Seon-Jin Kim

The temperature dependence of the wear resistance of iron-base NOREM 02 hardfacing alloy

Abstract The sliding wear behavior of an iron-base NOREM 02 hardfacing alloy was investigated in the temperature range of 25–300°C under a contact stress of 103 MPa (15 ksi). The wear resistance of NOREM 02 was comparable to that of Stellite 6 at temperatures below 180°C, and the dominant wear mechanism was mild oxidative wear. With increasing temperature, however, the wear mode of NOREM 02 changed abruptly to severe adhesive wear at 190°C and galling occurred above 200°C. It was found from microhardness variations beneath the worn surfaces that the abrupt decrease of wear resistance is due to the loss of work hardening ability. X-ray diffraction and TEM analyses of the worn surfaces showed that a strain-induced phase transformation from austenite to a ′ martensite took place below 180°C while not above 190°C. Thus, it was concluded the development of strain-induced a ′ martensite plays an important role in the wear resistance of an iron-base NOREM 02 hardfacing alloy.

Effects of alloying elements on the tensile properties and oxidation behavior of modified Zircaloy-4 in 360°C water

Abstract The effects of oxygen and tin contents on the tensile properties and oxidation behavior of modified Zircaloy-4 (Zry-4) were investigated. The modified alloys were prepared by changing the chemical compositions of Zry-4; the tin contents were decreased from 1.0 wt% to zero while oxygen increased from 0.1 to 0.8 wt%, 0.1 wt% Nb was added and the amounts of Fe and Cr were kept 0.1 and 0.2 wt%, respectively. The UTS increased significantly with the oxygen content. 0.2 wt% oxygen could compensate the decrease in strength resulted from the reduction of tin content from 1.0 wt% to zero. The weight gain in water at 360°C under a pressure of 180 bar increased with tin and oxygen contents. The alloy with tin and oxygen contents of zero and 0.1 wt% showed the lowest weight gain for 120 days of oxidation test.

The effect of manganese on the strain-induced martensitic transformation and high temperature wear resistance of Fe-20Cr-1C-1Si hardfacing alloy

Abstract Sliding wear tests of Fe–20Cr–1C–1Si–xMn alloys with x varying from 0 to 25 wt% were performed in air at temperatures of 25–450°C under a contact stress of 103 MPa. During wear at 25°C, low-Mn alloys containing less than 5 wt% Mn and high-Mn alloys containing more than 10 wt% Mn exhibited the γ→α′ and γ→e strain-induced martensitic transformations, respectively. As the test temperature increased, low-Mn alloys showed a wear transition from mild oxidative wear to galling at a temperature between 350°C and 400°C due to the absence of γ→α′ strain-induced martensitic transformation. In the case of high-Mn alloys, the mild oxidative wear was maintained until 450°C, which was the highest test temperature in this study, with the formation of strain-induced e martensite. It was considered that the γ→e strain-induced martensitic transformation could be more beneficial to the high temperature wear resistance of iron-base hardfacing alloy than the γ→α′ strain-induced martensitic transformation due to the higher Md temperature.

Photocatalytic properties of rutile TiO 2 acicular particles in aqueous 4-chlorophenol solution

The photocatalytic properties of TiO 2 rutile powder with acicular primary particles were characterized using the photocatalytic reaction in aqueous 4-chlorophenol (4CP) solution and compared to those of TiO 2 anatase with almost the same surface area of approximately 200 m 2 /g. The characteristics of commercial P-25 TiO 2 powders with the surface area of approximately 55 m 2 /g were also compared to rutile and anatase powders. The rutile phase powders surpassed both anatase and P-25 ones in decomposition rate for 4CP. The excellent photo-oxidative ability of the powder was dependent on the specific powder-preparation method, which led to a direct crystallization in aqueous solution, regardless of the crystalline structures of the powders.

Reliability Prediction of Long-term Creep Strength of Gr. 91 Steel for Next Generation Reactor Structure Materials

This paper focuses on reliability prediction of long-term creep strength for Modified 9Cr-1Mo steel (Gr. 91) which is considered as one of the structural materials of next generation reactor systems. A “Zparameter” method was introduced to describe the magnitude of standard deviation of creep rupture data to the master curve which can be plotted by log stress vs. The larson-Miller parameter (LMP). Statistical analysis showed that the scattering of the Z-parameter for the Gr. 91 steel well followed normal distribution. Using this normal distribution of the Z-parameter, the various reliability curves for creep strength design, such as stress-time temperature parameter reliability curves (σ-TTP-R curves), stress-rupture time-reliability curves (σ-tr-R curves), and allowable stress-temperaturereliability curves ([σ]-T-R curves) were reasonably drawn, and their results are discussed. (Received January 24, 2011)

EFFECT OF DISPERSED SiC PARTICLES ON THE OXIDATION OF AZ91D MAGNESIUM ALLOYS BETWEEN 420 AND 500°C IN AIR

AZ91D magnesium alloys with and without dispersed SiC particles were oxidized between 420 and 500°C in air. They oxidized to fine MgO oxide grains containing dissolved ions of Al. Their oxidation rates increased almost linearly, with an increase in the oxidation temperature and time. SiC particles did not oxidize during oxidation, and increased the oxidation resistance of the alloy through diminishing the exposed surface area. With the increase in the amount of the dispersed SiC particles from 5 to 10, and to 20 wt.%, the oxidation resistance progressively increased.

