• Journal of the Korean Ceramic Society
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• v.45 no.7
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• pp.387-394
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• 2008

The Physical property changes of calcined clay by carburization were investigated studied. The carburization mechanism is the penetration of carbon which occurred during incomplete fuel combustion into crevice of clay structure. The experiments for elasticity and freeze-thaw resistance were conducted, and the results can be summarized as follows: Dynamic modulus of elasticity and also freeze-thaw resistance of calcined clay by carburization treatment increased more than 92% after testing 300 cycle, which was more improved than 88% of calcined clay. Therefore, it can decrease the possibility of winter-sowing, which is one the weakness of calcined clay. It is on the basis of the fact that the porosity of calcined clay by carburization treatment is about 12%, which indicates smaller pore spaces comparing with the 14% of porosity of calcined clay and those values were calculated by apparent porosity show and also supported by SEM images. Infrared emissivity of calcined clay by carburization treatment and calcined clay were respectively 0.92 and 0.9l at $80^{\circ}C$. However, those values were 0.91 and 0.88 at $200^{\circ}C$, which means infrared emissivity of calcined clay by carburization treatment shows 3.6% higher than the calcined clay. Moreover, within the wavelength range from 3 to $7\;{\mu}m$, while the calcined clay had low infrared emissivity, the calcined clay by carburization treatment had increased infrared emissivity. It is inferred that it was affected by carbon element that has high infrared absorptivity within this wavelength range.

• Journal of Powder Materials
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• v.11 no.2
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• pp.124-129
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• 2004

Nanosized WC and WC-Co powders were synthesised by chemical vapor condensation(CVC) process using the pyrolysis of tungsten hexacarbonyl(W(CO)$_6$) and cobalt octacarbonyl(Co$_2$(CO)$_8$). The microstructural changes and phase evolution of the CVC powders during post heat-treatment were studied using the XRD, FE-SEM, TEM, and ICP-MS. CVC powders were consisted of the loosely agglomerated sub-stoichimetric WC$_{1-x}$ and the long-chain Co nanopowders. The sub-stochiometric CVC WC and WC-Co powders were carburized using the mixture gas of CH$_4$-H$_2$ in the temperature range of 730-85$0^{\circ}C$. Carbon content of CVC powder controlled by the gas phase carburization at 85$0^{\circ}C$ was well matched with the theoretical carbon sioichiometry of WC, 6.13 wt％. During the gas phase carburization, the particle size of WC increased from 20 nm to 40 nm and the long chain structure of Co powders disappeared.

• Transactions of the Korean Society of Automotive Engineers
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• v.2 no.6
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• pp.1-8
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• 1994

To enhance the strength of gears for transmission, Generally caburizing heat treatment is applied. But there are some problems in this technology the distortion of gears during heat treatment process, and the discontinuity of manufacturing process. For these reasons, the high frequency induction hardening process is widely used. This method is one of the surface hardening process to improve the wear resistance and fatigue life of the machine components. In this study, to compare the bending fatigue strength of caburized gear with that of induction hardened gear, bending fatigue testing of gears with two different cases was performed by using an electrohydraulic servo-controlled fatigue testing machine and double tooth bending fatigue test fixture. Fatigue life distributions at constant stress levels were established directly from fatigue data. For gear design, the fatigue strength distribution at specified life is more important. This distribution is obtained by statical transformation from fatigue life distribution. Reliability of bending fatigue strength was estimated by P-S-N curves and Weibull distribution.

• Journal of the Korean Society for Heat Treatment
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• v.5 no.4
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• pp.195-200
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• 1992

SCM 415 has been vacuum carburized in the carburizing pressure of 60-65kpa and the carburizing temperature of 1233k and 1273k after varied to 0-20 in the ratio of $N_2/C_3H_8$ and then diffusion treated for various times at 1123k. The results obtained from the experiment are as follows. 1. With increasing from 0 to 20 in ratio of $N_2/C_3H_8$ the sooting formation of surface after carburizing considerably decreased. 2. The hardness control and surface carbon content of carburizing surface has been modified by the addition of nitrogen to the propan. 3. The appoximate value of k is indirectry calculated at 1123k which results are obtained to $0.58{\times}10^{-2}(wt.%.S^{-1/2})$. 4. A great deal of propan by addition of nitrogen gas in carburizing gas was possible to saving without considerable change in case hardening depth. 5. The effective carburizing depth range is obtained to 0.8-1.1mm by diffusion temperature of 1123k after carburization at 1273k-3.6ks, and the surface hardness is increased as the increasing of $T_D/T_c$ in our experimental condition, and the maximum hardness as reachin distance from surface is decreased.

