Iron is a general word used to describe metals that have pure iron
as their main constituent. Sometimes the word iron is used as simile
of something hard like an "iron lady".
Most iron wares around us are not made of chemically pure iron but
are alloys, the most important of which is Carbon. Carbon is a big
factor in understanding the difference between Iron, Steel and Cast
iron. Adding some carbon to chemically pure iron makes steel. Add
even more and you'll make cast iron
Technically, iron means just that, chemically pure iron. Without
carbon, iron is very soft and ductile.
Iron becomes softer upon heating. So a smith can change its shape by
hammering but it can never be hardened by heat treatment.
Steel is an alloy of iron and carbon. The amount of carbon dictates
whether a steel is hard or it is tough. Adding Carbon makes the iron
harder. The more carbon the harder the steel. Carbon content in
steel usually falls a range between 0.3 ~ 1.5 % by volume. Iron
/carbon alloys within this range are called steels.
Steels can be forged and hardened by heat treatment. The high carbon
steel is harder than the low carbon steel. Because carbon content is
critical to hardening, the effect of heat treatment is big on high
carbon steel, and small on low carbon steels. In other words, high
carbon steels are more sensitive to tempering work. High carbon
steels undergo structural changes when heated and cooled rapidly
making them useful for items that require degrees of hardness. High
carbon steels appear some different phases that come from the
difference of the situation in tempering work. The result of
relation between sensitive steel and good tempering work makes
subtle appearances on blade surface.
Carbon content over 1.5 % make iron alloys brittle, non-ductile and
unable be worked by hammering. Also these alloys can't be hardened
by heat treatment. Such metals are easy to melt, and easy to break
by hammering. Therefore they are used only for casting work.
Therefore we can see that iron, steel, and cast iron form a family
based on the absence or degree of carbon content.
Although iron is not used for blades it can be used for tsuba and
other fittings like fuchi/kashira, kuirikata, and kojiri etc. Low
carbon steel is used for various kinds of tools and arms.
Most good tsubas are made of low carbon steel. Some "dubious" tsuba
are made of cast iron. (=> Tsuba of cast iron) High carbon steel is
used for cutting edges of blades and tools. Usually a blade is made
of combination with some kinds of steel. (=> construction) Sword
smiths can use either iron or cast iron as ingredients to produce
their own steels. They control the carbon content of the steel using
their forge. We call this home-made steel "Oroshi-gane".