Papers by Author: Nam Hyun Kang

Abstract: The separator is one of the most important parts in PEM fuel cells. Stainless steels are widely used as separator for its good mechanical properties and mass production. However, for a good chemical compatibility, stainless steels need to have high chromium content or surface treatment, which makes separator high cost. Low cost of separator is important for commercial use. In this study, conventional low carbon steel is used as base metal of separator. Low carbon steel is low at cost, but has poor chemical properties for separator. For a good corrosion resistance, low carbon steel needs to be surface treated. To make a uniform surface treated layer on low carbon steel, chromium is conventionally electroplated on the steel and thermally nitrided. Surface treated low carbon steel is investigated using microstructure and element analysis tools. Interfacial contact resistance and polarization test is applied for the properties of fuel cell separator. The results show that chromium nitrided layer uniformly formed on low carbon steel. And the surface treated steel showed a good corrosion resistance as a separator.

Abstract: The demand to replace Fe-V steel with Fe-Nb steel is evolving because of high costs of raw vanadium material. For the mass production of Fe-Nb steel, the most critical barrier is a poor impact toughness comparing with that of Fe-V steel. This study covers a microstructural investigation for ferrite grain size to explain the strength and toughness results as a function of V and Nb contents. The steel samples were made of three different compositions, i.e., Fe-V steel (Fe-0.05V-0.001Nb), Fe-V-Nb steel (Fe-0.014V-0.03Nb), and Fe-Nb steel (Fe-0.003V-0.033Nb). Rolling temperature to initiate was 1150°C for the all experiments. However, rolling temperature to finish was set differently for two conditions; 950°C and 860°C. The rolling to 860°C decreased the grain size for the ferrite phase and increased the impact toughness rather than the case of 950°C. The Fe-V-Nb steel exhibited similar value of the impact toughness with that for the Fe-V steel because of the low rolling temperature to finish, i.e., 860°C. The whole replace of V with Nb decreased the impact toughness significantly, however some extent of V content remained with Nb content showed the comparable toughness to the Fe-V steel by optimizing the controlled rolling process.

Abstract: For laser-GMA hybrid welds, the relationship between the process parameters and bead shapes was investigated by using the experimental design and the non-linear regression. The weld beads were fitted by two quadratic functions. By the statistical analysis, the primary bead was mainly affected by GMA welding parameters and the root bead was mainly by laser welding parameters. The root bead shaped deeper penetration and shallower width, as the laser power increased for the constant arc power. High laser power resulted in the martensite phase, therefore exhibiting the sudden increase of hardness as approached to the fusion line of the root bead.