As the carbon content in steel increases, what happens to the strength and ductility?

Increasing the carbon content in steel enhances its strength due to the formation of cementite and other hardening phases, which contribute to the overall hardness and tensile strength of the material. As carbon atoms are added to the iron matrix, they create interstitial solid solutions, making it more difficult for dislocations (which are defects in the crystal structure that allow deformation) to move. This increased resistance to dislocation motion translates into higher strength characteristics.

However, this increase in carbon content adversely affects the ductility of steel. Ductility refers to a material's ability to deform plastically without fracturing. Higher carbon content leads to a more brittle structure, primarily because the additional carbon phases hinder the movement of dislocations needed for plastic deformation. As such, while the material can support greater loads, it becomes less capable of undergoing significant deformation before failing.

In summary, with higher carbon content, steel benefits from increased strength but at the cost of reduced ductility. This characteristic makes high-carbon steels suitable for applications requiring high strength but less so for those needing malleability or formability.