• Journal of the Korean Society for Heat Treatment
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• v.29 no.3
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• pp.113-123
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• 2016

In the present work, we investigated the effects of the carbon potential on the formation of carbide at the carburized surface and anti-pitting fatigue strength in the supercarburized steels. Two low carbon steels with different Cr concentrations were adopted and the repeated supercarburizing treatment carried out with the different carbon potential conditions. The microstructure and carbides at the supercarburized surface were observed by using optical microscope and scanning electron microscope. The microhardness test was performed and the hardness distribution and the effective case depth at the supercarburized surface were discussed. The roller pitting fatigue test was carried out and the fatigue strength was evaluated with different the carbon potential conditions. The microstructure of the fatigue specimen surface was observed by means of scanning electron microscope and scanning transmission electron microscope. Depending on the chemical composition of the steels and the carbon potential condition, the resistance of temper softening and pitting failure was influenced due to the carbide distribution and the formation of coarse network carbide. Thus, it was confirmed that the control of the carbide formation is a key factor to improve the anti-pitting fatigue strength in the supercarburized steels.

• Journal of Ocean Engineering and Technology
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• v.25 no.6
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• pp.60-65
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• 2011

A low temperature plasma carburizing process was performed on AISI 316L austenitic stainless steel to achieve an enhancement of the surface hardness without degradation of its corrosion resistance. Attempts were made to investigate the influence of the processing temperatures on the surface hardened layer during low temperature plasma carburizing in order to obtain the optimum processing conditions. The expanded austenite (${\gamma}_c$) phase, which contains a high saturation of carbon (S phase), was formed on all of the treated surfaces. Precipitates of chromium carbides were detected in the hardened layer (C-enriched layer) only for the specimen treated at $550^{\circ}C$. The hardened layer thickness of ${\gamma}_c$ increased up to about $65{\mu}m$ with increasing treatment temperature. The surface hardness reached about 900 $HK_{0.05}$, which is about 4 times higher than that of the untreated sample (250 $HK_{0.05}$). A minor loss in corrosion resistance was observed for the specimens treated at temperatures of $300^{\circ}C{\sim}450^{\circ}C$ compared with untreated austenitic stainless steel. In particular, the precipitation of chromium carbides at $550^{\circ}C$ led to a significant decrease in the corrosion resistance. A diamond-like carbon (DLC) film coating was applied to improve the wear and friction properties of the S phase layer. The DLC film showed a low and stable friction coefficient value of about 0.1 compared with that of the carburized surface (about 0.45). The hardness and corrosion resistance of the S phase layer were further improved by the application of such a DLC film.

• Journal of the Korean Society for Heat Treatment
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• v.3 no.3
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• pp.13-20
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• 1990

SCM 21 and D.P steel were carburized in vacuum using prophane at a temperature ranging from 1228k to 1308k under a gas pressure ranging from 21.3kpa to 61.8kpa, and the following results were obrained. 1) D.P. steel has considerable efficiency in depressing the grain growth during the high temperature carburizing and it has fine structure even at 1268k for 14.4ks when carburizied. Therefore this steel is expected to be suitable for vacuum carburizing at a high temperature. 2) Case depth was increased as the carburizing temperature increases and it was 3.2mm at max, temperature of 1308k, for max, time of 14.4ks and under max, pressure of 61.8kpa. Thus vacuum-carburizing was considered effective for the materials which need case depth, which is necessary for machine structure use. 3) The rate of case depth of SCM 21 was faster than D.P. steel under same carburizing conditions and the increasing rate of the case depth was constant. 4) Case depth was increased as the gas pressure becomes high under same carburizing temperature. 5) Case carbon concentration, $C_s$, of SCM 21 obeys to a formula, $$C_s=kt^{1/2}+C_0$$ Where k is $2.15{\times}10^{-2}$($wt%.S^{-1/2}$) and this value is a little bit lower than that of SNCM 815.

