Columnar Ferrite Structure in Cast Iron Formed by Decarburization of White Cast Iron

摘要

Cast iron objects such as axes and other artifacts excavated from ancient sites generally have a layered structure with columnar ferrite on the outside, changing to pearlite and ledeburite with increasing depth, together with a minor fraction of dispersed graphite; this structure is the result of the heat treatment used in the decarburization process. The formation of the columnar ferrite structure has previously been explained by assuming that cast iron was produced using a metal mold and then decarburized.In this study, in order to clarify this issue, decarburization experiments were performed on white cast iron. Part of a white cast iron block, containing the surface that had been in contact with the mold during casting, in addition to a cut surface, was heated to 1273 K, held at that temperature for 96 h, and then slowly cooled to 873 K at 67 K/h (for 6h). It was found that columnar ferrite crystals grew in the direction perpendicular to the surface regardless of the solidification direction. The ferrite structure did not have a uniform crystal orientation, similar to the case for partially decarburized ancient cast iron. Therefore, this clarified that the issue was a misunderstanding without materials scientific grounds. In addition, when white cast iron was decarburized at 1123 K for 48 h and cooled to 873 K at 42 K/h, the overall thickness of the decarburized layers was close to that for ancient artifacts. Therefore, it is likely that in ancient times, the same type of cast iron and heat treatment conditions were used.