カーボニル焼結鉄のAr₃変態で現われるフェライト組織

抄録

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 ppm<C<150 ppm), and fine and equiaxed for higher contents (>150 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.