普通鋳鉄の焼入れ焼もどしによる機械的性質の改善

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Recently the superior natures have been required in mechanical properties, especially strength and toughness, as machine members.　Cast iron is one of the casting materials which have been popularly used as engineering materials, on account of its good castability. But generally speaking, cast iron is weak and brittle by the existance of flake graphite in it.　In this paper, applying quench-tempering to a pearlitic cast iron, the structual changes caused in it were first observed by an optical microscope. And the changes of tensile strength and hardness by quench-tempering were examined. Then aspects of fracturing of the quench-tempered cast iron were precisely investigated in bending, tension and rotating bending fatigue fracture,and the mechanism of improving mechanical properties by such heat treatment was considered. And following results were obtained:　(1) Heated at 900℃ for more 1 hour, flake graphites grow remarkably and simultaneously their edges grow round and their surface smoothes. Even if the heating time at the quenching temperature becomes longer, noticeable change can not be found in the matrix among large flake graphites. A thin layer of ferrite wraps each graphite.　(2) The highest tensile strength is obtained by heating at 900℃ for 3 hours, oil quenching, and tempering at 400-500℃. The specimen treated above is 1.45 times strong as the material as cast. This is caused chiefly by the change of state of graphite shown above in (1) and the change of matrix itseif.　(3) From the results it is explained that when quench-tempered cast iron is statically stressed, the chance of initiation and propagation of crack is reduced by the state of graphite and the properties of matrix. But when subjected to the repeated stress, the propagation of crack is affected by the structure of matrix rather than graphite.
1973-01-20

普通鋳鉄の焼入れ焼もどしによる機械的性質の改善

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