• 열처리공학회지
• /
• 제10권1호
• /
• pp.30-39
• /
• 1997

After changing the heat treating atmosphere of nitrogen gas, argon gas and vacuum, the nitrogen contents, microstructural changes, hardness and corrosion resistance of 0.25wt.%N alloyed super duplex stainless steel have been investigated in the temperature range from $1050^{\circ}C$ to $1350^{\circ}C$. The nitrogen content showed to be increased up to 0.36wt.% after heat treating the specimen in nitrogen gas at $1200^{\circ}C$, while the decrement of nitrogen content in vacuum atmosphere was shown down to 0.03wt.% at $1350^{\circ}C$. After heat treating in the mixed gas atmosphere of argon and nitrogen at $1250^{\circ}C$, the surface ${\gamma}$ phase existed as ${\alpha}+{\gamma}$ phase increased with increasing nitrogen gas content. The ${\gamma}$ single phase appeared at the surface above $80%N_2$ gas, while the surface ${\alpha}$ single phase was shown below $20%N_2$ gas. When heat treating the specimen in nitrogen gas at $1050^{\circ}C$, the hardness of austenite phases increased above Hv 40 at the surface layer compared to the hardness of the core parts, while decrement of denitriding effect caused to the hardness nearly unchanged between surface and the core parts after heat treating in vacuum atmosphere. The surface ${\gamma}$ single phase specimen showed superior corrosion resistance than the surface ${\alpha}$ single phase specimen. The surface ${\alpha}$ phase existed in the ${\alpha}+{\gamma}$ microstructure showed higher corrosion resistance after heat treating in the nitrogen gas atmosphere than the ${\alpha}$ phase heat treated in the argon gas and vacuum atmosphere.

• Corrosion Science and Technology
• /
• 제13권3호
• /
• pp.95-101
• /
• 2014

The pitting corrosion behaviors between the constituent phases in F53 super duplex stainless steel (SDSS) in acidified chloride environments were investigated using a critical pitting corrosion temperature test, a potentiodynamic anodic polarization test, and the microstructure analyses through a SEM-EDS and a SAM. As the solution annealing temperature decreased from $1150^{\circ}C$ to $1050^{\circ}C$, the ${\gamma}$-phase fraction increased whereas the ${\alpha}$-phase fraction decreased. The pitting potential and the critical pitting temperature increased with a decrease of solution annealing temperature, thereby increasing the resistance to pitting corrosion. The pitting corrosion of the SDSS was selectively initiated at the ${\alpha}$-phases because the PREN (pitting resistance equivalent number, PREN = %Cr+3.3%Mo+30%N) value of the ${\gamma}$-phase is much larger than that of the ${\alpha}$-phase, irrespective of the solution annealing temperature. The pitting corrosion was finally propagated from the ${\alpha}$-phase to the ${\gamma}$-phase. The decrease of solution annealing temperature enhanced the resistance to pitting corrosion greatly in acidified chloride environments due to a decrease of PREN difference between the ${\gamma}$-phase and the ${\alpha}$-phase, that is, a decrease of $PREN{\gamma}$ by dilution of N in ${\gamma}$-phase with an increase in the ${\gamma}$-phase volume fraction and an increase of $PREN{\alpha}$ by enrichment of Cr and Mo in the ${\alpha}$-phase with a decrease in the ${\alpha}$-phase volume fraction.

• 한국전기전자재료학회논문지
• /
• 제29권2호
• /
• pp.90-94
• /
• 2016

In this work, [$Pb(Mg_{1/2}W_{1/2})_{0.03}(Ni_{1/3}Nb_{2/3})_x(Zr_{0.5}Ti_{0.5})_{0.97-x}O_3-BiFeO_3$] (x=0.02 to 0.12) composition ceramics were fabricated by the conventional soild state reaction method and their microstructure and piezoelectric properties were investigated according to PNN substitution. The addition of small amount of $BiFeO_3$, $Li_2CO_3$, and $CaCO_3$ were used in order to decrease the sintering temperature of the ceramics. The XRD (x-ray diffraction patterns) of all ceramics exhibited a perovskite structure. The sinterability of PMW-PNN-PZT-BF ceramics was remarkably improved using liquid phase sintering of $CaCO_3$, $Li_2CO_3$. However, it was identified from of the X-ray diffraction patterns that the secondary phase formed in grain boundaries decreased the piezoelectric properties. According to the substitution of PNN, the crystal structure of ceramics is transformed gradually from a tetragonal to rhombohedral phase. The x=0.10 mol PNN-substituted PMW-PNN-PZT-BF ceramics sintered at $920^{\circ}C$ showed the optimum values of piezoelectric constant($d_{33}$), piezoelectric figure of merit($d_{33{\cdot}}g_{33}$), planar piezoelectric coupling coefficient($k_p$) and density : $d_{33}=566$ [pC/N], $g_{33}=29.28[10^{-3}mV/N]$, $d_{33{\cdot}}g_{33}=16.57[pm^2/N]$, $k_p=0.61$, density=7.82 [$g/cm^3$], suitable for duplex ultrasonic sensor application.

