Iron is a metal known from antiquity. Iron in its metallic form was probably collected from meteorites in the earliest days of civilization. But, smelting iron from hematite and other ores is more difficult than working with copper and tin. It requires a hotter furnace that must include a bellow or other means of forced air. Thus, a civilization's Iron Age follows its Bronze Age. We find smelted iron in Anatolia around 1500 BC, India 1400 BC, China 500 BC, and so on.
The simplest products of iron smelting are wrought iron, tough and malleable, and cast iron, hard and brittle. Cast iron requires a hotter furnace and for centuries was made only in China.
The creation of steel from iron requires adding controlled amounts of carbon. Some forms of steel are nearly as old as iron itself. A variety of steels was produced during the Middle Ages. Cheap mass-produced steel began in Sheffield, England, in 1855. Modern steels are carefully alloyed with small amounts of other metal for a huge range of properties. Chromium and nickel produce stainless steel, molybdenum hardens steel, manganese increases strength, and tungsten adds toughness.
Mining and Production
Iron is the second-most common metal in the earth's crust, behind aluminum. It occurs as an oxide in hematite, magnetite, and other ores. Significant mining (more than 1% of the total) occured in 2014 in at least a dozen countries, led by China (47%), Australia (20%), and Brazil (10%). An estimated 3 billion tonnes were mined worldwide in 2014, many times more than any other metal.
Iron ores are typically processed by crushing, grinding, and separating by flotation or magnets (magnetite is magnetic, hematite is not).
Once the iron oxide is separated from its surrounding rock, it must be smelted in a blast furnace. The oxygen is stripped away by exposing it to carbon monoxide (CO) obtained from partially-burned carbon (coke; treated coal). The resulting iron can be poured into molds and used as is; this is cast iron. Otherwise it is termed pig iron and moves to the next stage of production.
Most iron is destined to become steel. As described above, steel is produced from iron by carefully controlling the amount of carbon and alloying metals. Steel can be rolled into sheets, drawn into wire, and shaped into complex forms.
Properties and Uses
The variety of uses for steel can only be sampled here. Mild steel with about 0.2% carbon is simple and economic enough to manufacture that it has almost completely replaced basic wrought iron itself.
A harder steel with more carbon is used to form the beams, girders, and rails that are the skeletons of buildings and transport systems. Though the visible surface might be aluminum sheets or concrete, the strength comes from steel.
The major disadvantage of steel is that it easily rusts and eventually rusts completely. Stainless steel with 10% chromium is resistant to corrosion. Steel can also be plated with nickel or bonded to zinc (galvanized steel) for the same purpose.
Tool steel contains cobalt or tungsten for hardness, used in drill bits. Manganese steel with 12% alloy is very hard for extreme-force cutting blades. Vanadium and especially molybdenum are also used to harden steel.