| Summary: | 博士 === 國立成功大學 === 機械工程學系 === 87 === This study investigates the microstructure, mechanical properties and corrosion behavior of Inconel alloy 690 weldments by different filler metals and flux compositions of electrodes. This study includes two parts. In part one, I-52 and I-82 were used as the filler metals and GTAW was employed. In part two, the filler metal I-52 was coated with flux of 0.1~3.35 wt% Nb and 0.78~3.32 wt% Mn, for use as welding electrodes. SMAW was employed.
The experimental results of part one indicate that I-82 welds have a finer subgrain structure and a smaller dendritic spacing than I-52 welds. Mechanical test results demonstrate that the I-82 weldments have higher tensile strength with rupture occurring in the base metal. In comparison, the I-52 weldments have lower tensile strength, with rupture occurring in the fusion zone. Filler metal I-52 shows a better welding performance, a higher impact toughness and a better IGC resistance than I-82. Furthermore, a gradual increase in the hardness and percentage elongation of the weld is observed as the number of passes during welding increases. In addition, the HAZ has an obvious grain growth with network-like ghost grain boundaries (GGB). The HAZ shows no obvious hardness decrease. The GGB presence may be related to fracture not occurring in the HAZ.
Part two’s experimental weld results indicate primarily dendritic subgrain structure, with all of the tensile specimens rupturing in the fusion zone. A small HAZ with typical coarse grains, subsequently forming GGBs, is present. With increasing Nb, the welds tend to have a finer subgrain structure and smaller dendritic spacing. Nb-rich segregants in the form of small particles occur at interdendritic spaces and provide the sites for microvoid formation by rupture. With increasing Nb, the tensile strength and microhardness of the fusion zone appears to increase slightly, and the ductility decreases significantly. However, corrosion tends to initiate at interdendritic sites, and is worse with increasing Nb. With increasing Nb, the tensile rupture mode changes from slant (ductile) to flat (brittle) fracture.
With increasing Mn, the subgrain structure of the welds shows no obvious difference, the percentage elongation and amount of necking tends to increase, and the tensile rupture mode changes from flat (brittle) to ductile (ductile) fracture. Further, with increasing Mn, specimen tensile strength and the fusion zone hardness increase slightly.
For the same series of weld metals, the SMAW welds reveal finer subgrain structure, smaller dendritic spacing and higher tensile strength than the GTAW welds. However, the IGC resistance of SMAW is worse than GTAW.
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