How Iron and Steel Work

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How Iron and Steel Work
by Marshall Brain

If you had to pick one technology that has had a tremendous effect on modern society, the refining of iron and steel would have to be somewhere near the top of the list. Iron and steel show up in a huge array of modern products. Cars, tractors, bridges, trains (and their rails), tools, skyscrapers, guns, ships -- even the common steel can -- all depend on iron and steel to make them strong and inexpensive. Iron is so important that primitive societies are measured by the point at which they learn how to refine iron and enter the iron age!

Have you ever wondered how people refine iron and steel? You probably have heard of iron ore, but how is it that you extract a metal from a rock? If you have ever wanted to know all about iron and steel, then you will love this edition of How Stuff Works!

The Advantages of Iron
Iron is an incredibly useful substance for several reasons:

Relatively speaking, and especially when compared to wood or copper, iron is extremely strong.

By heating it, iron is relatively easy to bend and shape using simple tools.

Unlike wood, iron can handle heat, so you can build things like engines from it.

Unlike most substances, you can magnetize iron, making it useful in the creation of electric motors and generators.

Iron is plentiful -- 5% of the earth's crust is iron, and in some areas it concentrates in ores that contain as much as 70% iron.

It is relatively easy to refine iron using simple tools.

When you compare iron and steel with something like aluminum, you can see why it was so important historically. To refine aluminum you must have access to huge amounts of electricity. To shape aluminum you must either cast it or extrude it. Iron is much easier to deal with by comparison. Iron has been useful to man for thousands of years, while aluminum really did not exist in any meaningful way until the 20th century (fun fact - the tip of the Washington Monument is made of a 10 inch high aluminum pyramid rather than gold, because gold was less valuable than aluminum in 1884!).

An object like the flintlock rifle would be impossible to create without iron. Fortunately, iron can be created relatively easily with tools available to primitive societies. There will likely come a day when we become so technologically advanced that iron is completely replaced by aluminum, plastics and things like carbon and glass fibers. But right now the economic equation gives inexpensive iron and steel a huge advantage over these much more expensive alternatives.

The only real problem with iron and steel is rust. Fortunately you can control rust with paint, galvanizing, chrome plating or sacrificial anodes.

Iron Ore
To make iron you start with iron ore. Iron ore is nothing more than rock that happens to contain a high concentration of iron. One thing that gave certain countries an edge between the 15th and 20th centuries was the availability of iron ore deposits. For example, England, the U.S., France, Germany, Spain and Russia all have good iron ore deposits. When you think of the historical importance of all of these countries, you can see the correlation!

Common iron ores include:

Hematite - Fe2O3 - 70% iron
Magnetite - Fe3O4 - 72% iron
Limonite - Fe2O3 + H2O - 50% to 66% iron
Siderite - FeCO3 - 48% iron

Usually you find these minerals mixed into rocks containing silica.

Creating Iron
You can see in the previous section that all of the iron ores contain iron combined with oxygen. To make iron from iron ore, you need to eliminate the oxygen to create pure iron.

The most primitive facility used to refine iron from iron ore is called a bloomery. In a bloomery you burn charcoal with iron ore and a good supply of oxygen (provided by a bellows or blower). Charcoal is essentially pure carbon. The carbon combines with oxygen to create carbon dioxide and carbon monoxide (releasing lots of heat in the process). Carbon and carbon monoxide combine with the oxygen in the iron ore and carry it away, leaving iron metal.

In a bloomery the fire does not get hot enough to melt the iron completely, so you are left with a spongy mass containing iron and silicates from the ore (the bloom). By heating and hammering the bloom, the glassy silicates mix into the iron metal to create wrought iron. Wrought iron is tough and easy to work, making it perfect for creating tools in a blacksmith shop.

The more advanced way to smelt iron is in a blast furnace (see this extremely nice blast furnace animation). A blast furnace is charged with iron ore, charcoal or coke (coke being charcoal made from coal) and limestone (CaCO4). Huge quantities of air blast in at the bottom of the furnace. The calcium in the limestone combines with the silicates to form slag. At the bottom of the blast furnace, liquid iron collects along with a layer of slag on top. Periodically you let the liquid iron flow out and cool. Typically the liquid iron flows into a channel and indentations in a bed of sand. Once it cools, this metal is known as pig iron.

To create a ton of pig iron you start with 2 tons of ore, 1 ton of coke and half a ton of limestone, and the fire consumes 5 tons of air. The temperature reaches almost 3000 degrees F (1600 or so degrees C) at the core of the blast furnace!

Pig iron contains 4% to 5% carbon and is so hard and brittle it is almost useless. You do one of two things with pig iron:

You melt it, mix it with slag and hammer it to eliminate most of the carbon (down to 0.3%) and create wrought iron. Wrought iron is the stuff a blacksmith works with to create tools, horseshoes, and so on. When you heat wrought iron, it is malleable, bendable, weldable and very easy to work with.
You create steel (see the next section).

One of the very interesting properties of wrought iron is the ability to harden it by heating and then quenching it. Cooled slowly, iron is fairly soft. But quick-cooling it by queching red hot iron in water will make it much harder. This page goes into the process of hardening iron in some detail.

Creating Steel
Steel is iron that has most of the impurities removed. Steel also has a consistent concentration of carbon (0.5 to 1.5%) throughout. Impurities like silica, phosphorous and sulfur weaken steel tremendously, so they must be eliminated. The advantage of steel over iron is greatly improved strength.

The open hearth furnace is one of the most common ways to create steel from pig iron. Into an open hearth furnace goes pig iron, limestone and iron ore. It is heated to about 1600 degrees F. The limestone and ore forms a slag that floats on the surface. Impurities, including carbon, are oxidized and float out of the iron into the slag. When the carbon content is right, you have carbon steel. A variety of metals might be alloyed with the steel at this point to create different properties. For example, the addition of 10% to 30% chromium creates stainless steel which is very resistant to rust. The addition of chromium and molybdenum creates chrome-moly steel which is strong and light.

Another common way to create steel from pig iron is the Bessemer process.

Conclusion
When you think about it, there are two accidents of nature that have made it much easier for humans to move forward rapidly. One is the huge availability of something as useful as iron ore. The second is the availability of vast quantities of oil and coal to power the production of iron. This is an amazing coincidence, because without iron and energy, we would not have gotten nearly as far as we have today!

Links
You can learn a great deal more from the following links: