日本金属学会誌

J. Japan Inst. Metals, Vol. 41, No. 9 (1977),
pp. 920-926

Ferritic Structure of Sintered Carbonyl-Iron Obtained by the Ar3 Transformation

Hidenori Kuroki1, Yoshisada Kobayashi2 and Youichi Tokunaga3

1Faculty of Engineering, Kyushu University, Fukuoka. Present address: Faculty of Engineering, Hiroshima University, Hiroshima
2Graduate School, Kyushu University, Fukuoka. Present address: Kumagaya Works, Hitachi Kinzoku Ltd, Kumagaya
3Laboratory of Iron and Steel, Department of Metallurgy, Kyushu University, Fukuoka

Abstract:

The α structures of sintered irons containing 10∼300 ppm of carbon and oxygen, obtained by the Ar3 transformation, were observed by optical microscopy.
Standard samples were compacted from the powder containing 0.15% of carbon and sintered at 1100°C, followed by cooling at a rate of 30°C/sec. They had a density of 7.15±0.02 g/cm3 and contained 10∼20 ppm of carbon and 100∼250 ppm of oxygen. Another powder containing 0.75% of carbon was mixed with the one above mentioned to obtain the samples of higher carbon contents.
The α structures were coarse and columnar for lower carbon-contents (<40 ppm), coarse and equiaxed for medium contents (40 ppm150 ppm).
Taking into considaration a probable thermal-gradient within the sample during cooling and the subsequent nucleation of the α phase at lower-temperature portions, the experimental results could be explained as follows.
For lower carbon-contents, less long-range diffusion might be needed than for higher carbon contents, for the progress of the Ar3 transformation. Therefore, the massive α might grow more rapidly than, for example, the proeutectoid α in mild steel, resulting into coarse columnar structures.
For higher carbon-contents, envelopes, rich in carbon, should be produced ahead of the growing α grains, because of the difference in solubility of carbon between the γ and the α phases, as already proposed by Rathenau and de Jong. Then, according to the theory of solidification from liquids, the so-called constitutional super-cooling should be produced in the γ region close to the α / γ interface, leading to the nucleation of the α phase. This nucleation could result into fairly equiaxed structures. The above described mechanism might be applied to many irons of commercial purities with no columnar structure.


(Received 1977/03/14)

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