Over-saturated Oxygen in Japanese Iron Nails of Wakugi for Wooden Structure

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1 , pp Over-saturated Oxygen in Japanese Iron Nails of Wakugi for Wooden Structure Yasuko FURUNUSHI 1) * and Kazuhiro NAGATA 2) 1) Graduate School, Tokyo University of the Arts, 12-8 Ueno Park, Taito-ku, Tokyo, Japan. 2) Tokyo University of the Arts, 12-8 Ueno Park, Taito-ku, Tokyo, Japan. (Received on November 30, 2013; accepted on March 3, 2014) 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. KEY WORDS: Hocho-tetsu; Wari-tetsu; Wakugi; Okaji; Tatara; nail; refining. 1. Introduction Ancient Japanese low-carbon steel is called Hocho -tetsu or Wari-tetsu with about 0.1 mass% of carbon. Japanese iron nails, called Wakugi, had been used in shrines and temples until Meiji period and manufactured from Hocho-tetsu. Hocho-tetsu was made by decarburizing pig iron of Zuku in Okaji process. Zuku was produced in Tatara process. A remarkable characteristic of Hocho-tetsu is high corrosion-resistance caused by the formation of magnetite thin film on the surface of steel. Although they are often heavily corroded on the surface, their cores contain sound metallic iron that is free of oxide scaling. Igaki 1) carried out an enhanced corrosion test of the anodic polarization measurement in a natural aqueous solution using iron artifacts of various ages. Modern steel was poor in terms of corrosion resistance and showed heavy fluctuations on the passive current because of lack of stable passive film formation. Most old iron artifacts in the Kamakura period showed high corrosion-resistance and kept metallic luster under atmosphere for several months. Sugimoto et al. 2) found that the surface covered with uniform thin layer of Fe 3O 4 acted as a protective film to prevent from further oxidizing. W.E.O Grady 3) investigated the relationship between the corrosion resistance and the crystal structure, and showed that the structure of ferrite phase exhibited high corrosion resistance. Furunushi 4) focused on the oxide film formed on * Corresponding author: DZS03530@nifty.com DOI: the surface of a nail that was manufactured by blacksmith. Using a transmission electron microscope, the fine polycrystals of FeO were found in the unit size about 10 nm at the interface between the steel matrix and the oxide film of scale. Hocho-tetsu was much easier to forge-and-weld than modern steel. Nagata et al. 5) found that the interface of steel blocks are melted by the oxidation heat of steel at the interface in the time of forge-and-welding. Then, white sparks of Wakibana appear in the flame of charcoal burning in blacksmith s furnace. The interface of steel blocks is filled with molten FeO and generates CO gas bubbles. The growth of hemisphere gas bubble hits molten steel at the interface to produce small iron particles. The iron particles are oxidized in air to make Wakibana. The molten steel at the interface of blocks is near in equilibrium with molten FeO in about C. Nagata et al. 6) realized that in the final stage of Okaji process, steel block was oxidized by air, resulting in heating and melting itself by oxidation heat, leading to decarburization less than 0.1 mass%c at about C. In this state, molten steel is covered with molten FeO and near in equilibrium. After then, the steel block was quickly forged to make steel plates of Hocho-tetsu. Therefore, the oxygen concentration in Hocho-tetsu and forge-and-welded steel could be high. However, the conventional way to analyze oxygen content in Hochotetsu and forge-and-welded steel has been chemical analysis that includes oxygen in non-metallic inclusions as well as iron matrix. In the present work, The oxygen concentration in iron matrix without non-metallic inclusions in 2014 ISIJ 1074

2 Japanese nails has been measured by an electron probe micro-analyzer (EPMA). The effect of high oxygen concentration in Japanese steel on the high corrosion-resistance of Wakugi will be discussed. 2. Experimental Methods 2.1. Specimens The three Wakugi samples of No. 1, 2 and 3 were employed. No. 1 sample was used in the Saidai-ji temple in Nara Prefecture and manufactured in A.D.1300 of the Kamakura period. No. 2 sample was used in the Anumi Shrine in Ehime Prefecture and manufactured in A.D.1835 of the Edo period. The length of No. 1 was 150 mm and that of No. 2 was 170 mm. As shown in Fig. 1, they were formed in long pyramidal bars of steel with rolled head for No. 1 and bent head for No. 2, respectively. Their cross-sectional shape were squares of 6 5 mm for No. 1 and 8 7 mm for No. 2, respectively. Their surfaces were covered with a thin corroded layer of Fe 3O 4, FeOOH, FeO, and Fe 2O 3. The nail of No. 3 was used in the Bicchu-kokubun-ji temple, where the temple was reconstructed between 1821 and The shape was already reported in the previous report. 