How does carbon exist within the microstructure of steel?

In solid steel, carbon exists primarily in two forms within the microstructure:

• Solid Solution: Carbon atoms can be held in solid solution within the crystal lattices of austenite or ferrite. The face-centred cubic (fcc) structure of austenite provides more space for these foreign atoms than the body-centred cubic (bcc) structure of ferrite.12
• Carbides: Carbon may combine with iron to form iron carbide, known as cementite, or it may form carbides with other alloying elements such as titanium.1

These forms significantly influence the material's properties. For example, during cooling, austenite can transform into pearlite, a microstructure consisting of alternating platelets of ferrite and cementite. Additionally, rapid cooling or quenching can trap carbon in a supersaturated state to form martensite, a phase that provides high hardness.13

Note that in gray iron, the presence of silicon suppresses carbide formation, causing carbon to appear instead as graphite flakes or clusters.1