• Journal of the Korean Society of Industry Convergence
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• v.22 no.6
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• pp.681-688
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• 2019

SM45C steel, which is widely used for mechanical structure, was carburized at 870℃ for 4 hours and tempered at 300℃ and 400℃ for 1, 3 and 6 hours. The tempered materials were evaluated for tensile test, hardness test and impact test. In particular, the hardness and the absorption energy were evaluate the reliability by the Weibull statistical analysis. 300℃-1h specimen is considered to be the best heat treatment condition in the tensile stress and the observation of fracture surface. 300℃-1h specimen showed larger shape and scale parameter than the other specimens, and Rockwell hardness variance was small and showed the best characteristics. 400℃-3h specimen showed larger shape and scale parameter than the other specimens, the dispersion of impact absorption energy is small, and showed excellent characteristics.

• Journal of Welding and Joining
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• v.2 no.1
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• pp.4-17
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• 1984

Overlaid weld metals of austenitic stainless steel in a pressure vessel of power reactor are usually post-weld heated for a long period of time after welding. The PWHT is considered as a kind of sensitizing and it is important to check the soundness of the weld metal after PWHT, especially about the precipitation of carbides. The purpose of this report is to obtain information on the relation between the change of microstructure and Post-Weld Heat Treatment in the overlaid weld metals. Metallurgical aspects of the problem on austenitic stainless steel heated at $625^{\circ}C$, $670^{\circ}C$, $720^{\circ}C$ and $760^{\circ}C$ for 3, 10, 30, 100 and 300 hours have been investigated by means of optical-micrography, micro-hardness measurement, scanning electron microscope and electron-probe micro analysis. From the results obtained, the following conclusions are drawn; 1) The PWHT above $625^{\circ}C$ for a long time causes a diffusion of carbon atoms from low alloy steel into stainless steel, and consequently carbon is highly concentrated at the boundary layer of stainless steel. 2) C in ferritic steel migrated to austenitic steel and carbides precipitated in austenitic steel along fusion line. At higher temperatures, the ferrite grains coarsened in the decarburized zone. 3) In the change of microstructure of stainless steel overlaid weld metal, the width of carbides precipitated zone and decarburized zone increased with increase of PWHT temperature and time. 4) At about $625^{\circ}C$ to $760^{\circ}C$, chromium carbides, mainly $M_{23} C_6$, precipitate very closely in the carburized layer with remarkable hardening. 5) Precipitation of delta ferrite from molten weld metal depends on solidification phenomenon. There was a small of ferrite near the bond in which the local solidification time was short, comparing with after parts of weld metal. Shape and amount of ferrite were not changed by Post-Weld Heat Treatment after solidification.

• Journal of Welding and Joining
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• v.1 no.2
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• pp.45-52
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• 1983

Many pressure vessels for the hot H$\sub$2//H$\sub$2/S service are made of 2+1/4Cr-1Mo steel with austenitic stainless steel overlay to combat agressive corrosion due to hydrogen sulfide. Hydrogen dissolves in to materials during operation, and sometimes gives rise to unfore-seeable damages. Appropriate precautions must, therefore, be taken to avoid the hydrogen induced damages in the design, fabrication and operation stage of such reactor vessels. Recently, hydrogeninduced cracking (or Disbonding) was found at the interface between base metal and stainless weld overlay of a desulfurizing reactor. Since the stainless steel overlay weld metal is subjected to thermal and internal-pressure loads in reactor operation, it is desirable for the overlay weld metal to have high strength and ductility from the stand point of structural safety. In section III of ASME Boiler and Pressure Vessel Code, Post-Weld Heat Treatment(PWHT) of more than one hour per inch at over 1100.deg. F(593.deg. C) is required for the weld joints of low alloy pressure vessel steels. This heat treatment to relieve stresses in the welded joint during construction of the pressure vessel is considered to cause sensitization of the overlay weld metal. The present study was carried out to make clear the diffusion of carbon migration by PWHT in dissimilar metal welded joint. The main conclusion reached from this study are as follows: 1) The theoretical analysis for diffusion of carbon in stainless steel overlay weld metal does not agree with Fick's 2nd law but the general law of molecular diffusion phenomenon by thermodynamic chemical potential. 2) In the stainless steel overlay welded joint, the PWHT at 720.deg. C for 10 hours causes a diffusion of carbon atoms from ferritic steel into austenitic steel according to the theoretical analysis for carbon migration and its experiment. 3) In case of PWHT at 720.deg. C for 10 hours, the micro-hardness of stainless steel weld metal in bonded zone increase very highly in the carburized layer with remarkable hardening than that of weld metal.