Which element in steel primarily determines its hardness and strength?

The element in steel that primarily determines its hardness and strength is carbon. The presence of carbon in steel significantly influences its mechanical properties. When carbon is added to iron, it forms a stronger and harder alloy. This is because carbon atoms disrupt the regular arrangement of iron atoms in the steel's crystalline structure, which increases the material's hardness and tensile strength.

Higher carbon content enhances the strength of the steel, making it suitable for structural applications where durability and resistance to deformation are critical. The balance of carbon in steel is crucial; while increased carbon content can provide enhanced strength, it can also make the steel more brittle if taken too far.

In contrast, while elements like nickel and manganese can also affect the properties of steel, they are primarily used to improve specific characteristics such as toughness and corrosion resistance, rather than being the main determinant of hardness and strength. Iron, being the base metal, contributes to the overall structure but does not directly influence hardness in the same way that carbon does.