Papers by Author: Kee Sam Shin

Abstract: The P92 steels were aged at 632°C for 500hrs and 1,000hrs, and creep ruptured at 650°C~625°C with stress of 120~110 MPa. The creep rupture life (CRL) of the aged samples was decreased with the aged time. The microstructure of the P92 steel was observed as fine tempered lath with dispersion of Cr-rich M₂₃C₆ along various grain boundaries. Upon aging and creep, recovery of lath and precipitation of coarse W-rich Laves phase were characterized as the main microstructural change. The M₂₃C₆ is relatively stable upon the short-term aging and creep. Decrease of the CRL of the aged specimens is considered as the degradations of microstructure such as the recovery of lath due to the dislocation annihilation and precipitation of coarse Laves phase.

Abstract: The Nb-stabilized and Cu-strengthened austenitic stainless steel Super304H was aged at 600~700°C up to 20,000 hrs. Scanning electron microscope (SEM), and transmission electron microscope (TEM) were used to study the effect of aging on the microstructural evolution of the specimen, with focus on the precipitation behavior in relation to the temperature and time. Only NbCs in size range 2 μm ~ 50 nm were observed to be scarcely distributed in matrix in the as-received specimen. Upon aging, precipitation of σ-phase (~5 μm) and Cr-rich M₂₃C₆ (~1 μm) along grain boundary, and nanosized Cu precipitates (~65 nm) in the grain interior were formed. The size and fraction of the σ-phase and M₂₃C₆ increased with the increase of aging temperature and/or aging time, with higher sensitivity to the temperature. The size of Cu precipitates was relatively stable, while the fraction and number density increased with the aging temperature/time. The microhardness upon aging increased with increase of aging temperature/time, due to the precipitation of the nanosized Cu precipitates.

Abstract: 13Cr martensitic stainless steels are widely used in gas industry, which are usually manufactured by quenching-tempering treatment. Microstructural study of 13Cr steel through various heat treatments was carried out for determining the optimum parameters for industry manufacture. After quenching treatment at 975 °C for 20 min, precipitation-free martensitic structures were formed. During tempering, recovery of martensite through grain boundaries migration and dislocations annihilation was found to soften the steel. In addition, transformation of needle-like Cr₇C₃ carbides to the irregular shaped Cr₂₃C₆ carbides was observed when tempering temperature is above 710 °C. The phase transformation induced precipitation strengthening is discussed. The optimum heat treatment parameters of 13Cr steel for avoiding over tempering and the precipitation hardening are found.

Abstract: Effects of welding and long-term service on the microstructural evolution of superheater tubes of TP347H stainless steel used in power plants were investigated by optical microscope (OM), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), and transmission electron microscope (TEM). Analyses after welding or long-term service, showed fine NbCs in grains, which will precipitaion strengthen the matrix. When TP347 was long-term serviced in power plants, M₂₃C₆ formed preferentially on the grain boundaries and on twin boundaries, which was attributed to the embrittlement and the intergranular corrosion and fracture. The steam side had less recrystallization rate and more oxide compared to the fire side, which is part of the reason for the cracking from steam side to the fire side. And HAZ is more brittle than the matrix, because of α-Fe phase and coarse grains, due to which, cracks tend to initiate in the steam side of HAZ and propagate to the fire side.

Abstract: This paper proposes a semi-analytical method for the prediction of rolling force for rod rolling. The grooves of the passes are oval-round and round-oval. The mathematical model of rolling force is based on Shinkura and Takai’s model, but incorporates the effects of recrystallization and the alloy elements. Hot rod rolling experiments are conducted to examine the proposed semi-analytical method. The predicting values are compared with the mill log data, and they agree excellently over the whole rolling course. The predictions indicate that recrystallization affects the resistance of the material during hot rod rolling.

Abstract: The NiCoFeCrAl3 high entropy alloy coating with a little addition of C, Si, Mn, Mo has been succesively synthesized by laser cladding. The results show that simple solution phases of ordered BCC and a small fraction of FCC are obtained with fine equaixed dendrites morphology. Because the fine grain strengthening obtained by rapid solidification and the additived small atomic elements like C, Si further increase the distortion of the solid solution lattice, The microhardness of the coating reached above 800 HV and is 50 % higher than previous study on the similar composition by arc melting technique.

Abstract: In this study, the microstructural behavior of two kinds of 9-12% Cr steels (specimen A and B) used for power plants with and without W, Co, B which were subjected to aging during various time and temperature were investigated by electron microscope and related analytical techniques. The results indicate that there are four kinds of precipitates in these materials, i.e., M23C6, Nb-rich and V-rich MX, W-rich and Mo-rich Laves phase, and Z-phase. Upon aging, the area fraction of M23C6 increased whereas that of Laves phases decreased despite an increase in size. The area fraction of W-rich Laves phase was much higher than that of Mo-rich Laves phase, indicating that W addition compared to that of Mo addition is more powerful in the formation of Laves phase precipitation (specimen A). Specimen A aged at 700°C exhibited the formation of Z-phase. The formation of Z-phase is known to have a tendency of dissolving MX precipitates in similar alloys. The high activity of Cr at 700°C is believed to be the cause of Z-phase formation.

Abstract: The objective of this study is to determine the mechanism of the dramatic increase of impact toughness at low temperatures after post-weld heat treatment on weld joints. In this study, weld joints using two semi-automatic welding consumables were fabricated by flux cored arc welding with subsequent PWHT at 660°C for 65 min and 195 min, respectively. Tests of the tensile and yield strength, microhardness and impact toughness, were carried out. The microstructure was inspected by optical, scanning electron, and transmission electron microscopy in addition to compositional analysis using energy dispersive spectrometry. PWHT was observed to result in grain coarsening, sub-grain structure formation and decrease of the dislocation density. The increase of impact toughness is attributed to the relieved thermal stress, the inclusions and precipitations, softening of the structure, dislocation recovery and sub-grain structure.

Abstract: In Zr-Cu-Ni-Al bulk metallic glasses where there are no dislocations, localized plastic deformation in shear bands occurs largely by the formation and migration of defects such as voids, micropores, shear bands and local variations in composition. Thus, the investigation on defects is critical for the understanding and improvement of plastic deformation in metallic glasses. In this study, microstructures and nano defects in the Zr-Cu-Ni-Al BMGs are characterized by variety of techniques, such as X-ray diffractometry, high resolution transmission electron microscopy, scanning transmission electron microscopy and electron holography.

Abstract: Zr62Cu17Ni13Al8 in the supercooled liquid state is expected to be micro-formable at a relatively low stress. We used X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and quantitative high-resolution TEM (HRTEM) to investigate the microstructures of Zr62Cu17Ni13Al8 amorphous alloy after compression test. The alloy exhibited the homogeneous amorphous microstructure with some crystalline phases dispersed in the matrix. According to the XRD results, under the certain strain rate in the supercooled liquid state, the alloy showed higher crystallization at the higher heat treatment temperature. However, at the same heat treatment temperature, the alloy deformed under low strain rate showed higher crystallization. The β crystalline phase particles with spherical shape were detected by SEM and TEM. The sample with higher strain rate and temperature showed longer shear bands. Nano-voids formed by the coalescence of excess free volume in shear bands were investigated by quantitative HRTEM. Compared with the undeformed area, in the shear band, nanovoids were identified in the deformed area through quantitative HRTEM simulation.