Papers by Author: Jong Bong Lee

Abstract: The grain growth behavior and mechanical properties in the friction stir weld zone after post weld heat treatment (PWHT) have been investigated. As PWHT temperature increased, a normal grain growth of as-welded equaxied grains ceased and abnormally grown grains with elongated shape coarsened. Huge elongated grains changed into smaller equaxied grains at 500°C. In case of lower heat input condition, abnormal grain growth initiated faster due to smaller initial grain size. The weld zone with bigger initial grains had advantages to maintain the thermal stability at high temperature. The hardness near the weld zone was almost recovered to the 95% of the unaffected base metal at 500 °C and the weld zone under lower heat input condition resulted in the homogeneous recovery through the whole weld zone.

Abstract: The effects of nitrogen content on weld metal impact toughness in submerged arc welding were investigated and interpreted in terms of microstructural changes and solid solution hardening. The weld metal impact toughness in as-welded condition decreased with increasing nitrogen content from 110 to 200 ppm. The weld metal microstructure changed with increasing nitrogen content; ferrite with second phase increased at the expense of tough acicular ferrite. In addition to microstructural changes, the microhardness of acicular ferrite increased gradually with the nitrogen content. Therefore, the loss of impact toughness can be attributed to a combination of the effects of microstructural changes and solid solution hardening.

Abstract: The effect of welding conditions on defects, microstructure, mechanical properties and formability of CO₂welded 800MPa grade TRIP steel was investigated. Bead-on-plate welding was carried out under the various welding speed in 6kW with a shield gas of Helium. Extensive porosity was observed in the joints produced at a 4m/min of welding speed but was not observed in the joints when a welding speed greater than 6m/min was used. The maximum hardness was obtained at the weld metal and the HAZ near the weld metal and was the same regardless of the welding speed. The HAZ hardness near the base metal was reduced due to retained ferrite but was higher than that of the base metal. In a parallel tensile test to weld axis, the strength of the joints was higher than that of the base metal. Elongation was found to be lower than that of the raw material because a crack was initiated in the bead at the strength levels corresponding to the tensile strength of the base metal and was propagated perpendicular to tensile direction. The formability of joints was found to be 63% that of the raw material.