• Journal of the Korean Society for Heat Treatment
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• v.26 no.1
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• pp.14-20
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• 2013

Duplex stainless steel, which is a kind of stainless steel with a mixed microstructure of about equal proportions of austenite and ferrite, is generally known as a unique material with excellent corrosion resistance and high strength. However, toughness, strength, and corrosion resistance of the steel could be reduced due to precipitation of topologically closed packed phases such as sigma phase during cooling. In case of large forged products, they have strong possibility that ${\sigma}$-phase precipitates due to difference of cooling rate between surface and inner of the products. Investigation on sigma phase precipitation behavior of duplex stainless steel with change of cooling rate was carried out in this study. Forged SAF 2205 duplex stainless steel was used as specimens to examine the cooling rate effect. Dissolution behavior of sigma phase was also discussed through resolution test of duplex stainless steel containing lots of sigma phase. Experimental results revealed that impact energy was very sensitive to precipitation of small amount sigma phase. However, sigma phase could be removed by short term resolution treatment and impact resistance of the duplex stainless steel was restored.

• Corrosion Science and Technology
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• v.6 no.4
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• pp.164-169
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• 2007

Hydrogen induced cracking (HIC) was studied phenomenologically and the effect of microstructure on HIC was discussed for the steels having two different levels of nonmetallic inclusions. Steels having different microstructures were produced by thermomechanically controlled processes (TMCP) from two different heats which had the different level of nonmetallic inclusions. Ferrite/pearlite (F/P), ferrite/acicular ferrite (F/AF), ferrite/bainite (F/B) were three representative microstructures for all tested steels. For the steels with higher level of inclusions, permissible inclusion level for HIC not to develop was different according to steelmicrostructure. On the contrary, HIC occurred also at the martensite/austenite (M/A) constituents regardless of steel microstructure when they accumulated to a certain degree. It was proved that M/A constituents were easily embrittled by hydrogen atoms. Steels having F/AF is resistant to HIC at a given actual service condition since they covers a wide range of diffusible hydrogen content without developing HIC.

• Journal of the Korean Society for Heat Treatment
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• v.12 no.2
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• pp.117-128
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• 1999

This study was carried out to investigate the surface characteristics of SCM440 steel nitrided with various nitrogen contents for 7 hours at $520^{\circ}C$ by using micro-pulse plasma nitriding apparatus of hot wall type. The effects of oxidation treatment was also investigated on plasma nitrided in 30% nitrogen and post oxidized SCM440 steel at $500^{\circ}C$ in $H_2O$ atmosphere. The ${\gamma}^{\prime}-Fe_4N$ and ${\varepsilon}-Fe_{2-3}N$ phases were detected in compound layer of the nitrided steel. As the content of nitrogen in plasma gas increased with 30, 50, 70% on the micro-pulse plasma nitriding for SCM440 steel, the thickness of compound, diffusion layer and the surface hardness were increased. From the wear test results, the best wear resistance was appeared in the condition of ductile ${\gamma}^{\prime}-Fe_4N$ phase formed specimen at 30% nitrogen, whereas that of the treated with 50% and 70% nitrogen decreased owing to the exfoliation of brittle ${\varepsilon}-Fe_{2-3}N$ phase in the compound layer. On the nitrided and subsequently oxidized SCM440 steel, the surface layer consisted of $Fe_3O_4$, ${\gamma}^{\prime}-Fe_4N$, and ${\varepsilon}-Fe_{2-3}N$ phases. In these treatments, the dissolution of nitrides affect hardness and hardening depth in compound and diffusion layers. For the nitrided in 30% nitrogen and post oxidized specimen at $500^{\circ}C$ for 1 hour, the wear resistance was lower than that of the only nitrided one in 30% nitrogen but higher than those of the nitrided ones in 50 and 70% nitrogen.

• Journal of the Korean Geosynthetics Society
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• v.13 no.4
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• pp.109-117
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• 2014

Using the finite element analysis model presented in accompanying paper, parametric study was performed in this paper. Various parameters were considered such as the width of wheel loads-induced permanent plastic deformation, backfill, equivalent thickness and orthogonal characteristic of steel mats. The effects of these parameters were analyzed for vertical and rotational displacements, maximum moment and tensile stress. From the parametric studies, it is found that great vertical deflection and tensile stress above allowable flexural tensile strength are developed in steel mats by the wheel loads-induced permanent plastic deformation. Backfill or increasing the thickness of steel mats is a feasible solution on this problem.

