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What are Iron, Steel, and cast Iron?


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.

Cast Iron

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".





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