| Summary: | 碩士 === 大同大學 === 材料工程研究所 === 88 === Duplex stainless steel (DSS) is with a combination of better mechanical strength and pitting corrosion resistance than the single-phase austenite system, but the precipitated brittle phase as α phase formed during the cooling from welding procedure will rapidly decrease its mechanical property and corrosion resistance. In order to avoid the precipitation of secondary phases in the heat affected zone and welded pool, this study is going to study the vacuum brazing of the duplex stainless steel UNS S31803 (2205) and UNS S31200 (SuperDux64). There are two duplex stainless steel, and three fillers, copper foil, BNi-3 and BNi-7 Ni-based alloy fillers, used in this study to evaluate the effect of cooling rate and filler on the mechanical property and corrosion resistance. From the experimental results, the precipitation of α phase is obviously affected by the cooling rate. In air quench gave a cooling rate of 0.85℃/s, the precipitation of σ phase can be inhabited, for those studies duplex stainless steels; However, the precipitation of σ phase in UNS S31803 was strongly dependent upon the cooling rate. According to the results of lap tests; the strength of vacuum brazed joint using the copper foil as filler is the largest among those tests. Under the quenched state, the weight loss of UNS S31803(2205) DSS dipped in 6%FeCl3 solution at 450℃ for 24 hours is better than that of UNS S31200 (SuperDux64). When the cooling rate was lowered to 0.067℃/s, DSS was raised and then the corrosion resistance of UNS S31803 (2205) was decreases. Owing to the difference between the filler composition and duplex stainless steel, the preference corrosion was occurred at the interface after brazed joints were soaked in 6% FeCl3 solution for 24 hours.
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