Papers by Author: H.C. Kim

Abstract: The influence of solidification rates and Cr equivalents on the formation of the ferrite was studied by directional solidification in mod. (9-12)Cr-1Mo steels. It was found that the volume fraction of ferrite increased as increasing Cr equivalent and solidification rate. The volume fraction of the ferrite showed much higher at low solidification rates with the planar or cellular interface than that at high solidification rates with the dendritic interface. The volume fraction of ferrite in solidification microstructure showed much higher than that in extruded and heat-treated alloys. It depends on not only solidification rate and Cr equivalent but also the solidification fraction. At low solidification rates, there occurs segregation because the solid/liquid interface was planar or cellular, and it makes composition change with solidification fractions. The formation of ferrite has been discussed with Cr equivalent and solidification rate.

Abstract: Directional or single crystal technique was applied to enhance the ductility, and two phases of γ (Ni) phase or β (NiAl) phase in γ‘(Ni3Al) matrix were also considered to increase the strength and ductility. In this study, directionally solidified rods were prepared at the solidification rate of 50µm/s in 23-27 at.% Al-Ni alloys, and tensile strengths of these rods were analyzed at room temperature. Directionally solidified samples showed the γ dendrite fibers formed in the Ni3Al matrix in the hypo eutectic composition of 23 at.% Al, the γ‘ single phase in the eutectic composition of 24.5 at. % Al, and the β dendrite fibers in the γ‘ matrix in the hyper eutectic compositions of 25, 26, 27 at.% Al. The hypoeutectic alloy including γ dendrites with γ‘ matrix exhibited a large elongation of over 70% with ductile transgranular fracture at room temperature. With increasing Al contents, the γ dendritic microstructure changed to the β dendrite in the γ‘ matrix, which resulted in decreasing the elongation by increasing the volume fraction of the brittle β dendrites in the ductile γ’ matrix.

Abstract: The morphological evolution and growth mechanism of solidification interface with solidification rates were investigated in the Ni-base superalloy, IN738LC, by directional solidification and quenching technique under a relatively high thermal gradient of 20.5oC/mm. The planar interface of the MC-γ eutectic was found at the low solidification of 1 µm/s, and the dendritic interface formed above 5 µm/s. The dendrite lengths increased as increasing increasing solidification rate, and the dendrite tip temperature was close to the liquidus temperature at 50 µm/s. The carbide morphologies were blocky-type and rod-type in the planar interface of low solidification rates, and as the solidification rate increased, the carbide shape changed from script type to spotty type. The phase transformation temperatures from the dendrite to MC carbide and eutectic were estimated by DTA and by the solid/liquid interface morphology by directional solidification.

Abstract: Al addition is known to enhance corrosion resistance and high temperature properties in ferritic and austenitic stainless steels. Due to the addition of Al, formation and characteristic of nitride were studied in Al added 18Cr stainless steels. The phase diagram and segregation were estimated with addition of Al in 18Cr stainless steels by the Thermo-calc program. Formation behavior of the AlN phase was studied by observing the solid/liquid interface through directional solidification and the phase characterization was performed by XRD in the extracted precipitates from the matrix. It was confirmed that the AlN phase formed at the cell boundary from the liquid in 3 and 5 wt.% Al added 18Cr stainless steels.

Abstract: In order to exploit the anti-oxidation property of graphite mould, a new type of oxidation protective coating is produced by a pack cementation diffusion coating technique. To enable this material to be used at high temperatures, graphite moulds are coated with Si/SiC slips. The anionic dispersant is added to disperse the slip uniformly, of which the optimal amount is evaluated with viscosity. The graphite mold specimens are surface-modified at 100 °C for 10 minutes in a non-polar polymer aqua-solution, considering the uniform wettability of slip. The surface-modified graphite mold specimen shows better wettability than the nonsurface- modified graphite one when coating process is performed through the slip. The interface-reaction of the specimens is performed at 1450 °C in a reduction atmosphere. The microstructure and composition before and after the pack cementation are observed by SEM and EDS, and the phase identify was performed with XRD. The layer of specimens double-coated by the pack cementation and Si/SiC slip coating method is stable, and properties of SiC coating layer formed on the graphite mould surface are dependent on particle size of starting material, Si, and open-pore size of the graphite mould surface. It is found that larger particle size of Si and smaller open-pore size of the graphite mold were the preferable conditions for the interface produces an optimal reaction which anti-oxidation coating.