Appeared in Materials Australia Vol 43 No3, September 2010

Traditional Process Produces Lath Martensite Effectively
1A. K. Das, Go TAKAMI, Takuya OHBA, Shigekazu MORITO, Muneo YASO
Department of Materials Science, Shimane University, Nishikawatsu, Matsue, 690-8504,
1
JAPAN WAKOH Museum, Yasugi 692-0011, JAPANAbstract
The traditional Japanese sword is a highly regarded metal craft in the world from very earlytimes. Its exceptionally sharp cutting edge, wavy banding pattern (called hamon in Japanese)
toughness and durability are well appreciated. It is said that the Japanese sword cannot be
produced by ordinary steel as some traditional methods and techniques are employed in its
preparation process. Microstructural studies of the specimens taken from different stage ofpreparation show that initially the materials of the Japanese sword include ferrite bands in a pearlite
matrix. It has been observed that the structural morphologies change with the turns of successive
forge-fold operations gradually. In a complete sword the structural configuration is quite different; itshows martensite at the sharp edge and a combination of pearlite and ferrite in other regions. It has
been realized that repeated forge-fold operations as well as thermal quenching are attributed to theformation of structural morphology. In the present article the preparation procedure, microstructuraland a few mechanical properties of a traditional Japanese sword will be discussed.

dBackgroundA great deal of interest has been found in the recent years to study the micro-structural characteristicsof traditional wootz steel especially used in the fabrication of swords. In different parts of the world
different types of swords were used as weapons in the medieval age [1]. The extraordinary
mechanical properties, exceptionally sharp cutting edge and wavy banding pattern of those swords
are of special interest to the researchers. On the basis of archeological research and documentation
at least three methods of production of ancient steel-swords are recognized [2,3].

i) Japanesetechnique for the production of traditional swords, ii) „Eastern Damascus‟ techniques diffused in theIndian-Persian regions and iii) „Western Damascus‟ or „Welded Damascus‟ which has encountered asignificant success in the regions around the Mediterranean Sea.
Japanese swordsmiths have had their own recipe to prepare the swords. Many excellent Japaneseswords were found in the period of 11th-16th century in Japan but most of them are extinguished now.
In the recent years the new generations of old swordsmiths are trying to produce the sword of same
quality as those of the old sword. The swords made in Japan today are not, of course, intended for
actual use but its quality, design and durability have conferred the dignity of an aesthetic object to it inJapanese society. Swords of different size and shape of different ages are found in NBTHK (NihonBijutsu Token Hozon Kyokai-The society for preservation of Japanese Swords) in Tokyo, which works
for preservation, improvisation and study of the antique swords [4].
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Preparation Process of Japanese sword
The swords in Japan are made from a special kind of steel called tamahagane. The origin of thistamahagane comes from fine black iron sand called satetsu. The iron in this form is actually a mixtureof iron oxides such as- Fe2O3 Fe3O4, FeTiO3 etc. In order to produce steel from satetsu, oxygen isremoved and carbon is added. In Japan this is done in a traditional smelter called tatara, using
charcoal both as the fuel and as the source of carbon. When the smelter temperature is sufficiently
high, the oxygen introduced by the bellows reacts with the carbon present in the charcoal to form
carbon monoxide. The iron oxide in the ore then reacts with carbon monoxide to form pure iron andcarbon dioxide, which escapes as gas. In tatara process the steel produced is an inhomogeneousmass of steel- called tamahagane that can then be sorted out according to its carbon level. The blockof tamahagane is flattened into thin sheet by hammering and then cut into small pieces called heshi
chips. The heshi chips containing high carbon (brittle) and low carbon (ductile) are segregated andthen forge and fold several times separately to produce refined steels of expected level. Fig. 1 showsa schematic flow chart of preparation of materials of Japanese sword by cyclic forge-fold operations.

Figure1. Illustration of preparation of the materials of Japanese sword by cyclic forge-fold operations (schematic).
Formation Microstructural Morphology under Forge-Fold Process
The materials of Japanese sword in its different stages of preparation contain different types ofmicrostructures. In Figure 1 the A and B-type materials, initially contains a few ferrite bands in thepearlite matrix which become narrower and thinner gradually with the recurrence of forge-foldoperations. The C-type material (mixture of A and B) shows the different microstructure combined withpearlite and plate-like ferrite. Microstructure becomes little finer with increasing the turns of forge-fold2Appeared in Materials Australia Vol 43 No3, September 2010

operations but no major change are observed between C1 and C6 [5]. Formation of micro structuresin different stages of preparation observed by optical microscopy is shown in Figure 2.

