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Li, Kejian,Ma, Houyu,He, Yinsheng,Chang, Jungchel,Bae, Si-yeon,Shin, Keesam Elsevier 2017 Fusion engineering and design Vol.125 No.-
<P><B>Abstract</B></P> <P>High-chromium heat-resistant steels are widely used as key materials to improve the condition of steam pressure and temperature in the boiler tube system of the supercritical power plants. Material-related failures in boiler system have occurred mainly due to internal oxidation and corrosion. For the stable maintenance of boilers, determination of the corrosion mechanism, microstructural evolution and data collection pertaining to the oxidation is essential. In this study, the supercritical steam test was carried out on T92 steel at the temperatures of 600/650/700°C for 10,000/15,000/20,000h. The matrix of steels and the oxide scale layers were investigated after steam treatment using scanning electron microscopy, back-scattered electron microscopy and electron backscattered diffraction. The oxide scale layers from the outer surface to the matrix consisted of: i) an outer layer of nanoscale iron oxide (Fe<SUB>2</SUB>O<SUB>3</SUB>); ii) inner layer of microscale crystals of Fe<SUB>3</SUB>O<SUB>4</SUB>, and iii) a mixture of chromium oxide and chromite (Cr<SUB>2</SUB>O<SUB>3</SUB> and FeCr<SUB>2</SUB>O<SUB>4</SUB>) adjacent to the matrix.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The matrix and scale layers were subsequently investigated using various electronic microscope analysis. </LI> <LI> The order of scale layers from the surface to the matrix are: i) nanoscaled Fe<SUB>2</SUB>O<SUB>3</SUB>; ii) microscaled Fe<SUB>3</SUB>O<SUB>4</SUB>; and iii) a mixture of Cr<SUB>2</SUB>O<SUB>3</SUB> and FeCr<SUB>2</SUB>O<SUB>4</SUB>. </LI> <LI> The Cr-rich layer was considered as protective for the matrix, which was stable at 600°C and 650°C, but the thickness decreased at 700°C. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>(a) EBSD phase mapping of the scale layer, (b) SEM, (c) BSE, and (d∼g) EDS elemental mapping of the steam tested at 650°C for 10,000h.</P> <P>[DISPLAY OMISSION]</P>
Grain Growth and Precipitation in Nanostructured 304SS After Heat Treatment
Li, Kejian,He, Yinsheng,Ma, Houyu,Jung, Jine-Sung,Yang, Cheol-Woong,Lee, Je-Hyun,Shin, Keesam American Scientific Publishers 2017 Journal of Nanoscience and Nanotechnology Vol.17 No.10
<P>Specimens of 304 stainless steel (304SS) were ultrasonic shot peening (USP) treated for refined and gradient microstructure. The specimens were then heat-treated at 500 degrees C, 600 degrees C, and 700 degrees C for 2 hrs, respectively. The hardness and microstructure of the untreated, shot peened, and heat-treated specimens were investigated. Grain growth and nanoscale precipitation were apparent only in the 700 degrees C heat treatment specimen and the microstructural analysis was focused on that specimen. The gradient microstructure from the top were characterized as: (i) nanocrystalline layer, with very little grain size of similar to 200 nm, (ii) ultrafine grain layer in similar to mu m size with nanosize M23C6 in grain interior, and (iii) deformed coarse grain layer, with grains in similar to 50 mu m and the M23C6 were in the grain interior and boundaries. The nanoscale precipitates, distributed on the original lamellas and deformed twin boundaries, inhibited the grain growth, and strengthened the peening affected layers.</P>
Surface nanocrystallization of pure Cu induced by ultrasonic shot peening.
Li, Kejian,He, Yinsheng,Fang, Chao,Ma, Houyu,Kim, Jaeyong,Lee, Han-Sang,Song, Jung-Il,Yang, Cheol-Woong,Lee, Je-Hyun,Shin, Keesam American Scientific Publishers 2014 Journal of Nanoscience and Nanotechnology Vol.14 No.12
<P>Peening is mainly used as a method of surface treatment for microstructural modification in order to improve surface mechanical properties. The ultrasonic shot peening (USP) technique can cause severe plastic deformation with its high strain rate on the surface of metallic parts. However, systematic studies of microstructural refinement mechanism upon plastic deformation with consideration of alloy systems are rare. In this study, USP-treated Cu samples of 99.96% purity was examined using analytical techniques, Vickers microhardness test, electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). Results of EBSD and microhardness analyses indicated grain refinement with deformation structures and hardness increase down to 400 μm in depth upon treatment. Depth specific TEM analysis of the samples revealed the steps of the grain refinement process to the formation of randomly oriented fine grains.</P>
He, Yinsheng,Li, Kejian,Cho, In Shik,Park, In Gyu,Shin, Keesam American Scientific Publishers 2014 Journal of Nanoscience and Nanotechnology Vol.14 No.11
<P>In this work, an Al-7 wt.% Si alloy, which is widely used as the structural materials in the automotive and aerospace industries for their high specific strength, was subjected to ultrasonic nanocrystalline surface modification (UNSM) treatment. After UNSM treatment, the effect of UNSM on the microstructural evolution of both Al grain and the dispersed Si particles was studied by using scanning electron microscope (SEM) and transmission electron microscope (TEM). Experimental results show that the ultra-fine grain (UFG, - 400 nm in size) structure is developed in the top surface layer (up to - 15 μm in depth). The coarse Si particles were refined and well dispersed in the UFG Al matrix. Cross-sectional TEM observation revealed that the grain refinement mechanism involved the formation of new grain boundaries dividing the coarse grain into UFG structure. Nanotwin and nanosize Si were formed within the original coarse Si particles. The presence of dispersed Si particles in the Al matrix accelerated the Al grain refinement process.</P>