Papers by Author: Kyung Jong Lee

Abstract: The grain coarsening temperature in Nb microalloyed steels is investigated by multiphase-field model. In this study, the pinning force is treated as time-dependent using mean-field kinetics of precipitates including volume fraction and their size. The grain size is calculated with time under various temperature range. The grain coarsening temperature is determined by the ratio of the largest radius of grain vs. the average grain radius criteria (R_max / R_avg > 2.94) in two-dimensional growth. Through this model, it is possible to simulate grain growth in microalloyed steels more precisely.

Abstract: Inconel alloy 690 which contains high chromium concentration, has replaced Inconel alloy 600 because of its high resistance of stress corrosion cracking (SCC). Inconel alloy 690 is an austenite nickel-based alloy and it has intergranular chromium carbide (M₂₃C₆). Alloy should be maintained to be nearly free from fretting wear, corrosion, and hydrogen brittleness for a several decades. Main factors controlling deterioration are initial chromium carbide size and their distribution along austenite grain boundary and chromium concentration distribution inside of grain. The precipitated carbide along grain boundary are modeled by KJMA(Kolmogorov-Johnson-Mehl-Avrami) equation. The model is based on the classical nucleation theory, and Cr diffusion controlled growth followed by coarsening. The distribution of the chromium concentration near grain boundary with time is based on diffusion of chromium. The simulated results are compared with the experiments from literatures to confirm the validity of model.

Abstract: The phase transformation during continuous cooling in low carbon steel has been widely measured by dilatometer using the lever rule. However, the concept of lever rule has several limitations. In low carbon steels, it is observed that overlapped transformation region of multi-phase and inflection point of small amount of low temperature phase is hardly differentiated. First derivatives of LVDT during continuous cooling could be better way to identify the inflection point and transformation region of phases (especially low temperature phase). Furthermore, first derivative of LVDT could be expressed as the lattice parameter and phase fraction due to temperature. Therefore, phase transformation behavior is simulated by the analysis of first derivative of LVDT using Avrami equation from experimental LVDT. As a result, the start, finish temperature and the amount of each phase are determined. The method is also confirmed by OM and SEM.

Abstract: The phase transformation in steels has been widely measured by dilatometer using the lever rule. However, the concept of lever rule is no longer applied in case of multi-phase present. Furthermore, it is quite difficult to differentiate a low temperature phase from the others due to the small fraction change (10-3 - 10-4 fraction of original length) and the plastic deformation during transformation. The overlapping of first derivatives of LVDT of several continuous cooling dilatations could be better way to identify and to analyze low temperature phases. In addition, the length change is simulated by considering the lattice parameter changes due to the temperature, composition and phase as well as decomposition kinetics of austenite in order to verify the method suggested. By comparing the simulated length change with the measured, the first derivative of dilatation interfered could be separated for each phase. As a result, the start, finish and peak temperature and the amount of each phase are determined. The method is also confirmed by OM and SEM.

Abstract: It is generally accepted that Si promotes kinetics of polygonal ferrite due to thermodynamic factors such as Ae3 and maximum amount of ferrite formed. However, in this study, it was found that the difference between the measured rates of ferrite formation in C-Mn steel and Si added steel was much larger than that expected considering only thermodynamic factors. The classical nucleation theory with pillbox model was adopted to figure out what is the most controlling factor in formation of ferrite. The volume free energy change was calculated by use of the dilute solution model. The diffusivity of carbon (DC) was formulated as functions of C, Mn and Si by using experimental data. It was found that the volume free energy change was still predominant but the kinetic factors such as interfacial energy and the diffusivity of carbon by addition of Si were not negligible at lower undercooling. However, with increasing undercooling, the diffusivity of C was the most effective on the ferrite kinetics, though the ambiguity of treating interfacial energy was not yet clear.

