| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系 === 90 === Duplex stainless steels (DSSs) were increasingly used as structure materials in hard wearing environments due to their combination of high strength, resistance to localized corrosion, high temperature oxidation and stress corrosion cracking (SCC), and low price compared to nickel-base alloys. However, the high temperature workability of DSS was not good according to the difference of high temperature strength and hot ductility between α and γ microstructure. Edge crack was easily occurred in the DSS plate during hot rolling process. On the other hand, during the fabrication processes, such as in welding, etc., the austenite-ferrite ratio may be changed together with the precipitation of intermetallic compounds (ex. σ phase, χ phase, Cr23C6, Cr2N, γ2…etc.). All of these factors had the tendency to reduce both the mechanical property and corrosion resistance of the weldment. The effects of alloying elements and microstructure on the mechanical property and corrosion resistance of DSS have been treated in many literatures, in the main, qualitatively. However, systematical evaluation was also seldom. Therefore, the purpose of this thesis was to systematically and quantitatively study the effects of alloying elements and process parameters on the mechanical property and corrosion resistance property of DSS, especially in the Ni and N content, σ and γ, Cr2N, α/β ratio and cooling processes.
The experimental results showed that the edge cracking of DSS increased with increasing the value of CSI (crack sensitivity index). When the value of CSI was higher than 5.5, the duplex stainless steel was susceptible to edge crack. On the variation of alloying composition in steel melting process, all DSSs possessed good corrosion resistance property. The amount of σ phase would grow to 30~40%, when the time of aging treatment was prolonged. Nitrogen could inhibit the precipitation of σ phase, however, nickel had the same effect when the aging time was more than 100 h. Increasing σ phase deteriorated the corrosion resistance of DSS in sulfuric acid solution and ferric sulfate-sulfuric acid solution. The corrosion rate would present a saturation phenomenon when the volume of σ phase exceeded 10%.
In the simulated HAZ of DSSs, the reformed austenite would increase when the cooling time was increased. The grain size of the α phase and the amounts of Cr2N precipitated decreased with increasing γ content. Simultaneously, the extent of Cr2N precipitation was increased with decreasing nitrogen and nickel content and increasing the cooling rate. Pitting corrosion of Cr2N precipitates and selective dissolution of ferrite phases could assist crack initiation and induce SCC in the HAZ (heat-affected zone) of DSSs. IGSCC (intergranular stress corrosion cracking) could be found in the HAZ when GBA (grain boundary austenite) was present. The transformation from IGSCC to TGSCC (transgranular stress corrosion cracking) occurred as GBA was gradually replaced by WA (Widmanstatten austenite), IGA (intergranular austenite) or PTA (partially-transformed austenite). The presence of WA, IGA and PTA exhibited beneficial effect on SCC resistance by deviating the crack propagation path. Finally, the DSS treated by high temperature (900℃) ACC (accelerated cooling control) process has already gotten the similar corrosion resistance and strength as compared to those DSS achieved by traditional solution treatment.
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