How Does Carbon Steel Harden?
How Does Carbon Harden Steel?
Carbon Steel Is All Around Us
Steel isn’t just an alloy of carbon and iron, it’s one of the most vital parts of the structure of our modern world. Without it our skyscrapers would topple, our automobiles would crumble and industry would come to a halt around the globe. Many years have passed since the unique ability of carbon to harden steel was first discovered. So, we no longer think of the phenomenon of carbon in steel as revolutionary. But it is still interesting.
Carbon Is Added to Molten Iron to Make Steel
In solid form, pure iron molecules are arranged in a crystal structure Each crystal contains nine iron molecules—one in the center and eight on the outside in a cube shape. Arranged this way, the iron molecules can slide apart with relative ease. This is why iron is easy to dent, bend and mold. The word used to describe this quality is “ductility.” The ductility and durability of iron make it useful for many applications, but not those that require hardness, such as for tools and machine parts. To increase the hardness of iron, it is melted and carbon is added to the iron while it is molten.
Carbon and Iron Molecules Form a New Crystal Structure as They Cool
As the molten iron and carbon molecules cool, the carbon molecules situate themselves within the structure of the iron crystal. In this new arrangement, the molecules can no longer slide apart easily. The resulting crystal structure of steel is therefore stronger than pure iron.
The Amount of Carbon Affects Steel Hardness
The more carbon molecules that are in steel, the harder the steel will be. The hardest grades of steel contain as much as two percent carbon, and are used to make knives and other tools. Low carbon steels contain as little as 0.05 percent carbon, and are used as structural steel members. In between are carbon steels that are used for machine parts, springs and wires, among many other things. Steel may contain other elements such as manganese, chromium, copper or other metals , but none are as important for hardness as carbon.
Steel Is More Brittle Than Iron
Just as higher carbon content in steel means greater hardness, it also means the steel is less bendable, or ductile. Consequently, carbon steel is more brittle than iron, and more difficult to form into long wires and thin sheets. These dilemmas are solved, at least somewhat, by heat treating the steel.
Heat Treating Affects the Hadness of Carbon Steel
Alternately heating and cooling steel causes the carbon molecules to move in relation to iron molecules, so the crystal structure is changed yet again. The qualities of hardness and ductility change as well. There are several processes for heating treating steel. In each, different temperatures and different heating and cooling times are used to achieve particular molecular arrangements in the steel. By selecting the carbon content of steel and the heat processing methods, steels of different hardness and ductility can be made to suit a wide variety of purposes.
If you have questions about Carbon steel or other metal products, feel free to reach out to Carlson Manufacturing, Inc.