Over-saturated oxygen in japanese iron nails of "wakugi" for wooden structure

Abstract

Ancient Japanese low-carbon steel, called "Hocho-tetsu" or "Wari-tetsu", was made in "Okaji" process by decarburizing pig iron named "Zuku" produced in "Tatara" process. It is known that the low-carbon steel had higher corrosion-resistance and was much easier to forge-and-weld than modern steel. Japanese iron nails, called "Wakugi", were made from "Hocho-tetsu" and had been used in shrines and temples until the Meiji period. The low carbon steel tends rapidly to make thin film of magnetite, called "Kurosabi", on the surface to protect against corrosion under wet atmosphere or heating. The magnetite film is produced from the reaction of iron and oxygen. The oxygen and carbon concentrations in the iron matrix of ferrite in "Wakugi" were measured using EPMA to be about 0.15 to 0.38 mass% and 0.02 mass%, respectively. The oxygen concentration is over-saturated from the oxygen solubility of α Fe and γ Fe. The over-saturated concentration of oxygen in "Wakugi" was caused from "Okaji" process without deoxidation of steel.