Papers by Author: Sung Ho Kim

Abstract: In order to evaluate the effects of a pro-eutectoid ferrite content on the tensile strength and impact toughness of 2.25Cr-1Mo-0.25V steels, several samples with a different area fraction of ferrite in the range from 0 to 80% were prepared by a control of cooling rate from an austenitization heat treatment temperature of 930oC. The steels were then tempered at 690oC, followed by a heat treatment at 705oC as a simulation of postweld heat treatments. The results of microstructural observation indicated that the ferrite was uniformly distributed in the bainitic matrix. The results of tensile tests revealed that the tensile strength at room temperature and 450oC was slightly reduced with the ferrite content of up to 20%, but it was considerably reduced with the ferrite content of 80%. On the other hand, the results of impact tests at -60, -80 and -100oC indicated that the impact toughness was rapidly degraded with increasing ferrite content. These results attributed to the segregation of carbides in the bainitic matrix as a result of the formation of ferrite phase with low solubility of carbon. It is concluded that the ferrite content in the 2.25Cr-1Mo-0.25V steels could exert a sensitive inference on the impact toughness rather than on tensile properties.

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: This study deals with the fabrication of high strength ferritic/martensitic steels by a control of both the carbon concentration and the fabrication process parameters. The 9Cr-2W steels containing a carbon concentration of 0.05, 0.07 and 0.11 wt% were normalized at 1050oC for 1 h, followed by a tempering at 550 and 750oC for 2 h, respectively. The results of the tensile tests at room temperature indicated that the tensile strengths were increased with an increase of the carbon concentration from 0.05 wt% to 0.07 wt%, but no more increase was observed when the carbon concentration was increased further up to 0.11%. After a cold rolling from a 4 mm to a 1 mm thickness without/with an intermediate heat treatment and a final heat treatment, the results of the tensile tests exhibited that superior tensile properties were obtained when the fabrication processes were composed of a tempering at 550oC, and a cold rolling with several intermediate heat treatments. These results could be attributed to the finely distributed precipitates in the partially recrystallized matrix. The optimized carbon concentration and the controlled fabrication process parameters are thus suggested to fabricate a high strength 9Cr-2W steel sheet.

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: The Mg-6wt%Al-1wt%Zn alloy powders mixture produced by gas atomization were spark plasma sintered (SPSed). The SPSed sample was coated by plasma electrolytic oxidation (PEO) method, and the microstructure, mechanical and corrosion properties were investigated and compared to the normally sintered sample. The SPSed sample exhibited a microstructure of well-bonded grains with approximately 48 ,m in grain size, which was bigger than that of the normally sintered sample. The coatings on the SPSed sample were characterized by dense and porous layers, while, in the case of the normally sintered sample, the coatings had grown through the grain boundary of substrate with increasing the coating time. The surface roughness, thickness and micro-hardness of coatings on the SPSed sample increased gradually with increasing the coating time. The corrosion property of coatings on the SPSed sample had a tendency to improve with increasing the coating time in 3.5%NaCl solution. However, it was worse than that of coatings on the normally sintered sample.

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.