| Summary: | 碩士 === 國立臺灣海洋大學 === 機械與輪機工程學系 === 93 === Abstract
The purpose of this thesis duplex stainless steels have been wisely used in the oil, chemical and nuclear industries due to their high strength, good weldability, and high resistance to stress corrosion and pitting. The superior properties of these duplex stainless steels come primarily from the approximately equivalent amounts of austenite (γ) and δ–ferrite. However, this equivalent condition is destroyed by welding. Pulsing of the welding current is one approach to keeping the proportion of α and γ phase in the fusion zone. The environmental temperature significantly affects the fusion zone microstructure in pulsed GTAW weldments with the same heat input. Lower environmental temperatures lead to higher γ content. Also, the pulsed welding has the benefit of the depth/width ratio increase and a smaller heat affected zone.
The strain amplitude and strain ratio effect upon low cycle fatigue are investigated in 2205 duplex stainless steels. The results indicate that the microstructure of the FZ consists of large ferrite（δ）grains, grain boundary austenite（GBA）and intragranular austenite（IGA）structures. The trend of peak stress versus number of cycles for duplex stainless steel shows cyclic hardening at the initial few cycles, followed by cyclic softening to form a mixed mode of low-cycle fatigue. A secondary hardening is found only at the relatively high strain amplitude of 1.5% with either strain ratio R = - 1 or R = - 0.2.
Dislocation structures in fatigued DSS 2205 are classified into two types in strain ranges from 9×10−3 to 1.5×10−2, which correspond to the five regions in the cyclic stress–strain curve. Vein structures consisting of loop patches were observed at low strain amplitudes. These structures were mixed with labyrinth structures at intermediate strain amplitudes. At higher amplitudes, there is the formation of cellular structures, which became increasingly equiaxed and smaller with increases of strain amplitude and fatigue cycles.
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 changes 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 the cracks are initiated at the ferrite boundaries.
Dislocation structures in fatigued DSS 2205 are classified into two types in strain range from 9×10−3 to 1.5×10−2, which correspond to the five regions in the cyclic stress–strain curve. Vein structures consisting of loop patches were observed at low strain amplitudes. These structures were mixed with labyrinth structures at intermediate strain amplitudes. At higher amplitudes cellular structures are formed which became increasingly equiaxed and smaller with increase of strain amplitude and fatigue cycles.
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.
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