• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.21 no.2
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• pp.267-275
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• 2012

Austenite stainless steel 304 has properties of high resistance to corrosion and temperature changes. Therefore, this material is widely used in various of industries. However, when the material is subjected to heating and cooling cycles the forming accuracy, for example, the right angle associated with a sharp bend such as corner is lost. This phenomenon is caused by the reversion of the deformation-induced martensite into austenite when the temperature in increased. This result in misfit of a structure or an assembly, and an increase in residual stress. Hence, it is important to understand this process. In this study, to evaluate the mechanical behavior of the deformation-induced martensite and reversed austenite, a scanning acoustic spectroscope including the capability of obtaining both phase and amplitude of the ultrasonic wave (i.e., the complex V(z) curve method) was used. Then, the velocities of the SAW propagating within the specimens made in different conditions were measured. The experimental differences of the SAW velocities obtained in this experiment were ranging from 2,750 m/s to 2,850 m/s, and the theoretical difference was 3.6% under the assumption that the SAW velocity was 2,800 m/s. The error became smaller as the martensite content was increased. Therefore, the SAW velocity may be a probe to estimate the marternsite content.

• 한국해양학회지
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• v.25 no.2
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• pp.84-95
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• 1990

Observations by CTD castings, moored current meters and satellite imageries reveal some physical characteristics of the area around the tidal mixing front found in the mid-Yellow Sea off Korea. Tidal mixing is the greatest at the promontory of Taean Peninsula with a front around it. The front appears in April with the start of solar heating, becomes most clear in August and disappears in November with the start of surface cooling. In the north of the front, tidal fluctuations of temperature and salinity induced by tidal currents manifest the existence of the front, Differently from the usual tidal mixing front, the front in Kyunggi Bay is formed by presence of the water discharged from the Han River which meets the offshore water at the front. Near the surface cold center, vertically well-mixed zone extends to about 50 Km offshore from the coast, Farther south, this structure is generally retained but with lesser degree of vertical mixing. Within the relatively well-fixed coastal zone, the fresh water discharged from the Kum River makes another salinity front of smaller extent. At some places around this salinity front, an Upwelling-like feature is remarked.

• Tribology and Lubricants
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• v.36 no.3
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• pp.139-146
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• 2020

Global warming has led to an increase in demand of eco-friendly vehicles, such as electric cars, for reducing greenhouse gas emissions, and especially, regulating carbon dioxide generation. In addition, electric vehicles are equipped with an electric drive-type hermetic scroll compressor and a refrigerant, which exhibit current and future trends of using environmentally friendly refrigerants, including R-1234yf. In this study, polyol ester-based refrigeration oils are prepared via condensation esterification of polyol and fatty acids. The oils can be combined with R-1234yf refrigerant for applications in air conditioning and cooling systems of electric vehicles. The structure of synthetic polyol esters is confirmed via ¹H-NMR and FT-IR spectrum analysis, and the composition of the polyol ester is analyzed via gas chromatogram analysis. Furthermore, kinematic viscosity, viscosity index, total acid value, pour point, and color are analyzed as fundamental physical properties of the synthetic polyol esters. The compatibility and chemical stability of the synthetic polyol ester combined with the R-1234yf refrigerant are obtained via high temperature and high pressure oil-resistant refrigerant tests. The changes in the oil color and catalyst activity are observed before and after the experiment to determine whether it is suitable as a refrigerator oil.

• Journal of Welding and Joining
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• v.34 no.1
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• pp.75-81
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• 2016

This study applied laser surface melting process using CW(Continuous wave) Yb:YAG laser and cold-work die steel SM45C and investigated microstructure and hardness. Laser beam speed, power and beam interval are fixed at 70 mm/sec, 2.8 kW and $800{\mu}m$ respectively. Depth of Hardening layer(Melting zone) was a minimum of 0.8 mm and a maximum of 1.0 mm that exceeds the limit of minimum depth 0.5 mm applying trimming die. In all weld zone, macrostructure was dendrite structure. At the dendrite boundary, Mn, Al, S and O was segregated and MnS and Al oxide existed. However, this inclusion didn't observe in the heat-affected zone (HAZ). As a result of interpreting phase transformation of binary diagram, MnS crystallizes from liquid. Also, it estimated that Al oxide forms by reacting with oxygen in the atmosphere. The hardness of the melting zone was from 650 Hv to 660 Hv regardless of the location that higher 60 Hv than the hardness of the HAZ that had maximum 600 Hv. In comparison with the size of microstructure using electron backscatter diffraction(EBSD), the size of microstructure in the melting zone was smaller than HAZ. Because it estimated that cooling rate of laser surface melting process is faster than water quenching.

