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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,Yoo, Keun-Bong,Ma, Houyu,Shin, Keesam Elsevier 2018 Materials letters Vol.215 No.-
<P><B>Abstract</B></P> <P>A gradient-structured layer with initially nanoscale grain size increased gradually to original coarse microscale (∼50 μm) was fabricated on the surface of austenitic (γ) stainless steel SS304 <I>via</I> ultrasonic nanocrystallization surface modification (UNSM) peening. Modified cross-sectional and depth-specific plan-view (DSPV) sample preparation methods were explored with the analysis scales from macro to atom by synchrotron radiation XRD, EBSD, and TEM to determine the depth (strain) dependent deformation microstructures and grain refinement mechanism. The depth-dependent deformation microstructures were ascribed to strain-induced martensitic-transformed (SIMT) α′- and ε-martensite, deformation nanotwins (NT), and dislocation grids. The grain nanocrystallization mechanism was suggested as the formation of α′ and ε grain boundaries that divided the original coarse grains (CG) to the nanoscale. The strains also appeared to play a key role in the crystallographic orientation relationship (OR) of the γ/α′: Kurdjumov-Sachs (K-S) and Pitsch in low strain region and Nishiyama-Wassermann (N-W) in the high strain region.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A gradient-structured layer was fabricated on the surface of SS304 <I>via</I> UNSM peening. </LI> <LI> The depth-dependent microstructure was studied by the modified TEM sampling methods. </LI> <LI> The orientation relationships of the transformed martensite are depended on strain. </LI> </UL> </P>