Adding carbon to steel makes it harder because the carbon atoms disrupt the iron's crystal structure, making it more difficult for the iron atoms to move past each other.
Here's a breakdown of the process:
Why Steel is Normally Ductile
Pure iron has a crystal structure where atoms are arranged in an orderly pattern. This arrangement allows layers of iron atoms to slide relatively easily over one another when a force is applied. This property makes pure iron ductile, meaning it can be stretched and deformed without breaking easily.
The Role of Carbon
When carbon is added to iron, it forms steel. The carbon atoms are much smaller than the iron atoms and fit into the spaces (interstices) between the iron atoms in the crystal lattice.
Distortion of the Crystal Lattice
These smaller carbon atoms disrupt the regular arrangement of the iron atoms. This distortion of the crystal lattice makes it harder for the layers of iron atoms to slide past each other.
Increased Hardness and Strength
The impediment to the movement of atoms increases the steel's resistance to deformation, which translates to increased hardness and strength. In essence, the carbon atoms "lock" the iron atoms in place, preventing easy movement and making the steel much more resistant to dents, scratches, and bending.
Different Carbon Concentrations
The hardness of steel can be controlled by adjusting the amount of carbon added. Generally, higher carbon content leads to greater hardness, but also increased brittleness. Steels are classified by their carbon content, impacting their properties and applications:
| Steel Type | Carbon Content (%) | Characteristics | Applications |
|---|---|---|---|
| Low-Carbon Steel | 0.05 - 0.25 | Ductile, machinable, weldable | Car bodies, pipes, construction materials |
| Medium-Carbon Steel | 0.25 - 0.60 | Stronger, harder, less ductile than low-carbon steel | Machine parts, axles, gears |
| High-Carbon Steel | 0.60 - 1.25 | Very hard, brittle | Cutting tools, springs, high-strength wires |
| Ultra-High-Carbon Steel | 1.25 - 2.0 | Extremely hard, brittle, used in specialized applications | Knives, punches, dies |
Therefore, the addition of carbon to steel fundamentally alters the way the iron atoms interact, significantly increasing its hardness and strength by distorting the crystal lattice.