4) The size was 85 mm in length and mm in cross-section Experimental Procedure The iron nails were cut and mechanically polished. An electron micro-prove analyzer (EPMA) with a LaB 6 electron gun was used to measure oxygen concentration in steel samples. The EPMA can analyze the concentrations of elements in matrix with the area of 1 5 μm in every direction excluding non-metallic inclusions. The microstructures of non-metallic inclusions in the nails were observed using a scanning electron microscope (SEM). The hardness of the nails was measured using a Vickers Hardness(Hv) tester (Hv 300 g). 3. Results 3.1. Hardness and Non-metallic Inclusions of Wakugi The cross-sections of nails are shown Fig. 2 with Vickers hardness values. The gray parts in the figure are non-metallic inclusions that appear along the metal flow. The number of non-metallic inclusion in No. 1 sample is more than No. 2. The microstructures of both samples are single-phase ferrite crystals with carbon concentration of about 0.1 mass% or less. The Vickers hardness of No. 1 sample was in the range from 100 Hv to 135 Hv. The grain size at the 100 Hv area was about 100 μm, and 135 Hv, 20 μm. The Vickers hardness of No. 2 sample was in the range from 87 Hv to 186 Hv. The grain size at the 150 Hv area was under 10 μm. No. 3 sample as a reference was in the range from 115 Hv to 125 Hv. From these results, it is realized that carbon distribution in nails was not uniform and carbon content greatly fluctuates from place to place. Non-metallic inclusions, called Noro, were iron-rich slag with the composition of near fayalite. Figure 3 shows each grain and texture in the nonmetallic inclusions, the chemical compositions of which are shown in Table 1. These grains and textures were composed of FeO (No. 1, No. 3), 4) 2FeO TiO 2 (No. 2, No. 3), and textures, in fine dendrite structure. The grain of FeO consists of about 95 mass%(fe+o), and the grain of 2FeO TiO 2, about 97 mass%(fe+o+ti). The concentrations of other elements are at the almost same level. Fig. 1. Overview of Japanese iron nails. Fig. 2. Cross-sections and Vickers hardness of Japanese nails ISIJ

3 Fig. 3. SEM images of microstructure of the inclusions. Table 1. mass% atomic% Chemical composition of the non-metallic inclusions in the Japanese nails measured by EPMA (mass%). grain texture element No.3 4) No.1 No.2 Ti rich No.1 No.2 No.3 4) Si Mn P S Ti O Fe Al Mg Ca Total Ti O Fe Si O/Fe Concentrations of Oxygen and Other Elements Dissolved in Iron Matrix of Wakugi Table 2 shows the concentrations of oxygen and other elements in iron matrix of inner sample and near surface. The oxygen content of iron matrix was to mass% for No. 1 and 2 and mass% for No. 3 by EPMA. The metal structure of iron matrix in samples measured by EPMA is single phase of ferrite that has the maximum solubility of carbon of 0.02 mass%. The carbon concentration of No. 3 by chemical analysis in Table 3 was 0.04 mass%. The difference means that pearlite structure was partially in sample because of fluctuation of carbon concentration ISIJ 1076

4 4. Discussions 4.1. The Metallurgical Property of Japanese Nails Metallurgical investigations of nails were first performed on Horyu-ji temple nails by Nishimura et al. 7) and Horikawa et al. 8) Horikawa examined twenty-eight nails with the production time extends to 1800 from 607 years. The hardness distribution in these nails is in the range of 80 to 250 Hv. The hardness of No. 1 and No. 2 samples are lower than these nails. The reason could be that the metal structure of No. 1 and 2 samples is ferrite single phase Chemical Composition of Wakugi Table 3 shows the chemical composition of Japanese nails Table 2. element Chemical composition in iron matrix of the Japanese nails measured by EPMA (mass%). Surface Part Inner Part No. 1 No. 2 No. 3 4) No. 1 No. 2 No. 3 4) Si Mn P S Ti O Fe Al Mg Ca in published literatures. 