Abstract
The behavior of carbon deposition on Fe-S alloys containing 0-2.15 mass%S under the conditions of intensive carbon deposition was studied by thermogravimetry and metallography to elucidate the effect of sulfur in iron on carbon deposition in carburizing gas. Excessive sulfur over the solubility limit (0.001 mass%S) in iron at 923 K existed as FeS, which precipitated in the grain and along the grain boundaries. Two types of carbon, laminar carbon and filamentous carbon, was observed to deposit on iron and the 0.03 mass%S alloy. In the alloys containing more than 0.36 mass%S, however, only filamentous carbon deposited preferentially on FeS. Kinetic measurements and metallographical observations showed that sulfur in iron, particularly sulfur as FeS, retards carbon deposition and extends the induction period which is the time to take to cause the significant mass gain. FeS was desulfurized to become porous iron, so that in the 2.15 mass%S alloy where it precipitated three-dimensionally along the grain boundaries, its desulfurizing reaction proceeded for a long time because of the gas diffusion into the inside through porous iron. As a result, the 2.15 mass %S alloy had a very longer induction period and caused mass loss during the period.
