Materials Science Research International, Vo1.3, No.4 pp. 193-203 (1997) Review paper THE JAPANESE SWORD The Material, Manufacturing and Computer Simulation of Quenching Process Tatsuo INOUE Department of Energy Conversion Science, Faculty of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-01, Japan Abstract: Traditional methods of preparing a kind of steel called tamahagane used for the Japanese sword by tatara system and procedure of making the sword are briefly introduced with the discussions from the viewpoint of metallurgy and thermo-mechanical processing. Such traditional methods are also revealed to be consistent with the modern science and technology. The quenching process applied to the final stage of the procedure is focussed to explain how the pattern of blade, the deformation and residual stresses are induced by the computer simulation based on the theory of metallo-thermo-mechanics relevant to the coupled fields among temperature, microstructural change and stress/strain. Key words: Japanese sword, Quenching, Metallo-thermo-mechanics, Tatara system, Tamahagane, Iron sand, Sofri, Hamon 1. INTRODUCTION One of the most attractive and important stage of manufacturing the sword applied to the end of the The Japanese sword originally used as fighting process before grinding and polishing is quenching, weapon is now one of typical traditional crafts with which induces the characteristic deformation pattern artistic characteristics, and so many monographs have of bending called sori, and the formation of blade. been published in English[1-6]as well as in Japanese. The following parts treat some results of computer The sword is also interesting from the viewpoint of simulation of interesting bent shape of the sword and modern science and technology[7-26]since the way pattern of the blade simulated by a developed code of manufacturing the sword is really consistent with HEARTS•h [32-35]. the science as is same as other surviving traditional The code was accomplished based on the theory of products. Tawara, a professor of Japanese Sword metallo-thereto-mechanics[36-39]relevant to describ- Research Laboratory, the University of Tokyo, ac- ing the fundamental equations considering the cou- complished a monumental work in the framework of pling effect among microstructural change, temper- metallurgy[10]. Tawara measured the distribution of ature and stress/strain, which have been applied to carbon density, precipitation and hardness in the cross the simulation of heat treatment processes consider- section of the swords in relation with the pattern of ing phase transformation including quenching of the blade and sori representing the mode of deformation sword[19-26]. After discussing an paradoxical char- during quenching. Successive works were made by acteristics on the heat transfer coefficient between Bain[7],Suzuki[11-12], Tsuwa[13], and others. heated steel covered by a kind of thermally insulated Very few works on the sword were made, however, clay, called yakibatsuchi, and water as the coolant, from mechanical engineering aspect. Ishikawa[14-16 some results of simulation of a sword in the quenching discussed the mechanism of cutting objects from the- process are presented. ory of cutting and the shape of the sword from dynam- ics, and stress/deformation analysis after quenching 2. PREPARATION OF TRADITIONAL by the finite element method was carried out by Fuji- JAPANESE STEEL wara and Hanabusa[17-18]and the present authors[19- 26]. Almost all Japanese swords with some exceptions As is well known, the Japanese sword is normally are made of tamahagane steel, or noble steel, specially made of a traditional Japanese steel made of iron sand, prepared by the tatara system by use of iron sand, called tamahagane[27-31],and manufactured by a spe- but not by normal ore as seen in the old painting (see cial way, especially by folding the steel. In the first Fig. 1). and second parts of the paper, the process of prepar- Steels distributed in Japan before Meiji renovation ing the steel and the way of making the sword are in 1868 were produced by this method, while modern briefly introduced. system of iron and steel making had been developed Received August 29, 1997 193 Tatsuo INOUE Fig. 1. Old painting of Tatara system. in Europe. The amount of yearly production in the time was approximately 10,000 tons being equivalent Fig. 2. Cross sectional view of tatara furnace. to that of Great Britain[40]. The steel was used not only for swords, but also for guns, cutting tools, nails for construction of old temples and shrines, and other products necessary for ordinary life. Around the pe- riod, the tatara system was replaced by the modern western system except for providing the steel to sword smiths. The Iron and Steel Institute