Papers by Author: Joong Hwan Jun

Abstract: Thermal stability of  grains and tensile ductilities at room and elevated temperatures were investigated and compared for Mg-3%Zn-0.4%Zr and Mg-3%Zn-0.4%Zr-1%Bi alloys in hot-rolled state. The Bi-added alloy showed slightly finer-grained microstructure with enhanced thermal stability, which is closely associated with fine Mg-Bi compounds acting as obstacles for the migration of grain boundaries. The Mg-3%Zn-0.4%Zr-1%Bi alloy exhibited better tensile strength at room temperature and tensile ductilities at elevated temperature. Finer and more homogeneous grain structure with higher thermal stability would be responsible for the enhanced tensile properties in the Bi-added alloy.

Abstract: The surface cracking behaviors of Cu-bearing steels were studied under similar conditions of the direct hot charging processes. Several specimens with various Cu contents from 0.11 to 0.38% were prepared, the oxidation tests were performed at the temperature of 1100, 1150 and 1200°C, and the hot compression tests were conducted at 1050°C using the Gleeble 3500 in order to examine the Cu induced surface cracks. The surface cracks increased gradually as the Cu-rich phase above the critical level increased. The critical level of Cu-rich phase for the specimens of high Cu contents (0.38%) reaches with a thin scale layer. However, the specimens with lower Cu content (0.13%) the critical level of Cu-rich phase occurred with a thick scale layer. The oxidation potentials of steel were affected by temperature, time, atmosphere and other elements. The oxidation rates of Cu bearing steels decreased with increasing Cu contents. It is suggested that the oxidized scale thickness is the one of important factors inducing the surface cracks by Cu contained steels in direct hot charging process.