| Summary: | 碩士 === 國立海洋大學 === 機械與輪機工程學系 === 91 === Abstract
Due to their good mechanical and corrosion properties, duplex stainless steels are widely applied in petrochemical industry, chemical plants, marine environment etc. The strain amplitude and strain ratio effect upon low cycle fatigue are investigated in two kinds of duplex stainless steels. The results indicate that the microstructure of the FZ consists of large ferrite（δ）grain, grain boundary austenite（GBA）and intragranular austenite（IGA）structure. The tensile strengths of the two received duplex stainless steel and their weldments increase with increasing strain rate. However, their elongations decrease with increasing strain rate. The behavior of these duplex stainless steels and their weldments at all strain level exhibits appreciable hardening during the first few cycles, followed by softening with continued cycling. The fatigue life of SAF 2205 duplex stainless steel is superior to that of SAF 2507 duplex stainless steel. The GTAW welds exhibit poor fatigue strength and life as compared to the received metal. The fatigue strength of duplex stainless steel is sensitive to mean stress change at low strain amplitudes. The main fatigue crack of the weldments occurs at the base metal. The crack path propagates along the interphase boundaries, although most of cracks are initiated at the ferrite boundaries. In the aspect of corrosion resistance, SAF 2507 received metal is superior to SAF 2205 and AISI 304, but its weldment is sensitive to chloride ion in 3.5wt.% NaCl solution. SAF 2205 received metal and weldment are crucified easily in hydrochloric acid solution. The location where pitting exists has lower Cr and Mo content The pits are initiated at the ferrite-austenite grain boundaries, then propagate into the ferrite, and finally invade austenite.
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