435 片状黒鉛鋳鉄の靭性に及ぼす残留オーステナイト、黒鉛周囲のフェライトの影響

概要

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Three types of heat treated gray cast iron are prepared; 1st, AGI is made by austempering at 653K after heating at austenitizing temperature 1143K. AGI contains retained austenite (γ_R) about 35%. 2nd, Subzero treatment is used after austempering (AGI-SZ), that the specimen is remained about 5% of γ_R in the bainite matrix. 3rd, α-AGI is made by the austempering after slow cooling from austenitizing temperature to the range of the ferrite and the austenite coexistence temperature. In this process, the ferrite thickness around the graphite is about 15μm, and the matrix is the bainite structure. A high elongation on AGI is shown by stress induced transformation of γ_R. But, γ_R is not effective in improving Charpy impact strength. The precipitated ferrite around the graphite prevents the propagation of cracks and the improvement for impact strength. On the bending test, the acoustic emission (AE) counts are observed at high amplitude, if the ferrite does not exist around graphite. The matrix containing the ferrite layer around the graphite. an AE counts are less than that of the one without ferrite. As the results, the ferrite around the graphite is effective in absorbing the energy of a fracture.
2003-03-13