발간논문

Home > KJMM 논문 > 발간논문

Steel columns used in steel buildings are inclined to lose their strength when exposed to severe fire conditions, so fire resistance is required in most countries to protect against loss of life and building collapses. In Korea, the fire resistance of columns can be obtained by the fire test defined in KS F 2257-1, 7. The fire resistance of a steel column should be evaluated in terms of the column`s conditions, such as various section types (H-section, hollow-section), the column`s length and boundary conditions, and whether it is fixed or hinged. However, fire testing of steel columns is usually conducted on one standard-sized H-section over 3,000 mm, and the result is used as the column`s fire resistance. This is not a reasonable way to ensure that a building can withstand fire conditions. In this study, to evaluate the possibility of calculating the fire resistance of steel columns with material properties of high tensile strength of SS 400, both load-bearing fire tests and calculation of steel temperatures were carried out. The results of temperature calculation were very similar to those obtained by fire test.

keyword : metals, deformation, mechanical properties, tensile test, fire resistance

The aim of this study was to determine the effect of applied pressure and sintering temperature on the microstructure and mechanical properties for spark plasma sintering (SPS) from commercial pure titanium (CP-Ti) powders. Spark plasma sintering is a relatively new sintering technique in powder metallurgy which is capable of sintering metal and ceramic powers quickly to full density at a fairly low temperature due to its unique features. SPS of ?200 mesh or ?400 mesh CP-Ti powders was carried out in an Ar+H2 mixed gas flowing atmosphere between 650℃ and 750℃ under 10 to 80 MPa pressure. When SPS was carried out at relatively low temperatures (650℃ to 750℃), the high (>60 MPa) pressure had a marked effect on densification and grain growth suppression. The full density of titanium was achieved at temperatures and pressures above 700℃ and 60 MPa by spark plasma sintering. The crystalline phase and microstructure of titanium sintered up to 700℃ consisted of α-Ti and equiaxed grains. Vickers hardness ranging from 293 to 362 Hv and strength ranging from 304 to 410MPa were achieved for spark plasma sintered titanium.

keyword : metals, powder processing, mechanical properties, microstructure, scanning elctron microscopy, hardness, strength

To develop creep resistant die-cast Mg alloys, various alloying elements, including Ca, Ce, and Sr, were added to a Mg-Al alloy. The AXE710 alloy was produced on a 320 ton high-pressure die casting machine. The microstructure and creep properties of the alloy were examined. The creep behavior was investigated at 150℃ for stresses ranging from 50 to 100 MPa. The stress exponent was derived from the relationship between normalized secondary creep rates and compensated effective stresses. It was found to be 4.9, indicating that the dislocation climb is a dominant creep mechanism.

keyword : alloys, casting, microstructure, creep

The thermo-chemical treatment (TCT) process was applied to achieve surface hardening of CP titanium. The following three different surface modification conditions were tested so that the best surface hardening process could be selected:(a) PVD, (b) TCT+PVD, and (c) TCT+Aging+PVD. These specimens were tested and analyzed in terms of surface roughness, wear, friction coefficient, and the gradient of hardening from the surface of the matrix. The three test conditions were all beneficial to improve the surface hardness of CP titanium. Moreover, the TCT treated specimens, that is, (b) and (c), showed significantly improved surface hardness and low friction coefficients through the thickness up to 100㎛. This is due to the functionally gradient hardened surface improvement by the diffused interstitial elements. The hardened surface also showed improvement in bonding between the PVD and TCT surface, and this leads to improvement in wear resistance. However, TCT after aging treatment did not show much improvement in surface properties compared to TCT only. For the best surface hardening on CP titanium, TCT+PVD has advantages in surface durability and economics.

keyword : titanium, thermo-chemical treatment, oxy-nitriding, surface hardening

P122 steel, with a composition of Fe-10.57%Cr-1.79%W-0.96Cu-0.59Mn was arc-welded and oxidized between 600℃ and 800℃ in air for up to 6 months. The oxidation rates increased in the order of the base metal, weld metal, and heat-affected zone (HAZ), depending on the microstructure. The scale morphologies of the base metal, weld metal, and HAZ were similar because it was determined mainly by the alloy chemistry. The scale consisted primarily of a thin Fe2O3 layer at 600℃ and 700℃ and an outer Fe2O3 layer and an inner (Fe2O3, FeCr2O4)-mixed layer at 800℃. The microstructural changes resulting from heating between 600℃ and 800℃ coarsened the carbide precipitates, secondary Laves phases, and subgrain boundaries in the matrix, resulting in softening of the base metal, weld metal, and HAZ.