Figure 2. Microstructure observed in different stage of preparation.

However, in a complete sword as shown in Figure 3 the microstructure is different as some moretreatments are performed after assembling the A and B-type materials together. Consequently, lathmartensite morphology appears in the sharp edge which produces the glazy wavy banding pattern asobserved in Figure 4 and fine or course pearlite in the other regions [6, 7]. The lath martensite is atypical and unique morphology in Japanese sword which is rare in other swords [1, 2 & 3].
Final Processing of Producing Sword
There are many ways of producing Japanese sword depending on the swordsmiths. Most typical way
is that two types of C6 materials (ductile and brittle) are prepared depending upon their carbon leveland then ductile one is wrapped by brittle one to fabricate a sword. After having the shape of a sword
it is coated by a layer of clay mixed with charcoal (called Tsuchioki in Japanese). The clay coating isthinner near the blade, so that after heating up to 800°- 850°C and the following plunging in the water,
the blade can undergo a quenching process in order to produce the lath martensite, whose
microstructure is different from Damascus steel. Only the blade zone undergoes the phasetransformation which causes the lath martensite in the fast cooled region and the related thermal
expansion causes the particular bended shape of the typical Japanese swords [4].
3Appeared in Materials Australia Vol 43 No3, September 2010

Figure 3. A traditional Japanese sword prepared by Mr. Munehira Koyama in 1837. The length of the sword(blade) is 77.5 cm and curvature is 1.5 cm. The photograph was taken with the consent of Wakoh Museum.
※刀の写真の所有権は和鋼博物館にありますので，本原稿では解像度を落としています。

Micro hardness
MicroVickers

hardness

has

been

measured to know the trend of hardness
across the crosssection along the centerline. Figure 5 shows the hardness profileof the sword. The maximum hardness ofFigure 4. Top view of the sword (shown in fig.3) showing
wavy pattern along the sharp edge.

the sharp edge is 720-730 HV. ThisVickers Hardness (HV)

hardnesslevel

corresponds

tothe800

hardness of martensite which contains

700

0.78 mass% carbon [7]. It has been

600

found that the hardend area is limited to500

the400drastically beyond the sharp edge. As

300

the structural morphology is pearlite and200

ferrite outside the sharp edge which is

100

produced for slow quenching effect. The

0sharpedge

and

it

decreases

hardness profile shows good agreement
0510152025with the structural morphology [8].

Distance from sharp edge (mm)Figure 5 . Hardness profile across the cross section.
Summary

Micro structure of a traditional Japanese sword and its preparing materials subjected to successive
forge-fold operations were studied metallurgically. Microstructure of the preparing materials consistsof a combination of ferrite bands in pearlite matrix at the beginning stage. The ferrite bands become
finer with increasing the turns of forge-folding operations and scattered into pearlite matrix gradually.
In the complete sword, the sharp edge contains the lath martensite structure where as the other sideand the central part show the fine or coarse pearlite structure. The structural configurations resemble
4Appeared in Materials Australia Vol 43 No3, September 2010

to those of 0.6 mass% C steel [9] even though the concentration of carbon is different. Vickers microhardness measurement revealed that the sharp edge is comparatively harder than other sections ofthe sword. Present study elucidates that the tatara iron and its manufacturing procedure give thedistinctive features to Japanese swords which is different from ordinary steel. It is also notable thatJapanese swordsmiths utilized structurally graded materials without knowing details about it. Recentstudies are disclosing the hidden facts behind the old recipe.
Acknowledgements
The present work was financially supported by Tatara project of the Ministry of Education, Scienceand Culture of Japan Government and the Organization for the Promotion of Project Research in
Shimane University. The authors are grateful to Mr. Minagi, Mr. Takaiwa of Wakoh Museum, Mr.
Kubota and Mr. Kanaizumi of Hitachi Metals Ltd., Yasugi, Japan for helpful discussion. The authors
also acknowledge Mr. Sadanao Mikami, a famous Japanese sword maker for providing the samples
employed in present study. The photograph of the Japanese sword was taken under the courtesy ofthe authority of Wakoh Museum, Yasugi, Japan.
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