• 한국재료학회지
• /
• 제31권9호
• /
• pp.519-524
• /
• 2021

The impact properties of two austenitic Fe-23Mn-0.4C steels with different Al contents for cryogenic applications are investigated in this study. The 4Al steel consists mostly of austenite single-phase microstructure, while the 5Al steel exhibits a two-phase microstructure of austenite and delta-ferrite with coarse and elongated grains. Charpy impact test results reveal that the 5Al steel with duplex phases of austenite and delta-ferrite exhibits a ductile-to-brittle transition behavior, while the 4Al steel with only single-phase austenite has higher absorbed energy over 100 J at -196 ℃. The SEM fractographs of Charpy impact specimens show that the 4Al steel has a ductile dimple fracture regardless of test temperature, whereas the 5Al steel fractured at -100 ℃ and -196 ℃ exhibits a mixed fracture mode of both ductile and brittle fractures. Additionally, quasi-cleavage fracture caused by crack propagation of delta-ferrite phase is found in some regions of the brittle fracture surface of the 5Al steel. Based on these results, the delta-ferrite phase hardly has a significant effect on absorbed energy at room-temperature, but it significantly deteriorates low-temperature toughness by acting as the main site of the propagation of brittle cracks at cryogenic-temperatures.

• 한국세라믹학회지
• /
• 제39권1호
• /
• pp.51-56
• /
• 2002

고순도 알루미나 분말에 소결조제로서 초산 마그네슘과 질산 마그네슘을 1000ppm첨가하여 제작한 소결체에 대해서 소결조제로 사용한 MgO의 출발물질의 종류가 소결체 특성에 미치는 영향을 고찰하였다. 초산 마그네슘과 질산 마그네슘을 사용해서 제작한 소결체에 대해서 소결온도 1$600^{\circ}C$에서 상대밀도의 급격한 감소가 확인되었고 SEM관찰결과 소결조제로 사용한 두 종류의 출발물질을 첨가한 경우에 동일하게 소결체의 outer layer에서는 입성장 및 기공이 형성되어있고 inner layer에서는 알루미나 입계에 제2상의 존재가 관찰되었다. 이 시편의 EPMA을 이용한 원소분석결과로부터 소결온도 1$600^{\circ}C$에서 알루미나 입계에 존재하는 제2상은 Mg를 함유한 혼합물이라는 것을 알 수 있었다.

• 한국세라믹학회지
• /
• 제29권4호
• /
• pp.283-292
• /
• 1992

Duplex composites such as Y-TZP/Y-TZP-20 wt.% Al2O3 and Y-TZP/Y-TZP- 40 wt.% Al2O3 were made by mixing the sieve-shaked granules followed by isostatic pressing and sintering at 150$0^{\circ}C$ for 1 hour. So Y-TZP became matrix region and Y-TZP-20 wt.% Al2O3 or Y-TZP-40 wt.% Al2O3 became dispersed regions. In these composites, propagating cracks due to thermal shock always run into the dispersed region because these regions act as compressive zone due to low thermal expansion than matrix region. So duplexes having dispersed regions of Y-TZP-40 wt.% Al2O3 showed higher retained strength after thermal shock than matrix only composites because crack propagations were stopped more or less in the dispersed region. But when crack propagations were much more easy than matrix like Y-TZP-20 wt.% Al2O3 region, retained strength was decreased than the matrix only composites despite of the low initial strength.

• 공업화학
• /
• 제16권5호
• /
• pp.648-652
• /
• 2005

대기압하에서 소결이 어렵다고 알려진 초이온 전도체인 $K^+$-beta-aluminas를 attrition mill을 이용하여 약 350 nm의 평균 입도를 가지는 분말로 분쇄한 후, 시편을 일축가압 성형하여 상압 하에서 동일조성의 분위기 분말을 이용하여 소결하였다. 소결온도는 $1400^{\circ}C{\sim}1650^{\circ}C$까지 $50^{\circ}C$ 간격으로 소결하였으며, 각 소결온도에서 시간에 따른 소결특성을 살펴보았다. 또한 고온에서 입자 성장 속도를 조절하기 위하여 $1600^{\circ}C$에서 zone sintering을 실시하였다. $1600^{\circ}C$와 $1650^{\circ}C$에서 소결한 경우와 $1600^{\circ}C$에서 zone sintering한 시편의 경우, 각각 약 93%와 95%의 소결 밀도를 나타내었다. $1600^{\circ}C$ 이상의 온도에서 2 h 이상 장시간 소결할 경우, 입자의 거대성장으로 인하여 소결밀도가 90% 미만으로 감소하였다.

