• Journal of Welding and Joining
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• v.18 no.1
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• pp.59-69
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• 2000

The purpose of the present study was to investigate weld metallurgical phenomena such as primary solidification mode, microstructural evolution and hot cracking susceptibility in nitrogen-bearing austenitic stainless steel GTA welds. Eight experimental heats varying nitrogen content from 0.007 to 0.23 wt.% were used in this study. Autogenous GTA welding was performed on weld coupons and the primary solidification mode and their microstructural characteristics were investigated from the fusion welds. Varestraint test was employed to evaluate the solidification cracking susceptibility of the heats and TCL(Total Crack Length) was used as cracking susceptibility index. The solidification mode shifted from primary ferrite to primary austenite with an increase in nitrogen content. Retained delta ferrite exhibited a variety of morphology as nitrogen content varied. The weld fusion zone exhibited duplex structure(austenite+ferrite) at nitrogen contents less than 0.10 wt.% but fully austenitic structure at nitrogen contents more than 0.20 wt.%. The weld fusion zone in alloys with about 0.15 wt.% nitrogen experienced primary austenite + primary ferrite solidification (mode AF) and contained delta ferrite less than 1% at room temperature. Regarding to solidification cracking susceptibility, the welds with fully austenitic structure exhibited high cracking susceptibility while those with duplex structure low susceptibility. The cracking susceptibility increased slowly with an increase in nitrogen content up to 0.20 wt.% but sharply as nitrogen content exceeded 0.20 wt.%, which was attributed to solidification mode shift fro primary ferrite to primary austenite single phase solidification.

• Journal of Welding and Joining
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• v.32 no.2
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• pp.29-36
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• 2014

Recently, in order to enhance the function and usefulness of products, cladding of dissimilar materials that maximizes the performance of the material is being widely used in all areas of industry as an important process. Clad steel plate, produced by cladding stainless steel plate, an anticorrosive material, on carbon steel plate, is being used to produce pressure vessels. Stainless steel plate has good corrosion resistance, and carbon steel plate has good rigidity and strength; clad steel can satisfy all of these qualities at once. This study aims to find the ${\delta}$-ferrite behavior, mechanical properties, structure change, integrity and reliability of clad steel weld on hot rolled steel plates. For this purpose, multi-layer welding, repair welding and post weld heat treatment were implemented according to welding procedure specifications (WPS). In order to observe the mechanical properties and toughness of clad steel weld zone, post weld heat treatment was carried out according to ASME Sec. VIII Div.1 UW-40 procedure for post weld heat treatment. With heat treatment at $625^{\circ}C$, the hold time was used as the process variable, increased by intervals that were doubled each time, from 80 to 1,280 min. The structure of weld part was typical cast structure; localized primary austenite areas appeared near central vermicular ferrite and fusion line. The heat affected zone showed rough austenite structure created by the weld heat input. Due to annealing effects of heat treatment, the mechanical properties (tensile strength, hardness, impact value) of the heat affected area tended to decrease. From the results of this study, it is possible to conclude the integrity of clad steel welds is not affected much in field welding, repair welding, multi-layer welding, post weld heat treatment, etc.

• Journal of the Korea Institute of Military Science and Technology
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• v.6 no.4
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• pp.159-166
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• 2003

Microstructures and mechanical properties of VIM/VAR/VAR-processed Ni-based Alloy718 ingot were investigated. Vacuum arc remelting(VAR) results in chill grain zone, columnar grain zone, and equi-axed grain zone in the ingot due to the difference in local solidification processes. Different grain structures of the remelting ingot result in the different hot workability for the given cogging conditions. Experimental results on microstructural inhomogeniety and material flow behavior under billet cogging conditions were presented, and their potential effects on the billet cogging process are discussed.

• Proceedings of the KWS Conference
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• 2003.11a
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• pp.256-258
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• 2003

Hot-rolled and Extruded plates of AZ type magnesium alloys were successfully joined by friction stir welding(FSW). AZ31B-H24 and AZ61 plates with the thickness of 4mm were used, and the microstructural development in the stir zone were investigated using optical and scanning electron microscopes. The grain size of base metal and stir zone were investigated using the line-intersecter method. Hardness of the stir zone was remarkably increased due to very fine recrystallized grain structure both in AZ3l and AZ6l alloys. Tensile strengths of the FS welded Mg alloys AZ31 and AZ61 were strongly affected by formation of the intermetallic compounds, ${\beta}$-Al$\sub$12/Mg$\sub$17/.

