Supersaturation of carbon in austenite during carburization by CO gas

Fine particles of electrolytic iron in austenite state absorbed carbon from CO gas to be in supersaturation. The carbon content in austenite was estimated from the spacing of lattice planes of iron, which was measured by a high temperature X-ray diffractometer. The supersaturation was depended on the flow rate of CO gas and temperature. The solute supersaturation ratio of carbon was from 1.2 to 1.8 at the temperature between 1 452 and 1 599 K. Iron particles reduced from hematite by H₂ gas behaved as same as electrolytic iron particles. However, the iron particles reduced from hematite by CO gas were not in supersaturation of carbon and melted at the solubility of carbon in austenite state. The supersaturation can be explained by the decline of heterogeneous nucleation sites per unit volume. The spacing of lattice planes of reduced iron by CO gas abnormally increased and then decreased during heating. The reduced iron particles melted at the solubility of carbon in austenite and did not become supersaturation.