How the Carbon Content of Steel affects its Physical Properties
Metal has been an essential part of our history. It has allowed us to develop new technologies and industrialize, catapulting us into the modern world. The metal in the middle of it all is steel. Steel is an alloy composed of iron and carbon. The earliest evidence of steel being produced was believed to originate from 1800 BC. The people credited with being the first to mass produce high quality steel are the Chinese in the 3rd Century AD. One of the earliest techniques for making steel was known as “blister steel”. During the heating and reheating process carbon monoxide gasses were created in the steel causing bubbles to form, hence the name. Steel-making methods evolved over time from the previously mentioned “blister steel” method to a more modern electric arc method. Steel has all kinds of uses, such as in building, machines, tools, and etc. To be such a versatile metal it must have many different properties. The “magic” behind steel is that it’s properties actually change depending on the amount of carbon it contains. The percentage of carbon affects the metal’s properties including hardness, strength, and others.
Today, there are two ways to make steel widely used: using a basic oxygen furnace, BOS, or using an electric arc furnace, EAF. Both methods start similarly but have some key differences. For both the first thing to do is ironmaking, done by putting iron ore, coke, and lime in a blast furnace to create molten ore. In the BOS method scrap steel is added to the molten ore inside a converter, where oxygen is blown through the metal, dropping its carbon content between 0 and 1.5%. The EAF method adds scrap steel and feeds it into powerful electric arcs to melt and convert it into high quality steel.
Figure 1: An industrial electric arc furnace in use.
The key element in making steel is choosing what carbon content you have to create certain types of steel. Steel with a carbon content of lower than .25% is considered a low-carbon steel. A carbon content between .25% and .55% creates medium-carbon steel. Anything with a content higher than .55% carbon is a high-carbon steel. As carbon content increases the steel generally becomes harder and more brittle, which has both advantages and disadvantages.
Figure 2: A chart illustrating how carbon content affects physical properties such as strength and hardness.
High-carbon steel is incredibly hard and very resistant to wear and corrosion. These properties make it ideal for holding a cutting edge in sharp tools as well as other tools such as hammers and wrenches. High-carbon steel is not very ductile so it does have limited uses.
Figure 3: Knives made out of high-carbon steel.
Low-carbon steel isn’t as hard as high-carbon steel, but it has many properties making it more well suited for a variety of applications. It is more malleable, ductile, and tough than its higher-carbon counterpart, making it ideal for uses such as car parts and structural shapes. Low-carbon steels are also cheaper to produce than other variations.
Figure 4: Pipes made out of low-carbon steel.
The carbon content of steel affects its micro structures. These microstructures are ultimately what gives the metal its behaviors. There are many different types of microstructures which form depending on the amount of carbon present. The spaces between the iron atoms in steel are called lattices, and when a very small amount of them are filled with carbon atoms it is known as ferrite. Ferrite is soft, ductile, and is similar to pure iron. Ferrite comes in a body-centered cubic, or BCC, structure; which can be seen below.
Figure 5: An example of a body-centered cubic structure.
Raising the temperature will bring the steel to a phase known as austenite, where the gaps are a little larger, allowing for more carbon between the iron. Austenite also happens to come in a face-centered cubic, or FCC, structure seen below.
Figure 6: An example of a face-cent₩ered cubic structure.
As the austenite cools down, it creates a mix of ferrite and a new phase known as cementite. Since ferrite can only be .006% carbon at room temperature any parts with more than that become cementite. Opposite of ferrite, cementite is very brittle and hard. This mixture of both these phases is actually known as pearlite.