| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 92 === Abstract
Stress corrosion cracking (SCC) and corrosion fatigue cracking (CFC) behaviors of 22%Cr duplex stainless steel (DSS) were investigated. Slow strain rate testing (SSRT) and fatigue crack growth (FCG) rate measurement were employed in chloride solutions at high temperature to evaluate the SCC and CFC susceptibility. The effects of nitrogen content, phase, and environment in DSS on SCC and CFC behaviors are also discussed in this study. In addition, AISI 316L austenitic stainless steel (SS) and AISI 430 ferritic SS were also used for comparison purpose. The experimental results showed that the tensile strength and ductility of 22%Cr DSS increased with increasing amount of nitrogen content (in range of 0.103 to 0.195 wt%). Furthermore, nitrogen also caused increase of the microhardness of both ferrite and austenite. Similarly, nitrogen addition also increased the volume percentage and phase size of austenite in DSSs.
The pitting nucleation potential (Enp) of 22%Cr DSS after potentiodynamic polarization measurement in three chloride solutions with different cations at 100°C varies in following order: NaCl CaCl2 MgCl2. In 40wt% CaCl2 solution at 100°C, selective corrosion of ferritic phase in 22%Cr DSS was observed. Besides, pits in NaCl solutions could initiate in phase or at the boundary between and phase, and propagated in phase. On contrary, in 34wt% MgCl2 solution at 100°C, pits initiated preferentially in phase. However, the Enp increased with increasing nitrogen content in DSSs in all hot chloride solutions employed.
The slow strain rate testing results indicated that 22% Cr DSSs were resistant to SCC in 3.5 wt% NaCl solution at 80°C under two strain rates employed. But, dynamic strain aging occurred in 22% Cr DSSs during SSRT in preceding solution at higher strain rate. In hot 40wt% CaCl2 solution at lower strain rate, 22%Cr DSS with nitrogen content of 0.153wt% was more susceptible to SCC. However, slip dissolution and selective corrosion of ferrite participated in SCC process of 22%Cr DSSs. The ability of cation to promote SCC in chloride solution changes in the following order: Mg2+ Ca2+ Na+, for 22%Cr DSS; Mg2+ Ca2+ Na+, for 316L austenitic SS and 430 ferritic SS.
The FCG rate measurement results indicated that the FCG rate in 3.5wt% NaCl solution at 80°C was found to be the highest for 430 ferritic SS. The high FCG rate in 430 ferritic SS was associated with cleavage-like brittle failure. For 22%Cr DSS, accelerated FCG in hot NaCl solution was mainly attributed to brittle failure of the constituent ferrite phase. Furthermore, the results revealed that nitrogen in the range of 0.103 to 0.195 wt% did not significantly affect the FCG rate of 22% Cr DSS in air and in 3.5 wt% NaCl solution at 80°C, at 1 Hz. However, at a frequency of 0.1 Hz and in 3.5 wt% NaCl solution at 80°C, a higher FCG rate was found in the low-N DSS (0.103 wt% N) as compared with those with high nitrogen content (> 0.153 wt% N). As the nitrogen content increased, the content of ferrite phase, which was susceptible to hydrogen embrittlement, in DSS decreased. As a result, the corrosion fatigue crack propagation rate was reduced.
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