keyword : metals, welding, microstructure, oxidation, thermal analysis

The premature failure of a servovalve used for six months in a thermal power plant has been analyzed. The servovalve was made of stainless steel, containing 16Cr-0.44Mo, along with other elements. An overload of oil-supply pumping and an abnormal increase in the oil flux were observed during operation. A study revealed that erosion and corrosion could be the main causes of the failure. The visual examination of the servovalve did not show any appreciable damage. However, corrosion and erosion of the servovalve were observed using scanning electron microscopy (SEM). Upon examination of the servovalve, the corrosion was found to have occurred throughout the bushing and spool; however, erosion occurred at only the edge-side. In addition, the condition of the electrohydraulic control system (EHC) oil was investigated with respect to its satisfaction of the management standard.

keyword : alloys, deformation, corrosion, scanning electron microscopy, SEM, servovalve

Nanopowders of Fe3N and Si were fabricated by high-energy ball milling. A dense nanostructured 12Fe-Si3N4 composite was simultaneously synthesized and consolidated using a high-frequency induction-heated sintering method for 2 minutes or less from mechanically activated powders of Fe3N and Si. Highly dense 12Fe-Si3N4 with a relative density of up to 99% was produced under simultaneous application of 80 MPa pressure and the induced current. The microstructure and mechanical properties of the composite were investigated.

keyword : Fe-Si3N4, rapid sintering, hardness, fracture toughness

W-Ni-Fe heavy alloys have been used in various fields, such as kinetic energy penetrators and radiation shielding materials, due to their high density and good mechanical properties. In this study, the sintering of W-Ni-Fe alloys with various Ni/Fe ratios was demonstrated to improve the mechanical properties and penetration capabilities of heavy alloys by formation of interfacial phase. The microstructural changes and the mechanical properties of the W-Ni-Fe alloys after liquid-phase sintering were investigated. The Vickers hardness and tensile strength of the 95W1.3Ni3.7Fe sample, which had coated W grains by Fe7W6 phase (μ-phase), were 450 Hv and 1560 MPa, respectively. As a result, enhancement of the mechanical properties was considered to have uniformly generated μ-phase around W grains.

keyword : composites, sintering, microstructure, hardness test, compression test

The present work shows how the flexural displacement-induced fracture strength of silicon devices, whose back surfaces have wafer grinding-induced scratch marks, depends on the crystallographic orientation. Experimental results indicate that silicon devices with scratch marks parallel to their lateral direction (i.e. reference axis in this work) are very susceptible to flexural fracture, as compared to devices with marks which deviated from the direction. The 3-point bending test shows that the fracture strength of silicon devices having marks which are oriented away from the reference axis is 2.6 times higher than that of devices with marks parallel to the axis. It was particularly interesting to see that silicon devices with identical preferred marks even reveal different fracture strengths, depending on whether the marks are involved in specific crystal planes such as {111} or {011}, called cleavage planes. This work demonstrates that silicon devices with the reference axis-aligned scratch marks not existing on such cleavage planes can have higher fracture strength approximately 20% higher than those existing on the planes.

keyword : semiconductor, ceramics, machining, crystal structure, fracture

Silicon oxide thin films were deposited by using a plasma-enhanced chemical-vapor deposition technique to investigate the light emission properties. The photoluminescence characteristics were divided into two categories along the relative ratio of the flow rates of SiH4 and N2O source gases, which show light emission in the broad/visible range and a light emission peak at 380 nm. We attribute the broad/visible light emission and the light emission peak to the quantum confinement effect of nanocrystalline silicon and the Si=O defects, respectively. Changes in the photoluminescence spectra were observed after the post-annealing processes. The photoluminescence spectra of the broad light emission in the visible range shifted to the long wavelength and were saturated above an annealing temperature of 900℃ or after 1 hour annealing at 970℃. However, the position of the light emission peak at 380 nm did not change at all after the post-annealing processes. The light emission intensities at 380 nm initially increased, and decreased at annealing temperatures above 700℃ or after 1 hour annealing at 700℃. The photoluminescence behaviors after the annealing processes can be explained bythe size change of the nanocrystalline silicon and the density change of Si=O defect in the films, respectively. These results support the possibility of using a silicon-based light source for Si-optoelectronic integrated circuits and/or display devices.

keyword : oxides, plasma deposition, optical properties, atomic force microscopy, photoluminescence