J. Wadsworth, D.R. Lesuer /Materials Characterization 45(2000) 289- 95 ng a low-carbon core. The procedure used to make such a blade is shown in Fig. 4. This composite pproach allowed the swordmaker to achieve the desirable properties of both hardness and ductility in a single weapon. Often, one of these properties is only developed at the expense of the other. Tool steel econd, to produce the high-carbon part of the blade, the steel was subjected to multiple folding perations; this has given rise to the erroneous con- 5 cept that the swords contain millions of discrete layers. Thi experimentally examined in next section on modern lmcs Third, the development of the complex and Hardened beautiful surface markings was a consequence a selective surface heat-treatment process(known as yaki-ire), achieved in part by covering the blade with different thicknesses of clay; this gave rise to an intriguing combination of transformation pro- ducts and surface patterns. This was not only visible and aesthetic example of the skill of the (ultrahigh carbon steel) swordmaker, but it was also evidence that the edge of the blade had been hardened Fig. 4. Procedure used by Japanese blacksmiths to make The blade must have a recognizable pattern, called laminated tools by solid-state bonding ultrahigh carbon the hamon, where the structure changes from the hard steel, known as kawagane, to soft iron; including a cross martensite at the edge, called akiba, to soft pearlite section of a blade coated with various thicknesses of clay to The hamon is perhaps the most important aesthetic control temperature, with minimal clay coat at the edge to allow proper hardening(adapted from Bain [19)[14] feature of a blade, and the first thing sword aficiona dos look for, as it is essentially the swordsmiths signature. Kapp et al. [21] cite the work of B w. stage of manufacture of the Japanese sword in which Robinsons classic book The Arts of the Japanese steel of about 1.8%C is used Sword [22], illustrating 53 different hamon, each with The high-carbon steel (called kawagane, but also its own name(from the descriptive"straight irreg sometimes called magane)used for blademaking wa lar"to the more suggestive "chrysanthemum and principally prepared using a reduction process meth- water") and the name of the smith or school with od, in which iron, sand, and charcoal produced tama- which it is identified. (See Fig. 5 for an illustration of hagane. A fixed amount of iron ore and charcoal was broad classes of hamon patterns mixed and heated in air to 1200C. resulting in The Japanese swords external sheath has a similar products of molten pig iron, slag, and unmelted carbon content (i.e, it can be hypereutectoid, about ultrahigh-carbon steel (UHCS). The iron ore came 0.8-1%C)to that of the Damascus sword (also from so-called black sands known as satetsu(iron hypereutectoid, but in the 1.5-1.8%C range). This oxide). The carbon was added to the black sand in a similarity of composition is especially so in the early smelter called a tatara. When the pig iron and sla were allowed to separate by pouring, the end product was lumps of UHCS containing about 1.7%C(tama i Tylecote[20] has pointed out a similarity in this hagane). This material was then repeatedly forged regard between the Japanese sword composite design and folded until the appropriate shape and reduction and the much later development of"shear steel"in in carbon content was achieved through decarburiza Western Europe following the Industrial Revolution. tion. (It should be noted that, depending upon the The similarity lies in the fact that comparatively few arbon additions, temperature, and time at tempera- pieces of steel of different carbon content wer welded together to make a composite, single-edged, ture, the result of such a repeated folding and forgin blade which was finally heat treated. In shear steel process could be low-carbon steel. mild steel outer strips encased a high-carbon center blade involved a number trip-a design currently available in handmade First, the tama-hagane was repeatedly forged and knives by contemporary artisans. In fact, Tylecot folded to produce the kawagane, which becomes the goes as far as to say, "There is essentially no sheath or jacket steel. Second, the shingane, or low- difference in principle between a scythe [made from carbon core steel, was formed, also by a repeated shear steel], and a Japanese sword. folding procedure. Third, the low-carbon core wasing a low-carbon core. The procedure used to make such a blade is shown in Fig. 4. This composite approach allowed the swordmaker to achieve the desirable properties of both hardness and ductility in a single weapon.1 Often, one of these properties is only developed at the expense of the other. Second, to produce the high-carbon part of the blade, the steel was subjected to multiple folding operations; this has given rise to the erroneous con￾cept that the swords contain millions of discrete layers. This point is experimentally examined in the next section on Modern LMCs. Third, the development of the complex and beautiful surface markings was a consequence of a selective surface heat-treatment process (known as yaki-ire), achieved in part by covering the blade with different thicknesses of clay; this gave rise to an intriguing combination of transformation pro￾ducts and surface patterns. This was not only a visible and aesthetic example of the skill of the swordmaker, but it was also evidence that the edge of the blade had been hardened. The blade must have a recognizable pattern, called the hamon, where the structure changes from the hard martensite at the edge, called yakiba, to soft pearlite. The hamon is perhaps the most important aesthetic feature of a blade, and the first thing sword aficiona￾dos look for, as it is essentially the swordsmith's signature. Kapp et al. [21] cite the work of B.W. Robinson's classic book The Arts of the Japanese Sword [22], illustrating 53 different hamon, each with its own name (from the descriptive ``straight irregu￾lar'' to the more suggestive ``chrysanthemum and water'') and the name of the smith or school with which it is identified. (See Fig. 5 for an illustration of broad classes of hamon patterns.) The Japanese sword's external sheath has a similar carbon content (i.e., it can be hypereutectoid, about 0.8 ± 1% C) to that of the Damascus sword (also hypereutectoid, but in the 1.5 ± 1.8% C range). This similarity of composition is especially so in the early stage of manufacture of the Japanese sword in which steel of about 1.8% C is used. The high-carbon steel (called kawagane, but also sometimes called uagane) used for blademaking was principally prepared using a reduction process meth￾od, in which iron, sand, and charcoal produced tama￾hagane. A fixed amount of iron ore and charcoal was mixed and heated in air to 1200°C, resulting in products of molten pig iron, slag, and unmelted ultrahigh-carbon steel (UHCS). The iron ore came from so-called black sands known as satetsu (iron oxide). The carbon was added to the black sand in a smelter called a tatara. When the pig iron and slag were allowed to separate by pouring, the end product was lumps of UHCS containing about 1.7% C (tama￾hagane). This material was then repeatedly forged and folded until the appropriate shape and reduction in carbon content was achieved through decarburiza￾tion. (It should be noted that, depending upon the carbon additions, temperature, and time at tempera￾ture, the result of such a repeated folding and forging process could be low-carbon steel.) Forging the blade involved a number of steps. First, the tama-hagane was repeatedly forged and folded to produce the kawagane, which becomes the sheath or jacket steel. Second, the shingane, or low￾carbon core steel, was formed, also by a repeated folding procedure. Third, the low-carbon core was Fig. 4. Procedure used by Japanese blacksmiths to make laminated tools by solid-state bonding ultrahigh carbon steel, known as kawagane, to soft iron; including a cross section of a blade coated with various thicknesses of clay to control temperature, with minimal clay coat at the edge to allow proper hardening (adapted from Bain [19]) [14]. 1 Tylecote [20] has pointed out a similarity in this regard between the Japanese sword composite design and the much later development of ``shear steel'' in Western Europe following the Industrial Revolution. The similarity lies in the fact that comparatively few pieces of steel of different carbon content were welded together to make a composite, single-edged, blade which was finally heat treated. In shear steel, mild steel outer strips encased a high-carbon center stripÐ a design currently available in handmade knives by contemporary artisans. In fact, Tylecote goes as far as to say, ``There is essentially no difference in principle between a scythe [made from shear steel], and a Japanese sword.'' J. Wadsworth, D.R. Lesuer / Materials Characterization 45 (2000) 289±313 295