A macroscopic fracture surface examination was performed on the spheroidal graphite cast irons associated with fracture behavior of tensile specimens. Circumferentially notched round bar specimens with different notch acuity and unnotched round bar specimens were employed to obtain a wide range of stress concentration factor. All specimens were loaded up to break under tensile loading at room temperature. The mechanical test results indicate that the maximum nominal stress required to break the specimen increased slightly with increase of stress concentration factor for the spheroidal graphite cast irons because of low ductility compared with that for the structural steel. In contrast, the fracture strains of tensile specimens decreased markedly with increasing stress concentration factor. While, the macroscopic fracture surface appearance showed a remarkable difference depending upon the matrix structure of the spheroidal graphite cast iron. The area fraction covered by dimple was measured on the fracture surface using a scanning electron microscope for each specimen. For FCD 400 with ferritic matrix, the dominant fracture mechanism is dimpled rupture. FCD 450 with 10％ pearlitic matrix showed a mixed fracture appearance of dimpled rupture and cleavage fracture. Cleavage fracture become prevailing for FCD 500 with 32 ～42 ％ pearlitic matrix. Whereas, all the fractures for FCD 700 with 77 ～83 ％ pearlitic matrix were entirely by cleavage. Hence, it should be emphasized that the fracture behavior of the spheroidal graphite cast irons must be explained from the characteristic relation of dimple area fraction to the fracture properties developed for each cast iron with particular attention to the matrix structure.

The corrosion of steel in anaerobic seawater was examined using a 30 m³ steel tank. The water temperature in the tank was able to imitate the seasonal changes of natural seawater. The sulfides concentration in seawater within the tank was confirmed to change corresponding to seasons in which the maximum concentration was 1.38 mg・L ^─1 . Immersion tests for 10 days showed that the corrosion rate increased with increasing concentration of sulfide up to 1 mg・L ^─1 , and decreased rapidly in higher concentration than 1 mg・L ^─1 . This relation corresponded to the formation of the iron sulfide film. By continuous immersion tests, the corrosion rate was found to decrease with immersion time. However, the depth of the pit was 0.034 mm after 211 days. The rate of pitting corrosion was estimated to be 0.058 mm・y ^─1 , indicating that the corrosion rate is high even in an anaerobic environment. These results indicated that the corrosion behavior can not be understood based on the re sult of time tests.