Papers by Author: Young Min Kim

Abstract: In this study, we analyzed weld cracking in Ni-based superalloys. Alloys with about 64 wt.% Ni and 31 wt.% Ni were used for the weld metal analysis. Phase transition predicted by thermodynamic calculation were compared with experimental results. The weld metal’s microstructure was analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) mapping, and its mechanical properties tested. From EDX mapping, it was established that the alloying elements (Nb and Mo) were segregated around the weld crack as a result of the formation of a niobium carbide (NbC).

Abstract: Effects of acicular ferrite and retained austenite on the mechanical properties of bainite-base steels were investigated. Various morphology and volume fraction of constituent phases have been obtained by control of hot rolling conditions and alloy compositions. It has been shown that the steels containing retained austenite have better combinations of strength and ductility than the ones with no retained austenite. However, there is no noticeable change in DBTT by the incorporation of retained austenite in the microstructure since retained austenite exists as fine particles. On the other hand, DBTT of the steels are largely affected by the presence of acicular ferrite in the microstructure. EBSD analyses of fractured Charpy specimens show that cracks are deflected within the morphological packet of acicular ferrite, indicating its role in reducing the effective grain size of the steels.

Abstract: AZ31 and experimental ZMA611 alloys were strip cast into 2 mm thick strips. The as-cast AZ31 alloy strip consists of columnar dendrites. On the other hand, as-cast ZMA611 alloy strip shows equiaxed dendritic structure through the thickness of strip. These alloys were subjected to various thermomechanical treatments and their tensile properties were evaluated. Strip cast AZ31 alloy in H24 condition has equivalent yield and tensile strengths with similar ductility compared to commercial ingot cast AZ31-H24 alloy, indicating that strip casting is a viable process for the fabrication of Mg alloy strips. The ZMA611 alloy has a large volume fraction of fine dispersoid particles in the microstructure, resulting from the beneficial effect of strip casting on microstructural refinement. It has been shown that the ZMA611 alloy has superior tensile properties compared to commercial ingot cast AZ31-H24 alloy, suggesting the possibility of the development of new wrought Mg alloy sheets by strip casting.

Abstract: The yield ratio of various HSLA steels has been correlated with the materials constants of Swift equation. It has been shown that the materials constants, b and N, of Swift equation can be related to microstructural features such as the dislocation density and volume fraction of constituent phases. In particular, the constant b can be expressed as a function of volume fraction of constituent phases. It has also been shown that the yield ratio has a linear relationship with ln(b/N2). Since the microstructural features often have opposing effects on the values of b and N, careful control of microstructure is necessary to optimize the yield ratio and other properties. The possible way of decreasing the yield ratio without sacrificing other properties of HSLA steels is suggested based on the relationship between yield ratio and the materials constants of Swift equations.