Papers by Author: Sung Mo Hong

Abstract: In the present work, corrosion characteristics of ceramic dispersion strengthened (CDS) high-Cr stainless steel (SS) has been investigated. For the preparation of CDS SS with uniformly distributed reinforcements, TiC particles were mechanically activated with Cu powder by using high-energy ball mill machine, and then Cu-coated TiC particles were mixed with molten SS. Potentiodynamic polarization curves were experimentally measured on unmodified SS and CDS SS in 0.05 M NaCl solution. The values of corrosion potential Ecorr, corrosion current io, pitting potential Epit and polarization resistance Rp were quantitatively determined from the measured potentiodynamic polarization curves. From the result, it is concluded that CDS SS exhibits higher corrosion resistivity than unmodified SS.

Abstract: In this study the effects of harder ultrafine Al2O3 particles on the mechanical milling of ductile Cu flakes (200mesh, 70μm) have been investigated. The small hard Al2O3 particle in the mixture acted as an effective milling agent not only by inducing a drastic change of the Cu morphology from flattened flakes to equiaxed crystals but also by reducing the milling time required for a uniform dispersion of Al2O3 in the Cu matrix. This was more pronounced as the Al2O3 concentration increased. A critical concentration of the reinforcing Al2O3 required for a shape change was observed at the range of 8 to 9wt. %. All the hard Al2O3 particles were uniformly embedded in the ductile Cu matrix regardless of the concentration of Al2O3. By increasing the milling time, the weldment and fragmentation of the Cu flakes became significant and a uniformly distributed equiaxed composite structure (13wt.% Al2O3) was obtained at above 70 min..

Abstract: In this study the fatigue properties due to cavitation damage of flame-quenched 8.8Al-bronze (8.8Al-4.5Ni-4.5Fe-Cu) as well as current nuclear pump materials (8.8Al-bronze, SUS316 and SR50A) have been investigated by using an ultrasonic vibratory cavitation test. For this the impact loads of cavitation bubbles generated by ultrasonic vibratory device quantitatively evaluated and simultaneously the cavitation erosion experiments have been carried out. The fatigue analysis on the cavitation damage of the materials has been made from the determined impact load distribution (e.g. impact load, bubble count) and erosion parameters (e.g. incubation period, MDPR). According to Miner’s law, the exponents b of the F-N relation (Fb
N = Constant) at the incubation stage (N: the number of fracture cycle) were 5.62, 4.16, 6.25 and 8.1 for the 8.8Al-bronze, flame-quenched sample, SUS316 and SR50A alloys, respectively. At steady-state, the exponents b of the F-N curve (N: the number of cycles required for a 1μm increment of MDP) were determined as 6.32, 5, 7.14 and 7.76 for the 8.8Al-bronze, flame-quenched sample, SUS316 and SR50A alloys, respectively.

Abstract: This paper reports on the formation of tensile stress induced cracks by the application of a flame hardening to 12Cr steels and the monitoring of the flame hardening process at the desirable residual stress state. During the flame hardening of the steels elastic residual tensile stresses were typically generated due to a phase transformation of austenite into martensite and they became greater by increasing both the process temperature and cooling rate. Eventually the cracks were nucleated and propagated across the prior austenite grain boundaries by a generation of large tensile stress, which was accompanied by a drastic decrease in the tensile stress due to a stress relaxation.

Abstract: In this study, the movable flame hardening process of 12Cr steel for a uniform hardness and desirable residual stress have been investigated. For this, the temperature cycles have been controlled accurately as a function of the three processing variables, the flame intensity If, the scanning velocity Vs and the initial flame holding time th, where the standard surface temperature Ts,max was maintained at 960oC. The optimized conditions were Vs=0.68mm/s and th=67sec for the C3H8:O2=5:20
l/min, Vs=0.80mm/s and th=56sec for the C3H8:O2=6:24
l/min, Vs=1.01 mm/s and th=48sec for the C3H8:O2=7:28
l/min, and Vs=1.15 mm/s and th=39sec for the C3H8:O2=8:32
l/min. The optimally flame-hardened surface exhibited uniform distributions of the hardness and residual compressive stress over the treated area with moderate levels of 470~490HV0.2 in hardness and -300~-450MPa in residual stress, which were acceptable on the basis of the acceptance criteria of Siemens AG-KWU and GE Power Generation Engineering.

Abstract: In this study, the water drop impact erosion properties of 12Cr steel surface-hardened by the flame hardening process have been studied. For this, both the maximum erosion depth de,max and volume loss Ve with the number of cumulative impacts n have been investigated for the flame-hardened 12Cr steels with different hardnesses. Typically all the samples showed an erosion-time characteristic involving an incubation period initially followed by a steady state period. Compared to those for the as-received 12Cr steel, the flame-hardened ones showed an excellent erosion resistance to water drop impacts, showing a 2.2~2.8 times higher incubation time ti and 5~8 times lower erosion rate α. In the incubation period the as-received 12Cr steel was deformed by a ductile depression and ploughing, while the flame-hardened one by fatigue cracks and a brittle platelet deformation. In the steady state period the damage was progressed by a cleavage fracture for both the stages.