• Proceedings of the Korean Geotechical Society Conference
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• 2001.03a
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• pp.453-458
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• 2001

Copper slag is produced about 700,000 tons annually though copper refining process in Korea. In the paper, a laboratory investigation was carried out to estimate the geotechnical properties of copper slag and examine the feasibility of using the copper slag as a substitute for conventional construction materials and the improvement of the soft clay deposit. The specific gravity of copper slag is 3.45, and pH is 7.83. And the size distribution of the copper slag is well graded, so usage of copper slag will be extended in Geotechnical engineering fields. Copper slag has the permeability of 3.502${\times}$10￣$^2$cm/sec, which is satisfied with the criterion of sand drainage materials.. At the same time, it is thought to be suitable material for sand mat since it meets JIS of grain size distribution. The content of CaO from steel slag is about 40 percent while that of CaO from copper slag is about 5 percent. Based on this fact, copper slag has less hardening property compared to steel slag. Therefore, copper slag can be used as vertical drains, filters, and sand mats for improving the soft deposit.

• Steel and Composite Structures
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• v.17 no.4
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• pp.359-370
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• 2014

In this experimental and numerical study, the effect of plate thickness, the diameter of circular cutout, the distance between circular cutouts and rowing orientation angle effect (${\theta}$) on the buckling load of E-glass/vinylester pultruded composite beams with single and double circular cutouts, were investigated. The composite beam having 2, 4, and 6 mm thicknesses was produced as [Mat/${\theta}$ /Mat/${\theta}$ /Mat] by using pultrusion technique. Seven different fiber angles as $0^{\circ}$, $15^{\circ}$, $30^{\circ}$, $45^{\circ}$, $60^{\circ}$, $75^{\circ}$, and $90^{\circ}$ were chosen for investigation of rowing orientation angle. The distances between each circular cutout were selected as 15, 30, 45, 60, and 75 mm in the case of double circular cutouts. The diameters of circular cutouts were chosen as 2, 4, 6, 8, and 10 mm to investigate the effect of cutout size. The experimental buckling loads were compared with the results calculated from the numerical analysis. ANSYS 11 commercial software was used for numerical study. A good agreement was obtained between numerical and experimental results.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.3
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• pp.419-427
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• 2013

The catalytic converters for automobile exhaust systems are manufactured by inserting a mat-wrapped substrate into a stainless steel can. A residual pressure that is too high will initiate a fracture in the substrate. In contrast, a residual pressure that is too low will fail to hold the substrate in the acceleration or deceleration phase. Both the process capability and mat pressure on the substrate are predicted while considering the effect of the statistical variation in the dimensions of the parts. The validity of the solutions is then confirmed. A program using EXCEL combines a finite element analysis and process capability analysis in one program.

• Journal of Korea Foundry Society
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• v.15 no.6
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• pp.604-615
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• 1995

Austenitic stainless steel castings using expandable polystylene(referred to hereafter as EPS) patterns are often affected by distinctive defects associated with incomplete decomposition of the EPS as the molds are filled with metal. The quality of the castings, with particular reference to carbon pick-up in austenitic stainless steel is further influenced to a significant extent by such factors as reduced pressure, the additive by adding $Na_2CO_3$ in coating. The steel composition and microstructure were examined at the surface layer of castings, at depths of 1mm, by taking successive layers of swarf and analysis. In experiments, the carburizing atmosphere was neutralized, showing that the coating performed efficiently by decomposing almost instantly on heating and liberating $CO_2$. The upper parts of castings obtained using EPS patterns were slightly higher in carbon pick-up than other parts. Comparing the 316L and 304 stainless steel castings, qualitative and quantative differences could be found between the carbon pick-up behaviours as influence of the carbon content and alloying elements. Carbide former such as Cr makes carbon more soluble in the steel. This must make carbon pick-up in the surface layer but at the same time richer in carbon especially in the 304 stainless steel castings.

• Journal of the Korean Society for Heat Treatment
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• v.14 no.2
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• pp.81-88
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• 2001

The effects of post-oxidation and sulfnitriding treatments on the phase transformation in the nitrided case of tool steels have been studied. Dense and compact $Fe_3O_4$ layer was formed at the outer surface of nitride compound layer by post-oxidation treatment and multi layer of iron sulfide(FeS) was formed in the compound layer by sulfnitriding treatment. The surface hardness decreased because of formation of the soft oxide or sulfide at the nitride surface. Diffusion layer of nitride case was not affected by post-oxidation treatment or sulfnitriding treatment of nitrided alloy tool steels.

• Proceedings of the Korea Concrete Institute Conference
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• 2000.10b
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• pp.1181-1186
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• 2000

The crack of concrete induced by the heat of hydration is a serious problem, particularly in concrete structures such as biers, thick walls, box type walls, mat-slab of nuclear reactor buildings, dams or foundations of high rise buildings, etc.. As a result of the temperature rise and restriction condition of foundation, the thermal stress which may induce the cracks can occur. Therefore the various techniques of the thermal stress control in massive concrete have been widely used. One of them is prediction of the thermal stress, besides low-heat cement which mitigates the temperature rise, design change which considers steel bar reinforcement, operation control and so on. In this study, firstly it introduce the thermal cracks control technique by employing low-heat cement concrete, thermal stress analysis considering season. Secondly it shows the application of the cracks control technique like block placement.