• Bulletin of the Korean Chemical Society
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• v.21 no.1
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• pp.69-74
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• 2000

Applicability of Thermal field flow fractionation (ThFFF) was investigated for the analysis of masticated natural rubber (NR) adhesives produced bya hot melt mastication process. An optimum ThFFF condition for NR analysis was found by using tetrahydrofuran (THF) as a solvent/carrier and a field-programming. Low flowrate (0.3 mL/min) was used to avoid stopping the flow for the sample relaxation. Measured molecular weight distribution was used to monitor degradation of rubber during the mastication process. Rubber samples collected at three different stages of the mastication process and were analyzed by ThFFF. It was found that in an anaerobic process rubber degradation occurs at the resin-mixing (compounding) zone as well as in the initial break-down zone, while in an aerobic process most of degradation occurs at the initial breakdown zone. It was also found that E-beam radiation on NR causes a slight increase in the NR molecular weight due to the formation of a branched structure.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2007.04a
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• pp.71-74
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• 2007

Large Eddy Simulation(LES) has been conducted to insight interaction effects of turbulent flow and chemical reaction of a lean-Premixed swirl combustor. The unsteady turbulent flame is carefully simulated so that the motion of flow and flame can be characterized in detail. Fuel lumps escaping from the primary combustion zone move downstream and consequently produce local hot spots conveying large vortical structures in the azimuthal direction. The correlation between pressure oscillation and unsteady heat release is examined by the spatial and temporal Rayleigh parameter.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09b
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• pp.826-827
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• 2006

Copper infiltration is demonstrated as a viable solution to achieve higher mechanical properties by filling the interconnected porosities of a ferrous structure with copper infiltrant. This paper will present the results of a design of experiments study based on the selected processing variables in the copper infiltration process. The variables are the following: Infiltrating temperatures, infiltrating time at pre-heat zone and hot zone, the green density of iron part, the migration of copper into the iron part at different processing conditions. The results show the flexibility of the infiltration process to attain certain mechanical properties by changing the processing conditions.

• Journal of the Korean Society of Combustion
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• v.6 no.2
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• pp.29-35
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• 2001

Unsteady numerical study has been conducted on combustion dynamics of a lean-premixed swirl-stabilized gas turbine swirl injector. A three-dimensional computation method utilizing the message passing interface (MPI) parallel architecture, large eddy simulation(LES), and proper orthogonal decomposition (POD) technique was applied. The unsteady turbulent flame dynamics are simulated so that the turbulent flame structure can be characterized in detail. It was observed that some fuel lumps escape from the primary combustion zone, and move downstream and consequently produce hot spots. Those flame dynamics coincides with experimental data. In addition, basis modes of the unsteady turbulent flame are characterized using proper orthogonal decomposition (POD) analysis. The flame structure based on odd basis modes is apparently larger than that of even ones. The flame structure can be extracted from the summation of the basis modes and eigenvectors at any moment.

• Journal of Korea Foundry Society
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• v.15 no.3
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• pp.252-261
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• 1995

The purposes of this study were to investigate the microstructural changes in alloy ribbons of Al-Fe-Mo-Si quarternary system at $450{\sim}500^{\circ}C$, and to study the coarsening mechanism of fine precipitates. Using the hot stage in TEM, in situ microstructural changes in Al-4Fe-0.5Mo-1.5Si alloy ribbon and Al-8Fe-2Mo-1.5Si alloy ribbon have been examined successively up to 60 hours at $450^{\circ}C$ and $500^{\circ}C$. Cell structure in zone B of Al-4Fe-0.5Mo-1.5Si alloy ribbon was observed to collapse even in 10 minutes by in-situ heating at $450^{\circ}C$ and the size of precipitates in zone B increased twice in 60 hours. The precipitates in zone A of Al-4Fe-0.5Mo-1.5Si alloy ribbon showed slower coarsening rate than those in zone B by in-situ heating at $450^{\circ}C$. The precipitates in zone A of Al-8Fe-2Mo-1.5Si alloy ribbon increased 50% by in-situ heating at $500^{\circ}C$ in 50 hours compared to the initial precipitates while any microstructual change in zone B was not observed by in-situ heating at $500^{\circ}C$ up to 50 hours. Only the precipitates in zone A of Al-4Fe-0.5Mo-1.5Si alloy ribbon satisfied $r^3{\propto}t$ relationship of coarsening mechanism.

• Journal of the Korean Society of Propulsion Engineers
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• v.15 no.4
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• pp.26-34
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• 2011

The present work addresses structural characteristics of natural gas flames in a lean premixed swirl-stabilized combustor with an attention focused on the effect of the formation of recirculation zones on the combustion instability. It is known that the recirculation zone plays an important role in stabilizing a turbulent, premixed natural gas flames by providing a source of heat or radicals to the incoming premixed fuel and air. To improve our understanding of the role of recirculation zones, the flame structure was investigated for various mixture velocities, equivalence ratios and swirl numbers. The optically accessible combustor allowed for the application of laser diagnostics, and Particle Image Velocimetry(PIV) measurements was used to characterize the flame structure under both cold flow conditions and hot flow conditions. Dynamic pressures were also measured to investigate characteristics of combustion at the same time. The results indicates that the formation of recirculation zone is strongly related to the occurrence of thermo-acoustic instabilities.