Decarburizing Techniques of Cast Iron in Ancient and Pre-modern Times in View of Carbonic Diffusion

Decarburizing techniques of cast iron in old times were studied in view of reactive diffusion to clarify their mechanism. An ax excavated from remains of the Yayoi era shows decarburized microstructure of white cast iron. The time and temperature of decarburization calculated on the basis of reactive diffusion is considered reasonable for the era. Decarburization at solid-liquid coexisting temperature is regarded as a phenomenon of reactive diffusion which forms a solid layer. The rate of decarburization at this temperature, therefore remains low. Paddling of molten iron, which fractures the solid layer to replace it with fresh molten iron, seems to be an excellent technique for markedly increasing the decarburizing rate. Paddle Process of refining invented in England, as well as the refining process using reverberatory furnace to cast canons at the end of the Edo era in Japan, are considered to be advanced decarburizing processes by means of paddling. Sagé Process of refining in pre-modern times in Japan seems to be a decarburizing process which is accelerated by the included gas bursts at the front of the decarburized layer.