Cast duplex stainless steels of CF8M and CF8 are used in major components because of their superior characteristics, such as corrosion resistance, weldability and so on. But, these stainless steels are known to have tendency of thermal aging embrittlement after long term service. Therefore, mechanical properties and metallurgical structure were investigated using materials aged at 290-400°C up to 30,000 hours. As the results show, effects of thermal aging on mechanical properties and metallurgical behavior were identified. In addition, prediction method for Charpy absorbed energy and fracture toughness was established. The following results have been obtained. (l)It was recognized that Charpy absorbed Energy and fracture toughness tend to decrease and the tensile strength tend to increase with the increasing aging time. (2)It was confirmed that thermal aging embrittlement was caused by the phase separation in ferrite from the test results of APFIM. (3)In the degradation prediction model development the prediction model was applied to the material test data, including materials aged for 30,000 hours. As the results, the degradation prediction formulas for CVRT, CVHT, J{sub}{IC} and J{sub}6 were obtained. The toughness of cast duplex stainless steels during service could be estimated from chemical composition using this method.
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