• Journal of Ocean Engineering and Technology
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• v.25 no.6
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• pp.66-71
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• 2011

Recently, the production of shipbuilding and offshore plant industries, with a trend toward large structures, has led to an increased use of high strength ultra-thick plates. The use of ultra-thick plates increases the welding tasks, and the welding process generates distortion and residual stress in the weldment because of the rapid heating and cooling. Welding distortion and residual stress in the welded structure resulte in many troubles such as deformation and life deterioration. In particular, the welding residual stress has an important effect on welding deformation, fatigue, buckling strength, brittleness, etc. The purpose of this study was to evaluate the residual stress at a multi-pass weldment using an experimental method for EH36 high-tension steel. In this experimental method, AIS3000 was used to measure the residual stress of a welded part, HAZ, and base metal; EPMA and XRD were used to study the material properties.

• Composites Research
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• v.14 no.5
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• pp.1-11
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• 2001

The particulates reinforced metal matrix composites(PMMC) have a number of interesting mechanical properties. including high strength and good resistance to wear at high temperature and low thermal expansion. The equipment structure to obtain the homogeneous distribution in composites are proposed for the continuous pouring of reinforcement at the desired temperature. The particulates reinforced metal matrix composites(A357/SiCp) were fabricated by the process of the combined stirring method with the various fabrication process. The combined stirring method to niform distribution of particle is consisted of two stirring force both electro-magnetic stirring generated from induction heating and mechanical stirring with graphite stirrer. PMMC billets were fabricated with the volume fractions ranged from 0% to 20% and particle sizes ranged from 14${\mu}{\textrm}{m}$ to 25${\mu}{\textrm}{m}$. It is important to cont the size of primary $\alpha$-Al solid particles because it could become the cause of the particle pushing or capture phenomena from the fact that secondary dendrite arm spacing size depends on the cooling rate during the solidification in hypoeutectic Al-Si alloy. Therefore, the effect of primary $\alpha$-Al on the reinforcement distribution in matrix alloys has been investigated. The microstructure of PMMC fabracated with various volume fractions(0%, 10%, and 20%) and particle size were observed.

• Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
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• v.10 no.1
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• pp.21-26
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• 2012

Microstructural and mechanical property were evaluated at the medium-sized HT9 (12Cr-1MoWV) forged steel which was considered as primary candidate for the fuel cladding in sodium-cooled fast reactor (SFR). Material was forged at $1170^{\circ}C$ after the induction melting to make round bar as 160mm diameter, 7000mm length then the radial distribution of microstructure as well as microhardness was evaluated. The results showed that overall microstructure exhibited as ferrite-martensite structure, where small amount (2~3%) of delta ferrite was formed throughout the specimen and maximum 15% of transformed ferrite was formed at the center, where it gradually decreased toward the radial direction. Sensitivity analysis of the cooling curve and Time-Temperature-Transformation (TTT) diagram revealed that formation of transformed ferrite could be avoided when the diameter was decreased down to 120mm.

• Journal of the Korean Society of Propulsion Engineers
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• v.11 no.4
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• pp.38-45
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• 2007

A liquid rocket engine fuel-rich gas generator has been developed for the first time in the country, which can produce combustion gas over the rate of 4 kg/s at 900 K and 58 bar. The gas is not only for driving a turbopump but also for providing heat source for propellant supply tanks. The final design of the gas generator had been fixed based on the concept and preliminary development tests, and was validated through structure and heat transfer analysis. The manufacturing involved precision machining, surface finish, and special welding technique. The final assessment on the characteristics of ignition and combustion had been carried out for two different versions of injector heads. This concluded that the present product satisfies the development requirements such as spatial temperature distribution and the development has been successful.

• Journal of Welding and Joining
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• v.33 no.6
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• pp.6-12
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• 2015

Thermal fatigue life of the automobile exhaust manifold is directly affected by the restraint force according to the structure of exhaust system and bead shape of the welded joints. In the present study, the microstructural changes and precipitation behavior during thermal fatigue cycle of the 18wt% Cr ferritic stainless steel weld heat affected zone (HAZ) considering restraint stress were investigated. The simulation of weld HAZ and thermal fatigue test were carried out using a metal thermal cycle simulator under complete constraint force in the static jig. The change of the restraint stress on the weld HAZ was simulated by changing the shape of notch in the specimen considering the stress concentration factor. Thermal fatigue properties of the weld HAZ were deteriorated during cyclic heating and cooling in the temperature range of $200^{\circ}C$ to $900^{\circ}C$ due to the decrease of Nb content in solid solution and coarsening of MX type precipitates, laves phase, $M_6C$ with coarsening of grain and softening of the matrix. As the restraint stress on the specimen increased, the thermal fatigue life was decreased by dynamic precipitation and rapid coarsening of the precipitates.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.11a
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• pp.181-185
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• 2006

A liquid rocket fuel-rich gas generator developed for the first time in the country can produce combustion gas over the rate of 4 kg/s at 900 K and 58 bar. The gas can be used not only for driving a turbopump but also for providing heat source for propellant supply tanks. The final design of the gas generator has been fixed based on the concept and preliminary development tests, and was validated through structure and heat transfer analysis. The manufacturing involves precision machining, special surface finish, and welding techniques. The final assessment on the characteristics of ignition and combustion had been carried out through five combustion tests. This concluded that the present product satisfies the development requirements.