• 한국분말재료학회지
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• 제20권4호
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• pp.275-279
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• 2013

Fe-based oxide dispersion strengthened (ODS) powders were produced by high energy ball milling, followed by spark plasma sintering (SPS) for consolidation. The mixed powders of 84Fe-14Cr-$2Y_2O_3$ (wt%) were mechanically milled for 10 and 90 mins, and then consolidated at different temperatures ($900{\sim}1100^{\circ}C$). Mechanically-Alloyed (MAed) particles were examined by means of cross-sectional images using scanning electron microscopy (SEM). Both mechanical alloying and sintering behavior was investigated by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM). To confirm the thermal behavior of $Y_2O_3$, a replica method was applied after the SPS process. From the SEM observation, MAed powders milled for 10 min showed a lamella structure consisting of rich regions of Fe and Cr, while both regions were fully alloyed after 90 min. The results of sintering behavior clearly indicate that as the SPS temperature increased, micro-sized defects decreased and the density of consolidated ODS alloys increased. TEM images revealed that precipitates smaller than 50 nm consisted of $YCrO_3$.

• 한국재료학회지
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• 제9권10호
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• pp.985-991
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• 1999

Incoloy 800H, KSA (Kaeri Superalloy)-6, Inconel 600 및 Hastelloy C-276 합금의 용융염에서의 부식거동을 650~85$0^{\circ}C$ 온도범위에서 조사하였다. LiCl-Li$_2$O혼합용융염에서의 부식은 Li$_2$O에 의한 염기성 용해 기구에 의해 진행되며, 부식속도가 LiCl에서보다 훨씬 빠르게 나타났다. 혼합용융염 LiCl-Li$_2$O에서는 Ni기 합금의 부식속도가 Fe기 합금보다 빠르고, Mo와 W의 함량이 높은 Hastelloy C-276이 가장 빠른 부식속도를 나타내었다. 용융염 LiCl에서는 LiCrO$_2$의 단일 부식층이 형성되고, LiCl-Li$_2$O 혼합용융염에서는 산화물과 Ni의 2상구조의 다공성 부식층이 형성되었다.

• Journal of Welding and Joining
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• 제16권6호
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• pp.59-68
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• 1998

In this study, effects of solidification modes (primary $\delta$-ferrite, primary ${\gamma}$-austenite) on the pit initiation and propagation in the 304L and 316L austenitic stainless steel weld metals were investigated. The solidification mode of weld metal was controlled by the addition of nitrogen to Ar shielding gas. Through the electrochemical experiments (potentiodynamic anodic polarization and potentiostatic time-current transient test) and metallographic examination (microstructure and elemental distribution), the following results were obtained. The more the volume content of nitrogen in the shielding gas were, the lower critical current density for passivity was observed. In comparison with weldments solidified through the primary $\delta$-ferrite solidification mode and the primary ${\gamma}$-solidification mode, the former showed higher critical pitting potential and a longer incubation time for stable pit initiation than the latter. However, in the pit propagation stage the former exhibited a faster dissolution rate than the latter. These results were believed to ee related to the distribution of alloying elements such as Cr, Mo, Ni and S.

• Journal of Electrochemical Science and Technology
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• 제11권4호
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• pp.323-329
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• 2020

The cathodic hydrogen evolution ability of different pure metals and their long term galvanic corrosion behavior with pure Mg were investigated. The hydrogen evolution ability of pure Ti, Al, Sn and Zr is weak, while that of Fe, W, Cr, and Co is very strong. Initial polarization test could not completely reveal the cathodic behavior of the tested metals during long term corrosion. The cathodic hydrogen evolution ability may vary significantly in the long term galvanic tests for different metals, especially for Al whose cathodic current density reduced to 1/50 of the initial value. The anodic polarization shows that Al and Sn as alloying elements are supposed to provide relatively good passive effect for Mg alloy, while Ag can provide a slight passive effect and Zn has little passive effect.

• 한국주조공학회지
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• 제15권6호
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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.

• Nuclear Engineering and Technology
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• 제45권6호
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• pp.821-826
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• 2013

In the present study, the effects of various heat treatments on the microstructure and mechanical properties of dual phase ODS steels were investigated to enhance the high strength at elevated temperature. Dual phase ODS steels have been designed by the control of ferrite and austenite formers, i.e., Cr, W and Ni, C in Fe-based alloys. The ODS steels were fabricated by mechanical alloying and a hot isostatic pressing process. Heat treatments, including hot rolling-tempering and normalizing-tempering with air- and furnace-cooling, were carefully carried out. It was revealed that the grain size and oxide distributions of the ODS steels can be changed by heat treatment, which significantly affected the strengths at elevated temperature. Therefore, the high temperature strength of dual phase ODS steel can be enhanced by a proper heat treatment process with a good combination of ferrite grains, nano-oxide particles, and grain boundary sliding.