Preparation of Nanostructured TiO2 Powders with Additions of Various Metal–Chlorides for Efficient Photocatalyst

Preparation of nanostructured TiO2 powders for efficient photocatalyst was investigated through a direct crystallization from aqueous TiOCl2 solutions containing various metal–chlorides. The TiO2 powders without any additives and those added with Ni2+, Fe3+ and Nb5+ions, respectively, which has a similar cationic radius to Ti4+, were mainly crystallized to rutile phase, whereas those added with Al3+ and Zr4+ ions, which have a quite different positive ionic radius, were mainly crystallized to anatase phase. On the other hand, the secondary particles in the TiO2 powder consisted of acicular and spherical primary particles corresponding to rutile and anatase phases, respectively. From these results, it seems that the cationic radius of the additives could affect phase formation as well as morphology of TiO2 precipitates. Among the TiO2 powders prepared, Ni2+ doped powder, showing mainly rutile phase, had excellent photocatalytic ability in decomposition of 4-chlorophenol.

Study on the cavitation erosion behavior of Fe-based hardfacing alloys containing Mn, Ti and V for replacing Co-based Stellite 6

Stellite 6 has been used as hardfacing material of valve components in nuclear power plant industry and other applications. Stellite 6 contains more than 60 wt % Co and it was recognized that Stellite 6 used in valve hardfacing is one of the main sources of Co, which is the major contributor to build up the radiation field in nuclear power plants. Thus it is largely responsible for the occupational radiation exposure of plant maintenance personnel. Also the price of Co is expensive and highly varying. Therefore, there have been many efforts to develop Co-free hardfacing alloy We have also developed Fe-based new alloy, of which sliding wear and cavitation erosion resistance were almost equivalent to those of Stellite 6. In the present study, the effects of Mn, Ti and V on the cavitation erosion behavior of the alloy were investigated. The Fe-based new alloy containing V had similar cavitation erosion resistance to Stellite 6 but the others containing Mn and Ti were inferior to Stellite 6.

p-Si 기판에 성장한 BaTiO3 박막의 두께와 구조적 특성과의 관계

In this study, BaTiO3 thin films were grown by RF-magnetron sputtering, and the effects of the thin film thickness on the structural characteristics of BaTiO3 thin films were systematically investigated. Instead of the oxide substrates generally used for the growth of BaTiO3 thin films, p-Si substrates which are widely used in the current semiconductor processing, were used in this study in order to pursue high efficiency in device integration processing. For the crystallization of the grown thin films, annealing was carried out in air, and the annealing temperature was varied from 700˚C. The changed thickness was within 200 nm~1200 nm. The XRD results showed that the best crystal quality was obtained for ample thicknesses 700 nm~1200 nm. The SEM analysis revealed that Si/BaTiO3 are good quality interface characteristics within 300 nm when observed thickness. And surface roughness observed of BaTiO3 thin films from AFM measurement are good quality surface characteristics within 300 nm. Depth-profiling analysis through GDS (glow discharge spectrometer) showed that the stoichiometric composition could be maintained. The results obtained in this study clearly revealed BaTiO3 thin films grown on a p-Si substrate such as thin film thickness. The optimum thickness was 300 nm, the thin film was found to have the characteristics of thin film with good electrical properties.

CYCLIC OXIDATION OF Ti-48%Al-2%Cr-2%Nb-(0~1%)W ALLOYS BETWEEN 800 AND 1000°C IN AIR

Ti-48%Al-2%Cr-2%Nb-(0, 0.5, 1) at.%W alloys were synthesized via the powder metallurgical route, and cyclically oxidized at 800, 900, or 1000°C in air for up to 100 h in order to find the effects of W on their oxidation characteristics. At 800°C, they oxidized relatively slowly, and the scales were thin and adherent. At 900°C, the scales began to spall locally. At 1000°C, they spalled repetitively during oxidation. Cr, Nb, and W improved the cyclic oxidation resistance of TiAl alloys. The oxides formed were TiO2 and Al2O3, which contained the alloying elements of Cr, Nb, and W. Nitrides formed were TiN and Ti2AlN.