Masao Ibaraki1, Taira Okamoto1 and Hiroshi Matsumoto1
1The Institute of Scientific and Industrial Research, Osaka University, Osaka
A study was performed to make clear the influences of the freezing condition and the carbon content on the structure of metastable iron-carbon alloys frozen unidirectionally in a mold with a copper chill block. The carbon content of these alloys was within a range from 2.4% to 4.7%. The results obtained are as follows: (1) The width between the stalks of cellular dendrite of primary austenite in the hypo-eutectic alloys is inversely proportional to the square root of the cooling rate at the beginning of freezing of primary austenite and is independent of the carbon content. (2) The dendrite arm-spacing of primary austenite is inversely proportional to the fourth root of the cooling rate at the beginning of freezing of primary austenite and to the square of the carbon content. (3) The colony size of ledeburite is inversely proportional to the square root of the cooling rate immediately after the freezing of ledeburite. (4) The minimum lamellar spacing of ledeburite is inversely proportional to the square root of the freezing rate of ledeburite and is larger than that of ledeburite grown in a steady state. (5) The growth mode of ledeburite which is one of lamellar eutectics differs from that of ordinary lamellar eutectics, in which the lamellae tend to grow normal to the developing cellular solid/liquid interface. The lamellae in ledeburite grow straight without changing in the growth direction even for the cellular interface. In such a growth mode, the lamellar structure becomes unstable at the locally undercooled region and the branching of rodlike austenite from the lamellae must occur at the position near the colony boundaries.
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