of Japan constructed an experimental system of tatara in 1969[27-28]in Sugaya, Shimane Prefecture, and accumulated inter- esting data of steel making technology. Due to the lack of the steel for the sword, the Japanese Sword Mu- seum, Nippon Bijutsu Token Hozon Kyokai, started to organize the tatara system in Torigami, Shimane Prefecture, under the cooperation with Hitachi Met- als, Ltd. in 1977, and provides the steel of 3-4 tons every year. Iron sand with 2-5% content of iron mined from Chugoku Mountains, which includes the best quality Fig. 3. Kera, a block of blister steel. of iron sand in Japan, concentrated to the degree of 60% by magnets system, while the mineral dressing perature in the furnace is around 1200-1500•Ž lower method by gravity classification in flowing river had than the melting point of the steel, which follows that been adopted, which is no more popular to prevent wa- the reduction process of the partly molten state is oc- ter pollution problem. Such enriched iron sand (masa curred between iron oxide Fe2O3 and silica SiO2 con- satetsu) contains 8% of pure iron Fe and iron oxide tained in the clay of furnace. During the process, the Fe2O3 with very small amount of impurity such as initial thickness of 200-400mm of the furnace is re- 0.026% phosphorus P and 0.002% sulfur S being in- duced to 50-100mm. jurious for carbon steels. Chemical compositions are After taking out the slag from the bottom of the shown in Table 1. Here, alumina Al2O3 is so rare to be furnace followed by destroying the furnace, a block beneficial for low temperature refinement to be stated of blister steel called kera in sponge state with di- later. mension of 2.7m in length, 1m in width and 200- The enriched iron sand is supplied alternatively to 300mm in thickness and with 2-2.5ton containing steel the furnace with charcoal by hands. Figure 2 illus- of 1.5-1.8ton is obtained (see Fig. 3), while necessary trates the cross sectional view of the furnace under amount of iron sand and charcoal are respectively 8 operation with some drainage mechanism constructed and 13tons. (It is amazing that the block costs hun- to three meters under the ground. Only a difference dred thousand dollars, two hundred times much ex- of the system from the classical one in Fig. 1 is that pensive of normal steel!) electric motors are used for intermittent air blowing Steel produced on the both side of the block, where instead of manpowered bellows. the enough deoxidization is completed by air supplied Continuous burning is operated for 70h under the from kirokan (special wooden pipes) is called tama- direction of a murage, or chief foreman. The tem- hagane, or noble steel, which is spelled as mother of 194 THE JAPANESE SWORD Fig. 4. Process of manufacturing the Japanese sword 195 Tatsuo INOUE Table 1. An example of chemical composition of iron sand in virgin and enriched states. Table 2. Chemical composition of tamahagane, forged and core steels. metal in Japanese character. Other parts of the block The weight of the block decreases during the process with different chemical composition in Table 2 are also to approximately 700-100g in the final shape of the used for the sword making. The chemical composi- sword. tions of the best part of steel are 1.0-1.4%C, 0.02- A bar of shingane (core steel) with low carbon con- 0.03%P, <0.006%S, and 0.003-0.004%Ti, being very tent is wrapped by kawagane or hagane (skin steel) rare of sulfur and phosphorus even compared with in- with high carbon for which the tamahagane steel is dustrial carbon steel (see Table 2). normally used (see the cross sectional views in Fig. 4). The steel is cooled by cold environment since the op- This process is called tsukurikomi. After rough grind- eration is carried out in mid-winter followedby shat- ing by the smith himself, the sword is transferred to tering, and distributed to about 300 professional and the final process of yakiire (quenching), which is the registered sword smiths in Japan. main topics of numerical simulation in the following sections. Before quenching, a kind of clay, yakiba- 3. MANUFACTURING OF JAPANESE tsuchi, mixed by charcoal powder and so on is pasted SWORD on the surface of the blade to control the heat trans- fer intensity to be discussed in Sec. 6 as presented in The pieces of the steel with different carbon con- Fig. 6. tents are heated in the carburizing or decarburizing Most interesting situation is that the pasted clay is environment, termed jigane-oroshi. This process is thick on the ridge while thin on the blade part as il- made in the furnace burnt by charcoal and ash of lustrated in Fig. 7. Finally the quenching operation of rice straws with the blowing air sent by fuigo (blow- the sword heated up to 800-850•Ž into water is carried ers).