Abstract: Hydrogen redistribution caused by thermotransport in the Zr-1.0Nb-1.0Sn-0.1Fe alloy under the temperature gradient which is likely to be encountered between nuclear fuel cladding and primary cooling water (300-340
) was investigated. The heat of transport (Q*) of hydrogen was determined by using a steady state technique to evaluate the magnitude and direction of thermotransport of hydrogen in the alloy. The values of Q∗ were 23.1, 23.7 and 27.1 KJ/mol for the hydrogen concentration of 73.4, 75.8 and 94.3 ppm by weight respectively. In other words, hydrogen was transported from hot region to cold region and the value of Q∗ increased with increasing overall hydrogen concentration. The Zr-1.0Nb-1.0Sn-0.1Fe alloy had the smaller Q∗ value than that of Zircaloy-4 when compared with same overall hydrogen concentration. Thus, Zr-1.0Nb-1.0Sn-0.1Fe alloy has better resistance to the formation of hydride due to thermotransport than Zircaloy-4 does.

Abstract: The effect of Si on phase transformation was well known in dual phase steels. Si promoted the ferrite transformation and the enriched C in untransformed austenite prohibited the transformation at intermediate temperature range resulting in the formation of lower bainite and martensite at low temperature range. In addition, during continuous cooling with fast cooling rate, it was very hard to differentiate one phase from the others. In order to clarify the effects of Si on the austenite-to-ferrite transformation quantitatively, the start temperatures of bainite(BS) and martensite(MS) as well as ferrite(Ae3) and pearlite(Ae1) were calculated by thermodynamic analysis. LVDT measured by dilatometer and 1st differentiation peaks of LVDT were examined with microstructures, which gives a possibility of the phase separation. In CCT diagrams, it was also found that large austenite grain size(AGS) widened the gap between the transformation start(Ts) and end(Tf) when Si was added.

Abstract: The dilute solution model is quite widely used because the chemical potential is more easily defined than that in the sub-lattice model. In the present study, the thermodynamic model for the Fe-Mn-Si-Nb-Ti-V-C system was conducted by evaluating Wagner interaction parameters. The data used in this work was collected and modified by means of TCFE 2000-database in Thermo-Calc and up-to date references. The relationship of interaction parameters(L) in the sub-lattice model and Wagner interaction parameters in the dilute solution model was derived. The composition dependency of reference state and the higher order interaction parameters of the excess Gibbs energy were considered to evaluate Wagner interaction parameters. The equilibrium compositions of austenite and fractions of phases and the dissolution temperature of precipitates(NbC, VC, and TiC) were evaluated by the dilute solution model and compared with the results by the sub-lattice model.

Abstract: This work has attempted to find a new low temperature reduction process for fabrication of Cu nanopowder from fine CuO powder. For this purpose, we used electrochemical reduction method which is conducted in an electrolyte of NaCl aqueous solution at room temperature. It was found that ball-milled CuO powder (particle size ~100 nm and grain size ~40 nm) was completely reduced under the conditions of 20 V power, 0.5 mol NaCl solution and 2 h reaction time, producing Cu nanopowder (particle size ~80 nm and crystallite size ~25 nm). Simultaneously, we observed that sintering of nanopowders occurred during the reduction process, leading to agglomeration of nanopowder. Based upon the experimental results, the correlation between electrochemical reduction process and its related powder characteristics was discussed in terms of material transport.

Abstract: Anisotropy magneto-resistance and planar Hall-effect of ferromagnetic GaMnAs epitaxial films were investigated. The films were grown on 2 o off-cut GaAs (001) substrate in an optimized growth condition via low temperature molecular beam epitaxy. The GaMnAs layer revealed an easy axis along the (2x4) reconstruction direction of the substrate or along the off-cut direction. The large value of the anisotropy magneto-resistance ratio of ~7 % was realized by a well-alignment of the easy axis of the homogeneous ferromagnetic GaMnAs layer with the current. It also gives a very high planar Hall resistance ratio of ~500 %.