Papers by Author: Hwan Jin Sung

Abstract: Thermal fatigue is a complex phenomenon encountered in materials exposed to cyclically varying temperatures in the presence or absence of external load. Continually increasing working temperature and growing need for greater efficiency and reliability of automotive exhaust require immediate investigation into the thermal fatigue properties especially of high temperature stainless steels. In this study, thermal fatigue properties of 304 and 429EM stainless steels have been evaluated in the temperature ranges of 200-800oC and 200-900oC. Systematic methods for control of temperatures within the predetermined range and measurement of load applied to specimens as a function of temperature during thermal cycles have been established. Thermal fatigue tests were conducted under fully constrained condition, where both ends of specimens were completely fixed. Thermal fatigue property of STS 304 was superior to that of STS 429EM. Load relaxation behavior at the temperatures of thermal cycle was closely related with the thermal fatigue property.

Abstract: In the present study, powder injection molding (PIM) process, in which the porosity of sintered parts can be easily controlled, has been employed to produce specimens with the various porosities from the initial stage of sintering and to the stage of nearly full density. A series of tensile tests has been conducted on these specimens at room temperature to elucidate the effect of a wide range of porosity. The material used in this study was 17-4 PH stainless steel, which is the precipitation hardenable stainless steel containing 4% of Cu and well known to show high strength and the high corrosion resistance at the same time. The 17-4 PH stainless steel powders used in this study were produced by the high-pressure water atomization method. Based on the results of tensile tests on the specimens with the various porosity, a new approach to predict the elongation of sintered materials has been carried out and a new framework combining neck growth model and ideal pore model has been established.

Abstract: The effect of warm rolling under various conditions on the microstructure and mechanical property was investigated using an AZ31 Mg alloy sheet. Several processing parameters such as initial thickness, thickness reduction by a single pass rolling, rolling temperature, roll speed, and roll temperature were varied to elicit an optimum condition for the warm rolling process of AZ31 Mg alloy. Microstructure and mechanical properties were measured for specimens subjected to rolling experiments of various conditions. Warm rolling of 30% thickness reduction per pass was possible without any side-crack at temperatures as low as 200oC under the roll speed of 30 m/min. The initial microstructure before rolling was the mixed one consisting of partially recrystallized and cast structures. Grain refinement was found to occur actively during the warm rolling, producing a very fine grain size of 7 µm after 50% reduction in single pass rolling at 200oC. Yield strength of 204MPa, tensile strength of 330MPa and uniform elongation of 32% have been obtained in warm rolled sheets.

Abstract: In the present study, PIM process has been employed to produce sintered specimens with the various porosities from the initial stage of sintering and to the stage of nearly full density. A series of tensile tests has been conducted on these specimens at room temperature to elucidate the effect of a wide range of porosity. The material used in this study was 17-4 PH stainless steel. Based on the results of tensile tests on PIMed specimens with the various porosity, a new approach to predict the strength and the elongation of sintered materials has been carried out and a new framework combining neck growth model and ideal pore model has been established. By applying this new model to not only the results obtained in this study but also to previously published data, its generality and validity were sufficiently verified.