• 열처리공학회지
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• 제13권6호
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• pp.412-419
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• 2000

There has been a considerable attention on Invar alloys due to its low thermal expansion property. A low thermal expansion property of Invar alloys, lower than $10^{-6}$ near the room temperature, is attractive for electric transmission lines and precision machine tools. However, the expansion property of Invar alloys is limited below about 520K, and mechanical properties are relatively low to apply to electric transmission line. In order to improve mechanical properties in this alloy, Ti alloying element was added to the $Ni_{38}-Mo_2-Cr_1-Fe$ invar alloy. The microstructure Ti added alloy showed finer than that of the unalloyed one. It was found that the (Mo, Ti), Mo carbide formed by Ti addition obstacled grain growth by pinning effect and supplyed recrystallization sites during heat-treatment. Optimum heat-treatment conditions with Ti addition were also discussed in the modified Invar alloy.

• 한국재료학회지
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• 제23권10호
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• pp.574-579
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• 2013

The milling and particulate characteristics of Al alloy-$Al_2O_3$ powder mixtures for a reaction-bonded $Al_2O_3$ (RBAO) process were studied. A commercially available prealloyed Al powder with Zn, Mg, Cu and Cr alloying elements (7475 series) was mixed with a calcined sinter-active $Al_2O_3$ powder and then milled in centrifugal milling equipment for ~48 hrs. The Al alloy-$Al_2O_3$ powder mixtures after milling were characterized and evaluated in various ways to reveal their particulate characteristics during milling. The milling efficiency of the Al alloy increased with a longer milling time. Comminution of the Al alloy particles started with its elongation, showing a high aspect ratio. With a longer milling time, the elongated Al alloy particle changed in terms of its shape and size, becoming equiaxially fine particles. Regardless of the milling efficiency of the Al alloy particles, all of the Al alloy particles repeatedly experienced strong plastic deformation during milling, giving rise to higher density of surface defects, such as microcracks, and leading to higher residual microstress within the Al alloy particles. The chemical reactions, oxidation behavior and hydration behavior of the Al alloy particles and the hydrolysis characteristics of their reaction with the environment were also observed during the milling process and during the subsequent powder handling steps.

• 한국주조공학회지
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• 제20권5호
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• pp.323-329
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• 2000

The HSS consists of hard carbide and matrix of martensite, and so its characteristics of wear resistance, fracture resistance, and surface roughness are good. This study was undertaken to investigate the influence of Nb and V and manufacturing conditions on microstructural behaviors and characteristics in the HSS cylindrical specimens(90 $mm^{O.D.}$ ${\times}$ 60 $mm^{I.D.}$ ${\times}$ 50 $mm^H$) manufactured using VCC(Vertical Centrifugal Casting). In the specimen of Fe-2C-6Cr-1.5W-3Mo-4V alloy, the amount of MC carbide was increased and $M_7C_3$ carbide was decreased with the increase of V and Nb contents. The primary VC carbide was formed and followed by the rod-type eutectic MC carbide was formed in the cell boundary in 9%V added specimen. MC carbide was increased, and $M_7C_3$ carbide was decreased with the addition of Nb content. In the specimen containing more than 3%Nb, primary NbC carbide was formed within the cell of matrix. With increase in rpm, cell and carbides became fine, and amount of carbide $M_7C_3$ was decreased due to increase in cooling rate.

• 한국표면공학회지
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• 제31권1호
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• pp.54-62
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• 1998

In this study, the behaviorof ion nitriding of AISI 304 stainless steel was investigated using plasma ion nitriding system. The characteristics of ion nitriding, and their micsoctrucyures, and physical properties were investigated as a function of process parmeteds. important conclusions can be summarzied as follows. Firstly, it was found that growth of nitride layer in ion nitriding are mainly affected by N2 partial pressures and nitriding temperatures for AISI 304 stainless steel. The $N_2$<\TEX> partial pressure plays on important role in ion nitriding since it determiness the incoming flux of nitrogen species onto specimen surface. Nitriding thmprrature is also important besauseit determines the diffusion rates of nitrogen through nitride layers. While both parameters affects the characteristics rateding are controlled by nitridingen diffusion nitration profiles of N and alloying elements such as Cr and Ni are observed through niride layers. Secondly, nitride layer consists of the upper white laywe having various nitride phases and the underneath diffusion layers. The thickness of white layer increases with $N_2$<\TEX> partial pressures and nitriding temperatures. The thinkness of diffusion layer is increasting nitriding temperatures. Finally, nitriding of stainless steels steel show slighly low their corrsionce prorerties. However, passivation properties, which is normally observed in stainless steels, were still observed aftre ion nitriding.