Papers by Author: Woo Seog Ryu

Abstract: High Cr ferritic/martensitic steels are demanded to join using favorable welding processes with economical and metallurgical advantages in order to apply to the thick-walled reactor pressure vessel of a very high temperature gas cooled reactor. Narrow gap welding technology was adopted to weld a thick-walled 9Cr-1Mo-1W steel with thickness of 110mm. The welding integrity was checked by non-destructive examination, optical microscopy and hardness test, and the homogeneity through welding depth was checked by absorbed impact energy and tensile strength. The optimizing welding conditions resulted that a narrow U-grooved gap with almost parallel edges was sound in actual practice, and the coarse grain zone was minimized in the heat affected zone. The absorbed energy of 75±25 J through welding depth was acceptable in scatter band to check the uniformity through the welding depth. The ultimate tensile stress and yield stress were about the same through welding depth at 650±10 MPa and 500±10 MPa, indicating no difference through welding depth. Elongation was also almost same through depth, and the fracture surface was appeared as a normal. The weld metal had similar mechanical properties to base metal. The upper self energy of weld metal was 194J, and the ductile-brittle transition temperature was 30°C. The tensile behavior was the typical trend with temperature, and YS and UTS of weldment were slightly higher than base metal by nearly below 10%. Thus, it concluded that the soundness of the narrow gap welding of a thick-walled 9Cr-1Mo-1W steel was confirmed in terms of the welding uniformity through the depth and mechanical properties.

Abstract: The precipitation of nano Cr2N particles in high Cr FM steels has been studied. The nitrogen content of the FM steels was changed to form stable Cr2N particles. Tempering temperature was also changed from 500 oC to 800 oC to study the precipitation behavior of the Cr2N particles with the tempering temperature. The Cr2N particles remained as a stable phase at a higher tempering temperature by increasing the nitrogen content. The shape of these particle was a fine needle type which was very similar to V(C,N) particles. The size of some Cr2N particles was increased as the nitrogen content increased. But these precipitates were not dissolved or largely coarsened during a creep deformation at 600°C. So it seems that they may act as an effective obstacle against a dislocation glide during a creep deformation, thus contribute to an increase of the creep rupture strength in high Cr FM steels.

Abstract: To design HTGR components for up to 1000oC, their creep curves are necessary during a design process. In this study, the full creep curves were modeled by the nonlinear least square fitting method using the Kachanov-Rabotnov (K-R) creep model. A series of creep data was obtained experimentally under various stress levels for Hastelloy-X at 950oC, and the data was used to model the creep curves. The K-R model gave a poor description of modeling creep curves, but the modified K-R one, which has another variable, K in the K-R model, was in better agreement than the K-R one. It was found that the λ parameter in the K-R model was constant regardless of the stress variations. The λ value was about 3.9 for the K-R model and about 5.8 for the modified one.

Abstract: Tensile and fatigue properties were evaluated for base and welded type 316LN stainless steel. Welding methods were GTAW (308L, Ar environment) and GTAWN (316L, Ar + N2 environment). Yield strength of weld joint was higher than that of base metal but elongation of weld joint was lower than that of base metal. UTS of weld joint was slightly lower than that of base metal. Yield strength and elongation with welding method were almost same. Fatigue life of weld joint was lower than that of base metal but fatigue strength of weld joint was higher than that of base metal. Ferrite content was increased with welding. Fatigue life welded by GTAWN was better than that of GTAW at RT and 600°C. This fatigue life behavior was consistent with the behavior of ferrite content.

Abstract: The effects of thermal aging on microstructural evolution and mechanical properties are important in the understanding of the in-service behavior of ferritic/martensitic steels in advanced nuclear power system. Ferritic/martensitic steels have been aged at 600oC for times up to 20,000 hrs. The change of mechanical properties has been examined for these aged materials. The strength and hardness was hardly changed after the thermal aging at 600oC for 20,000 hrs in all specimens. The impact absorbed energy decreased with the aging time. But the decrease of the impact absorbed energy was larger at the early stage of aging in tungsten added steels. This is attributed to the formation of Laves phase. Nitrogen which is known to increase the creep rupture strength had no effect on the degradation of the microstructure and mechanical properties during thermal aging.

Abstract: SiC nanowires could be grown homogeneously within SiC fiber preforms by controlling the concentration of a reactant gas. The morphology and the growth behavior of the SiC nanowires were largely dependent on the degree of the reactant supersaturation. The SiC nanowires incorporated in the fiber preform increased the surface area at which the matrix deposition could take place, enhancing an efficiency of the matrix infiltration and altering the remaining pore structure of the SiCf/SiC composite.

Abstract: In the polymer impregnation and pyrolysis (PIP) process for the fabrication of SiCf/SiC composite, the curing process should be included to increase the conversion yields. During the curing process, unintended oxygen is introduced. Control of this oxygen is very important to obtain composites with a good high temperature stability. Using the electron beam curing process with full doses of 2~10 MGy and the pyrolysis process at 1300~1500oC, composites with an oxygen contents of less than 1 wt% could be obtained.

Abstract: Inconel 617 is a candidate tube material for high temperature gas-cooled reactors (HTGR). The microstructure and mechanical properties of Inconel 617 were studied after exposure at high temperature of 1050oC. The dominant oxide layer was Cr-oxide. The internal oxide and Crdepleted region were observed below the Cr-oxide layer. The major second phases are M23C6 and M6C types of carbides. The composition of M23C6 and M6C were determined to be Cr21Mo2C6 and Mo3Cr2(Ni,Co)1C, respectively, by EDS. These carbides are coarsened during exposure. M6C carbide is more stable than M23C6 at high temperature. There was not much change in mechanical properties after exposure at 1050oC for 1000 h.

Abstract: There has been a great progress in the development of heat-resistant silicon carbide ceramics, owing to the better understanding of composition-microstructure-properties relations. Based on the progress, it has been possible to fabricate heat-resistant SiC ceramics with improved fracture toughness. In this paper, three rare-earth oxides (Re2O3, Re=Er, Lu, and Sc) in combination with AlN were used as sintering additives for a β-SiC containing 1 vol% α-SiC seeds. The effect of intergranular phase, using Re2O3 and AlN as sintering additives, on the microstructure and mechanical properties of liquid-phasesintered, and subsequently annealed SiC ceramics were investigated. The microstructure and mechanical properties were strongly influenced by the sintering additive composition, which determines the chemistry and structure of IGP. The strength and fracture toughness of the Lu2O3-doped SiC were ∼700 MPa at 1400oC and ∼6 MPa.m1/2 at room temperature, respectively. The beneficial effect of the new additive compositions on high-temperature strength was attributed to the crystallization of the intergranular phase.

Abstract: La and Sr have recognized as elements of high level radioactive wastes immobilized by solid solution with CaTiO3. For forming solid solution with CaTiO3, the solution combustion synthesis (SCS) process was applied and the powder characteristics and sinterability were investigated. The proper selection of the type and the composition of fuels are important to get the crystalline of CaTiO3. When glycine or the mixtures of urea+citric acid with stoichiometric composition was used, the solid solutions of Ca(La, Sr)TiO3 were entirely produced. The combustion synthesized powder seemed to have a good sinterability with the linear shrinkage of more than 23% up to 1400oC, while that of the solid state reacted powder was less than 19% at the same conditions.