Development of high strength and toughness austemper ductile cast iron and application to its structural part materials

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This research aims at the development of atmospheric resistance austemper ductile cast iron which consider use in high strength, toughness, and the corrosion environment and practical use.The following experiment was carried out. The research clarified the best heat treatment conditions by austemper treatment of the Cu-Ni alloyed cast iron. The austemper treatment conditions and the elucidation of the mechanism evaluated strength and the impact characteristic by the instrumented Charpy impact test and the tensile test. Furthermore, the destruction behavior and the fatigue strength evaluation of the cast iron were experimented with the fatigue flaw growth resistance test. The evaluation of corrosion resistance was executed by in the ferric sulfate solutions and the atmospheric exposure examination.As a result of this research, the Ni-Cu alloyed cast iron obtained a remarkable impact characteristic with heat treatment. Strength and toughness reached absorbed energy of 20J by the austempering of 623K.As the crack initiation energy increases, the fracture load reached a high value. Crack propagation advanced gradual after reaching the maximum load value. An excellent high ductility and a high strength material were obtained by austemper treatment. Stress intensity factor (DELTA K) of the austempered cast iron enlarged the crack propagation resistance and showed the growth of a steady crack. Also, threshold stress intensity factor (DELTA Kth) showed a high value with austemper treatment. However, DELTA Kth was not able to be improved with the alloy material.Corrosion resistance of the Cu-Ni alloy cast iron have increased to two times that of non-alloy material in outdoor exposure test and in the ferric sulfate solutions.