Steel can be found everywhere. But what makes it so versatile?
Steel production in Iran. Source
Smelting simplified. Source
Steel is an alloy of iron and carbon. It is used already for centuries, starting with the first smiths forging daggers and nails in their coal foundries. This process lead to increased robustness of the rather soft iron.
Carbon steel accounts for around 90% of steel production. The rest is made up by alloy steels, which is known as stainless steel, but has a high variety of different mixtures of indigents.
The Iron-Carbon diagram shows the different mixtures of steel depending on carbon content. Wikimedia
Iron can have two different structures: alpha (α) and gamma (γ) iron. At room temperature, α-iron assembles a cubic structure, with 8 atoms forming the cube, and a 9th atom being in the middle. This is called cubic room centric.
At higher temperatures, above 1394 °C, it changes to γ-iron, a face-centered cubic structure. It has one atom at every corner and one on each face (think at a dice).
Alpha (α) and gamma (γ) iron structure. Wikimedia
The magic now happens when iron is heated while carbon is around: The γ-iron has more atoms, but also a bigger spacing between them. This makes it possible for carbon atoms to slip between them.
If the mixture is then cooled down and the iron switches back to the α-iron, cubic room centric structure, the carbon atom becomes locked between the iron atoms. The speed of cooling can increase the hardness of the steel, as more carbon is trapped.
If the metal is cooled down too fast and it took in much carbon, it can become so hard that it isn't elastic anymore, and will shatter like glass instead of bending.
Big pieces would need a long time to cool down; water increases the cooling and leads to a more homogenous material. Source
The main focuses on producing carbon steel are:
Other elements like sulfur or oxygen can make the steel brittle, susceptible to corrosion and harder to machine. A manufacturer strives to drive these amounts as low as possible, without increasing the costs too much.
Being more expensive than carbon steel, alloys with components additional to carbon bring a lot more abilities to this material.
Adding molybdenum, manganese, chromium, or nickel, in amounts of up to 10% by weight, the hardness of the metal is improved. While it is vulnerable to rust, it is often cheap enough to add a surface treatment instead of choosing a more expensive alloy. This can be done my painting or galvanic treatment, which covers the surface with a thin layer of non corrosive metal.
High strength low alloy steel has additions that are usually < 2%. Adding 1.5% manganese can increase the strength for a relatively low price increase. They combine a lower price with a higher strength, and can so offer something in between carbon and stainless steel.
Stainless steels contain at least 11% chromium, often combined with nickel. This gives stainless steel its famous resistance to corrosion. By adding other components the steel can also become non magnetic, harder, have a different color, stronger, more elastic, lower density and more. With adding more components, the material also gets a lot more expensive. That explains while normal carbon steel is so popular.
It is often possible to find out if the steel is stainless by checking it with a magnet. If the magnet doesn't stick, it is stainless steel. If it does stick, you might have a polished piece of carbon steel.
The greatest benefits of steel are:
After characteristics, there will be a closer look on the ways steel is made, formed and used in the industry.
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