Steel is a metallic alloy whose major constituent is iron. Carbon steels are the most widely used metallic alloys, but there are many specific alloys used for their specific properties. One classical definition is that steels are iron-carbon alloys with up to 2.1% carbon. Presently there are several classes of steels in which carbon is an undesirable alloying element. Examples are interstitial free steels. A more recent definition is that steels are iron-based alloys that can be plastically formed. The importance of carbon in most steels results from its effect on steel properties and phase transformations. With the increased carbon, steel is harder and has a much higher tensile strength than iron, but is also more brittle. The first steels were probably created accidentally when iron sword blanks were heated in charcoal forges.
While the ferrite allotrope of iron, which is normally what we see as wrought iron, will not absorb much carbon, when heated to a higher temperature it assumes a mostly austenite structure, allowing it to absorb up to 5% carbon and become pig iron. Additional metals are usually added to carbon steel to change its characteristics. Nickel in steel adds to the tensile strength, chromium increases the hardness, and vanadium also increases the hardness while reducing the effects of metal fatigue.
After the heating process, the cooling of the steel must be controlled in order to control its crystal structure. This is known as quenching, which converts austenite into varying proportions of martensite, ferrite, cementite and bainite. The steel must be cooled quickly in order to achieve the desired crystal form, but cooling too quickly will cause the metal to crack. Water cools the steel too quickly, so oil is traditionally used.
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2 Production methods
3 Steel producers
Types of steel