• Nuclear Engineering and Technology
• /
• 제35권1호
• /
• pp.14-24
• /
• 2003

This work investigates the effects of Cr$_2$O$_3$ and oxygen potential on grain growth and densification of UO$_2$ pellets. Powder mixtures of UO$_2$ and 0.03-0.4wt% Cr$_2$O$_3$ were pressed and sintered in 3 different gas atmospheres: the $H_2O$-to-H$_2$ ratios were 5$\times$10$^{-4}$ , 1$\times$10$^{-2}$ and 3$\times$10$^{-2}$ In the first gas atmosphere the Cr$_2$O$_3$ contents below 0.2 wt% have an insignificant effect on grain size, but the Cr$_2$O$_3$ contents more than 0.3 wt% promote grain growth in the inner zone of a pellet but not in the outer zone. In both the second and third atmospheres, the grain size increases with the Cr$_2$O$_3$ content. With the same level of Cr$_2$O$_3$ content the grain size is larger in the second atmosphere than in the third. Sintering behavior and developed microstructure are discussed in terms of the reduction of C$r^2$O$^3$ to Cr, the dissolution of C$r^2$O$^3$ in UO$_2$, and liquid phase sintering.

• Advances in materials Research
• /
• 제1권1호
• /
• pp.51-74
• /
• 2012

A powder metallurgy-based rapid consolidation technique, Plasma Pressure Compaction ($P^2C^{(R)}$), was utilized to produce near-net shape parts of gamma titanium aluminides (${\gamma}$-TiAl). Micron-sized ${\gamma}$-TiAl powders, composed of Ti-50%Al and Ti-48%Al-2%Cr-2%Nb (at%), were rapidly consolidated to form near-net shape ${\gamma}$-TiAl parts in the form of 1.0" (25.4 mm) diameter discs, as well as $3"{\times}2.25"$ ($76.2mm{\times}57.2mm$) tiles, having a thickness of 0.25" (6.35 mm). The ${\gamma}$-TiAl parts were consolidated to near theoretical density. The microstructural morphology of the consolidated parts was found to vary with consolidation conditions. Mechanical properties exhibited a strong dependence on microstructural morphology and grain size. Because of the rapid consolidation process used here, grain growth during consolidation was minimal, which in turn led to enhanced mechanical properties. Consolidated ${\gamma}$-TiAl samples corresponding to Ti-48%Al-2%Cr-2%Nb composition with a duplex microstructure (with an average grain size of $5{\mu}m$) exhibited superior mechanical properties. Flexural strength, ductility, elastic modulus and fracture toughness for these samples were as high as 1238 MPa, 2.3%, 154.58 GPa and 17.95 MPa $m^{1/2}$, respectively. The high temperature mechanical properties of the consolidated ${\gamma}$-TiAl samples were characterized in air and vacuum and were found to retain flexural strength and elastic modulus for temperatures up to $700^{\circ}C$. At high temperatures, the flexural strength of ${\gamma}$-TiAl samples with Ti-50%Al composition deteriorated in air by 10% as compared to that in vacuum. ${\gamma}$-TiAl samples with Ti-48%Al-2%Nb-2%Cr composition exhibited better if not equal flexural strength in air than in vacuum at high temperatures.

• 한국압력기기공학회 논문집
• /
• 제19권2호
• /
• pp.84-92
• /
• 2023

As temporary storage facilities for spent nuclear fuels in domestic nuclear power plants are expected to be saturated, external intermediate storage facilities would be required in the future. Spent nuclear fuels are stored in metal canisters and then placed in a dry environment within concrete or metal casing for operation. In the United States, the dry storage method for spent nuclear fuels has been operated for an extended period. Based on the corrosion experiences of dry storage canisters in chloride environments, numerous studies have been conducted to reduce corrosion in welds. With the construction of intermediate storage facilities in Korea for spent nuclear fuels expected near coastal areas adjacent to nuclear power plants, there is a need for research on the corrosion occurrence of welds and mitigation methods for canisters in chloride environments. In this paper, we measured and compared the residual stresses in the Heat-Affected Zones (HAZ) after electron beam welding (EBW) and gas tungsten arc welding (GTAW) processes for candidate materials such as 304L, 316L, and duplex stainless steel(DSS). We investigated the possibility of microstructure control through the application of surface modification processes using friction stir processing (FSP). Corrosion tests on each welded specimen revealed a higher corrosion rate in EBW welds compared to GTAW. Furthermore, it was confirmed that corrosion resistance improved due to phase refinement and redistribution of precipitates when FSP was applied.