fill material in a through-Si-via (TSV) subjected to thermal loading was investigated. The Cu filling of the TSV was accomplished using pulse periodic reverse (PPR) electroplating. To study the extrusion, TSVs of varying via pitch were filled with Cu by electroplating. Defect-free Cu filling of the TSV was obtained at a Cathodic Current Density (CCD) of -5 mA/cm². The Cu-filled TSVs were subjected to annealing at 450 ℃ and the extrusion heights were measured. Microstructural characterizations were performed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The experimental results were also validated using finite element analysis (FEA). The results indicated that as the distance between via holes, i.e., pitch, decreased from 40 to 20 μm, the extrusion heights were found to increase. In other words, the extrusion height increases due to the mutual influence between vias when the spacing of the vias is reduced. The simulated extrusion heights of the Cu-filled TSVs were in good agreement with the experimental results. The FEA simulation results also indicated an overall increasing tendency of extrusion heights when via pitch decreased. (Received January 4, 2018; Accepted April 13, 2018)

keyword : extrusion, through-silicon-via, electroplating, finite element method, thermal expansion coefficient

Electroplated copper has been used as wiring in electronic circuit boards and as cathode materials for secondary batteries, and its usage has expanded into the construction industry because of its convenient production and high economic value. During the electroplating process, various parameters, such as crystal phase and size, can affect its mechanical properties and surface characteristics. Electroplating parameters were applied with the Taguchi experimental design method, and the electroplated copper layer was analyzed to observe changes in properties with various parameters. Among the parameters, current density significantly affected mechanical properties such as tensile strength and elongation. The size of the crystal grain decreased with the current density, which resulted in a 47% increase in tensile strength. Tensile strength and elongation tended to decrease when SPS(bis(3-sulfopropyl)disulfide)) was added as additives, and high elongation was observed when the hydroxyethyl cellulose(HEC)/SPS ratio was three times or more. When the concentration of sulfuric acid was high, the strongly preferred direction of crystal growth was parallel to the (111) plane. (Received February 26, 2018; Accepted April 9, 2018)

keyword : electroplating, parameter, current density, taguchi method, mechanical properties

(Ce_1-zNd_z)_yFe_4-xCo_xSb₁₂ skutterudites were prepared by encapsulated melting, annealing, and hot pressing. The formation of a marcasite phase (FeSb₂) was significantly suppressed by partially filling the voids with Ce/Nd and making a Co substitution at the Fe site. Both the Hall and the Seebeck coefficient values were positive, indicating that all specimens had p-type characteristics. The electrical conductivity decreased with increasing temperatures, as in a degenerate semiconductor. The Seebeck coefficient increased with the charge compensation of Co and filler content of Nd; however, the electrical conductivity decreased. The partially double-filled specimens had a higher electrical conductivity and lower Seebeck coefficient than the completely double-filled specimens, due to the increase in carrier concentration. The lattice thermal conductivity was expected to decrease via partial double filling with Ce and Nd; however, it did not change significantly because of the comparable atomic masses and ionic radii of Ce and Nd. Therefore, the partially double-filled specimens had lattice thermal conductivities comparable to those of the completely double-filled ones. Co substitution had an influence on the ZT values due to the change in the intrinsic conduction onset temperature, which decreased with increasing Co content. Thus, the (Ce_1-zNd_z)_yFe_4-xCo_xSb₁₂ skutterudites are a promising material for thermoelectric power generation in the mid-to-low temperature regions. (Received March 14, 2018; Accepted April 23, 2018)

keyword : thermoelectric, skutterudite, partial double filling, charge compensation

We developed a multi-scale modeling framework using the thermodynamic UW1 database of the Fe-Mn-Ni-Si quaternary system. To obtain input data for the phase-field model, such as mobility and interfacial energy data, we used DICTRA software and the TC-PRISMA package with the commercial TCFE8 and MOBFE3 databases. Using the developed framework, we evaluated the stability of single precipitates and compared the results to those from the thermodynamic model. We also investigated the precipitate interactions of paired particles of identical and different phases. (Received February 5, 2018; Accepted April 18, 2018)

keyword : particle stability, particle interaction, phase-field modeling