Steel is a refined form of iron that has a controlled amount of carbon added to it. It has also been smelted with various minerals to help remove impurities at specific temperatures. There are several grades of steel, and other metals may be alloyed with the steel, depending on what its eventual use will be. The smelting of iron into steel was developed to an efficient industrial process only in the 19th century.
Iron occurs naturally in rock as an oxide ore (iron mixed with oxygen). Other impurities such as sulphur, manganese and phosphorus may be present. Iron was first mined 4,000 years ago. It supplanted the use of bronze (an alloy of copper and tin) when it was discovered that iron was harder and did not bend or lose its edge as easily.
Iron ore is smelted in a furnace with blasts of air and a carbon material added, typically coke. (Coke is coal that has been heated without oxygen to burn away sulphur and other impurities.) The addition of carbon atoms to a crystalline iron molecule increases malleability and hardness.
Types of Iron
The easiest kind of iron to produce is "wrought" iron, which is iron ore smelted at a temperature high enough to bond carbon atoms to between 0.2 to 0.8 per cent of the molten iron. This type of iron is hard but malleable enough to be hammered and shaped. As blast furnaces were improved for higher heat, iron with a higher carbon content of between 3 per cent and 4.5 per cent could be smelted; this was known as "cast" iron, which was immediately poured into moulds. It was harder but brittle, and could not be shaped or hammered.
Steel quality is dependent on smelting iron at a precise temperature with a controlled amount of carbon and other alloys. The carbon content of steel ranges from between 0.2 per cent and 1.5 per cent. While a higher carbon content hardens steel, it also makes it brittle. This can be offset by a variety of methods involved in heating and cooling the steel, called "tempering."
Steel is a crystalline structure of iron molecules interspersed with carbon molecules. This is properly known as "cementite." The hardness and malleability of steel depends not only on the carbon content, but on how the carbon and iron molecules are arranged to one another. Internal stresses in the steel's crystalline structure will increase or decrease depending on the temperature it is subjected to and the rate at which molten steel is cooled.
Grades of Steel
Carbon steels are divided into low, medium, high and very high. Low and medium carbon steels are malleable and can be welded. High and very high carbon steels are difficult to weld and require additional heat treatments and machining. Other steels are stainless steel (an alloy of steel, chromium and nickel), titanium steel and manganese steel.
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