Iron is found abundantly in many parts of the world. In fact the Earth's core is made mostly of iron and acts as a giant magnet, which is why compasses point north. The Earth's crust - the part that humans can get at - also contains plenty of iron, although after centuries of mining the most accessible iron deposits have already been dug up, and miners are having to either dig deeper or go to remote locations.
The iron ore that miners dig up is almost always in the form of iron oxides, compounds of iron and oxygen. To convert iron oxide to iron, the iron oxide must be smelted. The iron ore is heated to at least 800°C in the presence of carbon monoxide gas. Under these conditions a chemical reaction takes place in which the carbon monoxide is transformed into carbon dioxide, while the iron oxide is tranformed to elemental iron.
The carbon monoxide gas is produced by burning charcoal or some other fuel. Thus fire serves a dual purpose in iron smelting: it provides both the heat and the carbon monoxide needed to convert iron oxides into iron.
The earliest type of iron produced by humans was wrought iron. To produce it, an iron worker heats iron ore in a very hot fire, between 800°C and 1000°C. At these temperatures iron ore doesn't melt, but it does become soft and spongy. The iron worker beats the ore with a hammer. This beating action is called forging, and it serves several purposes. It drives the impurities, or slag, out of the ore. (Slag is mostly silicon compounds, or to put it less technically: bits of rock.) The beating action also allows smelting, by encouraging the soft ore to combine with carbon monoxide from the fire, which transforms the iron oxide to iron. Furthermore, repeatedly beating the metal can change the metal's properties, for example, making it stronger. And of course, the iron worker also beats the iron with a hammer to get the metal into the desired shape.
Since it isn't possible to drive out all of the slag, wrought iron is generally up to 2% slag. It is softer than other kinds of iron or steel, though harder than copper or bronze. (Hardness means that a metal is difficult to scratch, dent, or bend out of shape, and also that if sharpened it keeps its edge well.) Wrought iron is ductile - if you exert enough force on it, it will bend rather than shattering. It is resistant to rust. A disadvantage of wrought iron is that it can only be made into simple shapes, since it has to be beaten into shape with a hammer.
To make cast iron, iron ore is heated to temperatures of 1150°C to 1350°C. At these temperatures the iron becomes liquid. The slag also becomes liquid and floats to the top, and can be poured off. The liquid ore easily combines with carbon monoxide gas from the fire to form iron. The liquid iron is then poured into molds, which allows the iron to be formed into any desired shape.
Cast iron is harder than wrought iron, but it is also more brittle (if you strike it hard enough it will shatter rather than bend) and prone to rust.
Most steel-making processes use cast iron as a starting material.
Steel is a group of iron alloys with between 0.3% and 2.1% carbon, which have properties such as hardness and resistance to rust which make them useful to people.
There are many types of steel, and there are a few different processes for making it. The starting point of most steel-making processes is cast iron. During the steel-making process nearly all impurities in the metal are removed, and the carbon content is lowered. Sometimes other metals, such as nickel or chromium, added.
Modern-day steels are grouped into four categories:
All iron or steel contains some carbon. If a thin sliver of iron or steel is scanned using a very powerful microscope, tiny chunks of carbon can be seen. Wrought iron is less than 0.08% carbon, while cast iron is 2.1% to 4% carbon, and steel has between 0.3% and 2.1% carbon.
How does the carbon get in the iron? A small amount of carbon may be naturally present in the iron ore, but more is added during the smelting process. During smelting, some coal (coal, whether charcoal or mineral coal, is carbon) dissolves into the iron.
Cast iron has a high carbon content of around 4%, while steel has 2.1% at most. Steel-making processes use cast iron as a starting point, so part of any steel-making process is to lower the carbon content. One way to do this is by blowing oxygen through hot liquid iron, which removes the carbon by converting it to carbon monoxide or carbon dioxide.
Iron forging is the process of repeatedly beating a piece of iron or steel with a hammer while it is hot. Repeatedly striking the metal can change its properties, making it stronger or harder.
The reason forging works is that all iron or steel contains some carbon or other impurities. Forging changes the shape and structure of these impurities, and thus changes the properties of the metal.
Forging is necessary in the production of wrought iron. However forging is also sometimes done with cast iron or with steel, in order to improve the metal's properties. In the past forging has been done by a person holding a hammer, or by trip hammers powered by flowing water; in modern-day factories forging is done by trip hammers or other machines powered by electricity.
Forging iron or steel to improve its properties is a fiddly, tricky process. To get the perfect result, it's necessary to find the perfect temperature and the perfect duration of forging, and these depend on the particular iron or steel being forged; therefore iron and steel forging processes are usually worked out through trial and error.