| Summary: | 碩士 === 臺灣大學 === 材料科學與工程學研究所 === 95 === The research aims for the stress corrosion cracking characteristics of SAE 8620 alloy steel in chloride aqueous. The abilities against the corrosive environment are observed in the different parameters of heat treatment. The experiment method includes the slow strain rate test and constant K tension test for the crack propagation. In addition, it is also attested from the microstructure and electrochemical properties. From the results of slow strain rate test, in the room temperature, the yield strengths decrease with the increasing concentration of chloride ions, but the decreases in ductility are not conspicuous. In general, the mechanical force is predominant factor that causes fracture. The 80℃ chloride aqueous principally leads to the decrease in ductility, but no matter what condition of tension test, the fracture graphs appear the ductile characteristics.
In the constant K tension test, the same length of pre-crack is established, and then the crack propagation is measured. In the result, the crack propagation is the most rapid in the specimen quenched and then tempered in 300℃. The crack propagation in the specimens quenched and tempered in 400℃ and 500℃ is secondary, and there is almost no crack propagation in the normalized specimen.
Overall, in the heat treatment of quenched and tempered in 400℃, the SAE 8620 alloy steel maintains high yield strength and ultimate tension strength, and the crack propagation is relatively slower than the heat treatment of quenched and tempered in 300℃. Besides, there are comparatively better electrochemical properties in the heat treatment of quenched and tempered in 400℃.
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