Seunggab Hong

Hybrid Friction Stir Welding of High-carbon Steel

A high-carbon steel joint, SK5 (0.84 wt% C), was successfully welded by friction stir welding (FSW), both without and with a gas torch, in order to control the cooling rate during welding. After welding, the weld zone comprised gray and black regions, corresponding to microstructural variation: a martensite structure and a duplex structure of ferrite and cementite, respectively The volume fraction of the martensite structure and the Vickers hardness in the welds were decreased with the using of the gas torch, which was related with the lower cooling rate.

Effect of Restraint Stress on the Precipitation Behavior and Thermal Fatigue Properties of Simulated Weld Heat Affected Zone in Ferritic Stainless Steel

Abstract Thermal fatigue life of the automobile exhaust manifold is directly affected by the restraint force according to the structure of exhaust system and bead shape of the welded joints. In the present study, the microstructural changes and precipitation behavior during thermal fatigue cycle of the 18wt% Cr ferritic stainless steel weld heat affected zone (HAZ) considering restraint stress were investigated. The simulation of weld HAZ and thermal fatigue test were carried out using a metal thermal cycle simulator under complete constraint force in the static jig. The change of the restraint stress on the weld HAZ was simulated by changing the shape of notch in the specimen considering the stress concentration factor. Thermal fatigue properties of the weld HAZ were deteriorated during cyclic heating and cooling in the temperature range of 200to 900due to the decrease of Nb content in solid solution and coarsening of MX type precipitates, laves phase, M 6 C with coarsening of grain and softening of the matrix. As the restraint stress on the specimen increased, the thermal fatigue life was decreased by dynamic precipitation and rapid coarsening of the precipitates.Key Words : Thermal fatigue, Ferritic stainless steel, Heat affected zone, Precipitation

Liquation behavior in the weld HAZ of high Si nodular iron

In the present study, liquation behavior including the constitutional liquation phenomena in the weld heataffected zone (HAZ) of nodular cast iron with 4% Si was investigated. The thermal cycle of the weld HAZ was simulated using a Gleeble simulator at various peak temperatures and a fast heating rate of 1000 °C/sec was selected to observe the constitutional liquation phenomenon. Differential thermal analysis has shown that the eutectic constituents of γ + Laves and γ + Fe3C begin to melt at 1120 °C and 1140 °C, respectively. Meanwhile, constitutional liquation by γ/graphite reaction occurred above 1140 °C, due to carbon enrichment around the graphite nodules.