Smelting of the ore by direct reduction
The earliest method known to man for obtaining iron from oxide ores was
by direct reduction with charcoal. In the direct reduction process iron ore is
mixed with charcoal and heated in a simple furnace. Oxygen from the air is
admitted at the base of the furnace and combines with the hot charcoal to
form carbon dioxide. This reaction is exothermic and the carbon dioxide
reacts almost immediately with the surrounding hot charcoal to form carbon
monoxide. It is the carbon monoxide which reduces the iron ore higher up in
the furnace, where, as the reaction proceeds, the reduced iron forms porous
globules. These globules gradually descend to the hotter regions of the furnace
and eventually become pasty, forming sponge iron on cooling. The
sponge irons resulting from ancient direct reduction processes contain many
impurities such as slag inclusions and unreduced oxide [ 1 ].
For over a millennium, in Japan, direct reduction was the first step in the
manufacture of a Japanese sword. The ore most commonly used was fine
sand iron which was collected from river banks. The wooded regions of Japan
where the iron oxide-bearing sand could easily be collected, became famous
for the making of swords. One such area is Bizen in Okayama Prefecture (to
the west of Kobe; nowadays the area of Bizen is very famous for its potteries
whose kilns are fired with pine).
The furnace was made of some heat-resistant bricks with a layer of clay
spread inside it. Wood and rice straw were burnt in the furnace initially, to
dry the clay and particularly the furnace bottom. Once the furnace was dry,
charcoal was added and a fire started with the aid of oxygen from the air,
often supplied by bellows. The charcoal was usually made from red pine
because of its lower water content. When the fire was well-lit sand iron would
be placed in the charcoal. Charcoal was subsequently added from time-to-time
to regulate the temperature which could be judged through a window in the
furnace wall. Slag was run-off in a slag runner near the bottom of the furnace.
Typically after about seven hours, the reaction was completed and the furnace was destroyed. The sponge iron, which was in a depression in the
base of the furnace, was then removed after clearing away the debris. For a
furnace of about one metre in height a yield of about 18 kg of sponge iron
would have resulted from charging the furnace with about 70 kg of sand iron
and 11 huyo of charcoal (a huyo is a rice straw sack of about a quarter of a
cubic metre capacity). Each crafsman had his own secret additions to the
furnace smelting ingredients besides charcoal and sand iron. Thus, in old
texts references can be found to additions of old nails and broken pottery.
Depending on the maximum furnace temperature and the amounts of
charcoal, sand iron and other additions, the resulting sponge iron could have
had widely differing carbon contents. For example, if the furnace temperature
had been between 800°C and 1000°C, ductile sponge iron containing about
0.05% carbon would have resulted, whereas, if the temperature had been
between 1100°C and 1200°C, sponge iron containing between 0.8% and 1.8%
carbon would have formed. At furnace temperatures in excess of about
1300°C, the sponge iron would have melted completely dissolving between
1.8% and 3.5% carbon; this iron was extremely brittle and hard. Sponge iron
formed at the intermediate temperature of about ll00°C was the metal that
would eventually end as the outer covering of the sword, including the
hardened cutting edge. A typical composition of this steel was: 1.33%C,
0.006%S, 0.014%P, with traces of Cu and Mn. This steel is remarkably pure
when compared with modern Western steels which contain silicon, manganese
and sulphur.

Quenching
The final sword was required to have a very hard zone in the vicinity of the
cutting edge and a soft interior, therefore some means of selective quenching
was required. This was accomplished by covering the entire sword with a thick
coating of clay. The clay was removed from the regions near the cutting edge,
see Fig. 6, in a particular pattern, the pattern often depending on the individual
swordsmith. The sword was then placed in a simple charcoal furnace and
heated to above 800°C. Then the sword was plunged into water. The clay
acted as an insulator and only the region not covered with it was cooled at a
fast enough rate to transform directly from unstable austenite to martensite.
The cooling rate of the ductile core was very much slower than that of the
cutting edge and therefore the resulting microstructure would have been that
typical of a hypo-eutectoid steel, that is pro-eutectoid ferrite plus a little
pearlite.
Fresh river water was reputed to have been the best quenching medium
and only a single quench was used. The temperature of the quenchant was a
well-guarded secret and, depending on the individual swordsmith, the water
may have contained certain secret additions which could have included soft,
straw and rice husks. It seems highly probable that quenching was only
carried out at a particular time of year when the river water was at the
required temperature.
Possible quenching defects would have included quenching cracks and
'half-moon' or 'half-beak' cracks. If any of these defects were discovered on
the sword, after cleaning off the clay and oxide, the sword would have been
discarded. Also due to the quenching stresses the curvature of the blade (sori)
would have changed, therefore the initial curvature before quenching would
have been forged with this in mind.

Grinding and final polishing
After quenching, the sword was scraped clean of clay and the cutting edge
was cleaned to some extent of the oxide. The sword was examined carefully
by the swordsmith so that he could check that no quenching defects were
present. If necessary, the curvature of the sword was carefully adjusted by
pressing the back of the sword against a hot copper block. After this, the
sword was ready for grinding and polishing.
Apparently little is known about these final operations, however it is
known that at least ten different grinding stones of differing coarsenesses were
utilized.
After final polishing, the true beauty of this complex series of operations
could be seen. Referring to Fig. 7, the parts of the sword that were originally
covered with clay during quenching are highly reflective. On close examination,
these surfaces can be seen to have a characteristic texture or mechanical
fibering. This fibering is the result of the lamination processes and consists of
inclusions and elongated grains. The inclusions are spread along the welded
interfaces (parallel to the cutting edge) resulting from lamination. The sizes
of these inclusions depend upon the number of laminations made.
The areas of the sword near the cutting edge that were not covered with
clay are martensitic, and after polishing, have a characteristic diffuse whitish
or milky appearance. These regions of martensite are called nioi.
Finally, one of the most interesting regions of the sword is the transition
zone between the hardened and unhardened regions. This zone is usually a
fine line called the nie line, whose microstructure usually consists of a few
relatively coarse areas of martensite in pearlite. The areas of martensite appear
as very small bright spots and as the angle of the blade is changed relative
to the light source these spots of martensite flash. The bright spots give the
nie line an intriguing appearance and add to the value of a sword.
The final stage in the making of any sword would have been the engraving
of the tang with the swordsmith's name. A handle and a hand guard (tsuba)
were then attached and a scabbard fitted. The numerous adornments to a
Japanese sword are a study in themselves!

Fig. 2. (a) Grooving a forge-welded block; (b) opening the groove and bending the block;
(c) forge-welding the folded block. This sequency of operations was repeated as many as
thirty times.

Fig. 3. Bending of the grooved block by the traditional method (from Ref. [ 3 ] ).

Fig. 4. The forge-welding of the core to the tool steel and the stages in the manufacture
of the composite sword (after Ref. [ 2 ] ).

Fig. 6. Section of a blade showing the clay coating which has been selectively removed
from the sword blade, allowing only the region at the blade to transform to martensite
on quenching.

Fig. 7. ~t~.e appearance of part of a finished sword showing the mart~nsitic area of the
blade and the transition zone between the nioi and the more slowly cooled back

References
1 The Making, Shaping and Treating of Steel, 9th edn., United States Steel Corp., 1971.
2 E.C. Bain, J. Iron Steel Inst., 200 (1962) 265.
3 J. Honma and K. Sato (Eds.), The Making of Japanese Swords, Tokuma -- Shoten (in
Japanese), 1966.
4 N. Ogasawara, Japanese Swords, 3rd edn., Hoikusha Publishing Company Ltd., 1975

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看不懂英文不行

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