On the Gas Decarburization of White Pig Iron Used for White Heart Malleable Cast Iron

Abstract

The decarburization of white pig iron such as used for making the white heart malleable cast iron was studied mainly in the exothermic atmospheres of CO-CO₂-H₂-H₂O system converted from propane. The process of decarburization of white pig iron under various heating conditions was considered by comparing the decarburization behavior of the exothermic gas containing hydrogen about 10% converted at air ratio 14 with that of the wet hydrogen containing water vapour 4% and that of the mill scale forming atmospheres of the simple CO-CO₂ system around it. The results obtained were as follows.
In the flow of an exothermic gas, the decarburized amount of white pig iron was increased through a rising of propane-air ratio for producing the gas up to 10, but it was nearly independent of the variation of the air ratio in the range of 10 to 17 due to the concentration difference for carbon diffusion in the austenite phase reaching its maximum (when heated at 950°C for 4hours in the gas cooled at 18°C after converted at 1000°C).
The decarburized weight in each atmosphere was decreased slightly in the order of the flow of wet hydrogen, the flow of the exothermic gas and the mill scale owing to the rising of the concentration difference for carbon diffusion in austenite due to the rising of surface carbon content with carbon potential of the atmospheres. It was observed that the surface structure of the decarburized layer which formed in each atmosphere in the experimental range 750 to 1100°C was uniform single ferrite only, so that the surface carbon content was kept to within the limit of the solid soiubility of carbon in α iron phase.
Free cementite in the white pig tends to be stabilized and its graphitization becomes difficult with an increasing of the hydrogen amount in the decarburizing atmospheres. The rate of dissolution of graphite into unsaturated austenite as caused by decarburizing reaction was relatively small compared with that of cementite, hence the rate of decarburization was lowered with the progress of graphitization.