The purpose of this study is to estimate temperature distribution in the vicinity of weld interface during a friction welding process involving an upset process. On the base of a simple model of friction heat input, a non-steady heat conduction analysis was carried out by finite element method. As a result from a comparison of the estimated temperature distribution with the experimental data, it turned out that the friction heat input model that allowed for the effects of temperature and linear velocity on the friction coefficient was appropriate. This heat input model could simulate adequately the change in friction heat input and temperature distribution in a friction welding process. As a result, the relationship between burn-off length and temperature distribution in upset process has been explained and also the relationship between temperature distribution and width of heat-affected zone has been obtained. This heat input model allows us to estimate temperature distribution in friction welding, from friction pressure, rotation speed and the thermal property of base metal, even where a friction-welding machine does not have a function of torque measurement.

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Weld simulation of heat-affected zone (HAZ) was performed to investigate the mechanism by which austenite affects the toughness of super duplex stainless steel. Thermal cycles of various peak temperatures in the range from 1373 K to 1673 K corresponding to the HAZ were applied to SAF2507 super duplex stainless steel specimens. Charpy impact test was carried out using the specimens after the weld simulation, and the fracture surfaces were observed by SEM using three-dimensionally reconstruction technique. Austenite content decreased with increasing the peak temperature when the peak temperature exceeded 1473 K and the impact value decreased with increasing the peak temperature and decreasing the austenite content. The thermal cycle of the peak temperature of 1673 K corresponding to weld bond region caused decreasing of austenite content which was 22% lower than that of the base metal. The ductile-brittle transition temperature was measured. As a result the temperature increased rapidly in the weld bond region, the peak temperature of which exceeded 1623 K by the grain growth of ferrite matrix occurring subsequently to the completely dissolution of austenite. The morphology of the fracture surfaces after impact testing at 77 K showed cleavage fracture of ferrite. The {100} orientations of cleavage fracture facets were measured using three-dimensional images of the fracture surfaces and the results were visualized as the orientation color maps. The results showed that there were cleavage fractures consisting of a few facets parallel to each other. It was considered that a few facets existed in one ferrite grain. It was concluded that Widmanstätten austenite divided the large fracture into smaller cleavage facets in a ferrite grain and then suppressed the degradation of bond toughness of duplex stainless steel.

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High frequency induction heating was applied to the partial austenitization treatment of the Interior Permanent Magnetic (IPM) rotor using Fe-17.6mass%Cr-2.0 mass%Ni-1.0mass%Al-0.5mass%C alloy with ferromagnetic ferrite (α) and M₂₃C₆ carbide structures, and high saturation magnetization (J) of 1.46T. Twelve bridges at the outer edge of the circular rotors were simultaneously heated by high frequency induction heating due to the increase in eddy current and Joule's heat at the bridges. The heat-treated bridges mainly consisted of paramagnetic retained austenite (γR) and a small amount of ferromagnetic martensite (α') phases with Vickers hardness number (H_v) of 274-309. The retained austenite fraction, X_γR, at the bridges was estimated to be 0.70-0.80 from the relation among austenitization temperature, H_v and X_γR. In addition, the J value at the bridges was estimated to be 0.16-0.27T from the relation between X_γR and J values: J = 0.72X_γR² -2.19X_γR+1.45. This estimated J value was almost consistent with the experimental value of 0.20T.

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The present work deals with a preferential grain growth process in a localized region utilizing local heating method in order to fabricate some unique microstructures different from those fabricated in the homogeneous way of microstructure evolution. A Monte Carlo simulation of grain growth under a heterogeneous temperature gradient, i.e. spot heating, was performed. Steep temperature gradient brought about a preferential grain growth in the higher temperature region, showing that the local heating was effective for the control of grain structure of polycrystalline materials. Such type of preferential grain growth became less significant under the mild temperature gradient. Local heating of pure copper foil with 0.2mm in thickness utilizing laser beam was performed by changing the irradiation conditions. In the case of 200W for laser power and 18mm/s for sweep velocity, some grains were observed to have larger grain sizes than their surrounding grains, suggesting a possibility of preferential grain growth in the localized region.

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