How does the addition of carbon affect the properties of steel?

The addition of carbon to steel significantly influences its properties, primarily increasing its hardness and tensile strength. Carbon acts as a solid solution strengthener, meaning it interferes with the movement of dislocations—defects in the crystal structure of metals that allow them to deform. When carbon atoms are introduced into the iron matrix, they create obstacles for dislocation motion, which results in a stronger and harder material. This is a key reason why carbon steel is widely used in applications where strength and hardness are essential, such as in construction, automotive, and manufacturing industries.

Furthermore, the proportion of carbon can be precisely controlled to tailor the properties of the steel for specific applications. Higher carbon content typically leads to greater hardness and strength but usually at the expense of ductility. Though this does somewhat decrease the material's ability to deform under stress, the trade-off is often desirable in many engineering applications.

While the addition of carbon can influence other properties—such as ductility and thermal conductivity—these effects do not define the primary significance of carbon in steel. The enhancement of hardness and tensile strength stands out as the most important factor when evaluating the role of carbon in steel alloying. This is why the correct answer highlights the increased hardness and tensile strength that result from carbon addition