| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 96 === This research mainly focuses on the effect on corrosion resistance and stress corrosion resistance of Inconel 690 Alloy after Inert Gas Tungsten Arc Welding（GATW） and Laser Beam Welding（LBW） process. The effect on both characteristic caused by variables such as measurement of welding heat cycle and sensitization was discussed. Modified Huey’s test and electrochemical test were used in experiment in order to probe into the difference of corrosion resistance. Meanwhile, slow strain rate test（SSRT） was used to evaluate stress corrosion cracking（SCC） resistance of weldment by GTAW and LBW processes.
The temperature measurements showed that the heating rate (3309˚C/sec) and cooling rate (90~157˚C/sec) of the weldment in LBW process were much higher than in GTAW process. During cooling procedure the LBW process, it only took 1.5~1.9 seconds for the LBW weldment to pass through the sensitization range which is much shorter than the 19 seconds needed in GTAW process. The high cooling rate is favorable for depressing Cr-carbides precipitation along the grain boundary which improves the corrosion resistance of weldments. The corrosion resistances of fusion zone and weld decay zone have all decreased after GTAW and LBW process. LBW weldment will cause less corrosion in fusion zone and intergranular corrosion cracking（IGC） in weld decay zone during the early stage of corrosion, the state of corrosion will not worsen in time; On the contrary, the state of corrosion in fusion zone and weld decay zone will worsen as the time duration increases. Polarization of weldments by GTAW and LBW processes both of their current density for passivation increased. The effect caused by GTAW process is more obvious than which caused by LBW. The result of double-loop electrochemical potentiokinetic reactivation（DL-EPR）showed that IGC has occurred in the weld decay zone of GTAW weldment. The result of SSRT showed that the tensile strength in fusion zone of LBW weldment is higher than base metal, and the IGC in weld decay zone did not crack because of the stretch；IGC in fusion zone of GTAW weldment leads to early fracture, and the IGC in weld decay zone clearly stretched and cause the material to crack. Intergranular stress corrosion cracking（IGSCC） in this section is still possible.
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