4,8,9) The concentration of Ti was over 0.01 mass% in the Japanese nails made from hocho-tetsu that was produced from pig iron by Tatara furnace using iron sands. The concentration of Mn in the nails was lower by two orders of magnitude than that of modern blast furnace iron. Also, the concentration of sulfur in the nails was observed to be very low. The low concentrations of these elements come from high oxygen potentials of about atm at C in Tatara furnace and of about atm at C in Okaji furnace that are near the equilibrium oxygen potential with iron and FeO, while oxygen potential in a modern blast furnace is low of about atm at 1500 C in equilibrium with carbon and CO gas. Thus, in the blast furnace, gangue in iron ore is easily reduced and dissolved into pig iron produced. In addition, it should be noted that the iron produced using charcoal has less sulfur that makes iron brittle Oxygen Concentration in Iron Matrix of Wakugi EPMA with LaB 6 electron gun used in the present work had high accuracy in quantification. As the resolution area of secondary electrons is about 5 nm, inclusions with the size of 25 nm can be recognized but inclusion with the size less than 25 nm could not be recognized. Figure 4 shows the iron matrix and FeO inclusions with the size of about 1 3 μm including slightly Si, P and Ti elements. The iron matrix composition of nails of No. 1 and 2 measured by EPMA is shown in Table 2. The analysis was carried out at the center and near the surface of samples, respectively. The oxygen concentration of the iron matrix ranged from to mass% and approximately lager by one order than modern steel. The data in literature 4) shows the oxygen concentration of mass% for Bicchu-Kokubun-ji and mass% for Konko-in that Table 3. Chemical composition of the Japanese nails (mass%). Wooden structure period AD C Si Mn P S Ti O Ref. Horyu-ji Kon-do Asuka/Nara tr < ) tr ) tr. tr ) Byodo-in Heian tr ) tr ) Horyu-ji Kon-do Kamakura tr ) Horyu-ji Kon-do Edo < ) Byodo-in tr ) Edo Bicchu-Kokubun-ji (1821) ) Konko-in(ori) ) Edo ) Senjyu-ji Daigo-ji tr ) Otsuka-shuzou SLCM(Yakushi-ji) ) Gendai b.f.steel(sphc) < ) Note: Oxygen concentrations were analyzed by chemical analysis ISIJ

5 Fig. 4. SEM images of the fine inclusions of FeO including slightly Si, P and Ti. The mark + is the measurement points. Si concentration in the black area (B) is relatively larger than (A). P in (A) is relatively larger than (B). Fig. 6. Chronological change of oxygen concentration in Japanese nails and modern steel analyzed by chemical analysis and EPMA Chronological Change of Oxygen Concentration in Japanese Nails Figure 6 shows the chronological change of oxygen concentration in nails. 4,8,9) The nail of Horyu-ji temple in Nara city dated in A.D. 607 has low oxygen concentration of mass% with 0.10 mass% of carbon. The state of liquid steel at high temperature is in equilibrium with δ -Fe, as shown in Fig. 5. After then, the oxygen concentration in nails chronologically increased to about 0.2 mass%. The state of most of nails is out of liquid iron and has non-metallic inclusions. The chronological change in the state of steel from the Nara age to the early age of Heian could be due to the development of technique of Okaji process, that is increase in operation temperature. In order to increase temperature and to proceed decarburization of steel in Honba of the second stage of Okaji, iron was oxidized and produced more FeO and non-metallic inclusions. Fig. 5. Carbon and oxygen concentrations in Japanese nails with phase diagram of Fe C O system at C. oxygen concentrations are analyzed by chemical analysis and affected by non-metallic inclusions in nails. Figure 5 shows the relation between the carbon and the oxygen concentrations in nails measured by chemical analysis and EPMA reflecting on the phase diagram of Fe O C system at C. The white circles are the concentrations determined by chemical analyses. The samples with the oxygen concentration less than 0.04 mass% are in liquid iron phase and δ Fe. The other samples are, however, out of liquid phase and have the oxygen concentration including non-metallic inclusions. The data by EPMA analysis are also in liquid phase. In Okaji process, a steel block was heated over C and partially melt by the oxidation heat of iron. After carbon concentration in steel decreased less than 0.1 mass%, the block was taken out to forge and quickly formed Hocho-tetsu in plate. Thus, the state of decarburized steel remained oxygen concentration in liquid iron into matrix without non-metallic inclusions Over-saturated Oxygen in Steel during Okaji and Forge-and-welding Processes In Tatara process, steel Kera and pig iron Zuku were produced. Pig iron and low quality of steel Bugera in these products were decarburized with air by hand blowing to produce steel plates with low carbon concentration, called Hocho-tetsu or Wari-tetsu. The decarburization in premodern refining process, called Okaji was composed of two stages of Sageba and Honba. 10) In the Sageba, pig iron was decarburized to steel of about 0.7 mass%c, called Sagegane and the yield was almost 100%. In the Honba, the Sagegane was decarburized to about 0.1 mass%c, called Orosigane and the yield was 60 to 70%. The Orosigane was spontaneously forged to make plates without deoxidation. Japanese steels of Watetsu produced by Tatara process, such as Tama-hagane of high quality steel and Hocho-tetsu, have been characterized by fluctuation of carbon concentration. Therefore, Watetsu is forge-andwelded by hammering to make a plate. The fluctuation of carbon concentration in steel dispersed into fine areas. In the forge-and- welding process, steel plates are heated up to about C by charcoal burning. When Wakibana 5) of white sparks are appeared in flame, blacksmith takes out the plates to forge and welded each other. White sparks of Wakibana are small iron particles oxidized by air. The iron particles come from the interface of steel plates where 2014 ISIJ 1078

6 the surface of steel is heated near C and melted by oxidation of iron. FeO melt produced by oxidation of iron reacts with carbon in steel to generate CO bas bubbles. The growth of CO gas bubble is accompanied with production of fine iron particles. White sparks of Wakibana always appear in flame when Japanese steel is melted and accompanied with oxidation of iron to produce FeO melt. FeO melt contacts with molten steel near C. Therefore, the oxygen concentration in iron matrix becomes about 0.2 mass% and FeO inclusions are included. As shown in Table 3, the oxygen concentration in modern steels is very low because of deoxidation of molten steel by aluminum before solidification. However, there is not deoxidation process to make low carbon steel in Okaji process High Corrosion-resistance of Wakugi The maximum solubility of oxygen in δ Fe is mass% at C, mass% in γ Fe at C and lower in αfe than γ Fe. Oxygen concentration in Japanese steels of Watetsu is always in over-saturation. Under 560 C, α Fe with over-saturated oxygen decomposes into ferrite and magnetite by some triggers, such as humidity and heating. Oxygen in iron is a surface active element and tends to concentrate at the surface of αfe plate. Therefore, the surface of Hocho-tetsu with over-saturated oxygen is quickly covered by magnetite film at a trigger. The thin and uniformly dense oxide film of magnetite acts as a protective layer that behaves high corrosion-resistance. Over-saturated oxygen also precipitates fine oxide particles, mainly wüstite of FeO in steel during forge-and welding process. These fine particles could be a material to form some beautiful patterns on Japanese swords as well as metallic crystals of metallography. 5. Conclusions Japanese nails were mainly composed of ferrite and have over-saturated oxygen of about 0.15 to 0.38 mass% in the iron matrix. The nails were made from Hocho-tetsu that was produced in Okaji works. In Okaji process, pig iron and low grade steel block produced in Tatara works were heated and melted by charcoal burning and decarburized by air near C. The decarburized steel block was immediately forged to form the plates of Hocho-tetsu with fluctuated carbon concentration. Over-saturated oxygen in iron matrix of nails quickly forms a thin film of magnetite on the surface of plate that is caused of high corrosion-resistance. Acknowledgement Authors thank Dr. Takako Yamashita, JFE Steel Corp, for her help with our experiments. REFERENCES 1) K. Igaki: Kobunkazai-no-Kagaku, 28 (1984), 18. 2) K. Sugimoto, S. Matsda, M. Isshiki, T. Ejima and K. Igaki: J. Jpn. Inst. Met, 46 (1982), ) W. E. O Grady: J. Electrochem. Soc., 127 (1980), ) Y. Furunushi: Tetsu-to-Hagané, 91 (2005), No. 1, 91. 5) K. Nagata, T. Watanabe and N. Kugiya: Tetsu-to-Hagané, 97 (2011), No. 12, ) K. Nagata and T. Watanabe: Abstract of BUMA8, D-03, ISIJ, Tokyo, (2013). 7) H. Nishimura and N. Aoki: Tetsu-to-Hagané, 41 (1955), No. 3, ) K. Horikawa and Y. Umezawa: Tetsu-to-Hagané, 48 (1962), No. 1, 4. 9) O. Umezawa: Bull. Iron Steel Inst. Jpn., 6 (2001), No. 10, ) K. Tawara: Old Smelting Method of Iron Sand in Meiji Period, Maruzen, Tokyo, reprint with come by M. Tate, Keiyousha, Tokyo, (2007) ISIJ